Explanations in evolutionary theory¹

Introduction

Countless people have followed the recent debates and trial in Harrisburg, Pennsylvania, USA on the teaching of biological evolution versus intelligent design in public schools in the United States as well as the earlier one in the state of Arkansas, USA on teaching biological evolution versus scientific creationism. In both cases the courts declared against the teaching of intelligent design and scientific creationism as being religious, not scientific, and hence violating the separation of church and state in the United States Constitution. Most interesting in these two trials was the spectrum of meanings given to the Theory of (Biological) Evolution which varied so much that one could be surprised that any decisions could be reached at all in these trials. Most likely these court decisions were reached, not on the basis of understanding what is meant by biological evolutionary theory, but on a conclusion that intelligent design and scientific creationism are not scientific theories (Brockman 2006).

A major foundation for this diversity in the meanings of the theory of evolution goes right back to 1859 and the publication of Darwin's ‘On the Origin of Species’, namely that:

Over the decades as more and more was learned about evolution, many evolutionists stated that Biological Evolution was no longer a theory, but was factual or a fact (= an objective empirical observation) which confused the issue even more as a sharp difference exists between scientific theories and objective empirical observations. In most of the statements that evolution is a ‘scientific fact’, the author actually meant that historical evolutionary theory (= the general notion that living organisms descended with modifications from a common ancestor) is so exceedingly well tested (= well corroborated, Popper 1959; 1968: 32–34) that it can for all intense purposes be accepted as factual. However, it is still better to state that historical evolutionary theory is an exceedingly well corroborated theory and that massive counter tests supported by strong empirical objective observations are needed to disprove it. Facts, as used in science, are quite different from theories and the two are best kept strictly separated.

The major themes to be addressed in this essay have been nicely summarized a quarter-century ago by Mayr (1982: 8) in the introductory chapter of his The Growth of Biological Thought where he wrote: ‘‘As a consequence, some exceedingly confused accounts of the history of biology have been published by authors who did not understand that there are two biologies, that of functional and that of evolutionary causations. Similarly, anyone who writes about ‘Darwin's theory of evolution’ in the singular, without segregating the theories of gradual evolution, common descent, speciation, and the mechanism of natural selection, will be quite unable to discuss the subject competently.’’ Today the problems are much the same as when Mayr penned this passage in the late 1970’s with the addition that most scholars do not comprehend the distinction between nomological-deductive (N-DE) and historical-narrative explanations (H-N·Es) and hence that major differences exist between nomological and historical theories of evolution. Mayr hinted at this last difficulty when he wrote (Mayr 1982: 27): ‘‘Every evolutionist who has had a discussion with lay people has been asked: ‘Has evolution been proven?’ or ‘How do you prove that man descended from apes?’’ But he did not emphasize that these represent quite different scientific questions and require different modes of explanation, the first being nomological-deductive and the second historical-narrative. Later in this introductory chapter, Mayr argued (pp. 71–76) for the importance of historical narratives and for the need of a philosophy of biology separate from that of the physical sciences. Yet he never pushed these ideas to the logical conclusion that a distinction must be made between nomological and historical theories of evolution, most likely because of his strong aversion to law-like statements in science and hence to nomological explanations. Possibly Mayr equated functional explanations with nomological-deductive and evolutionary explanations with historical-narrative which is largely true although this relationship does not hold quite so simply (Bock 2004a). Or possibly because Mayr did very little work in functional biology, he was not sufficiently versed in the nuances of nomological-deductive explanations (N-D·Es) and overlooked their role in evolutionary biology. Or possibly, he restricted his view of biology to evolutionary explanations and considered all other aspects of biological study (= strictly functional explanations) as physical sciences.

Because Darwin always referred to his ideas as my theory (Mayr 1982: 8; 1985: 757) – always in the singular – almost all workers accepted the existence of only single theory of biological evolution which was regarded as being strictly historical. Most specialists and laity alike have been and still are mainly interested in the historical evolutionary theory, that is, in the history of life, or in modern terms –‘The Tree of Life’. This is nicely shown by the history of evolutionary studies in which biologists accepted rapidly the theory of common descent (= historical evolutionary theory), but ignored or rejected Darwin's theory of natural selection as well as gradualism and speciation (= nomological historical theory; see Mayr 1985). Many biologists, then as well as now, do not care about nomological theories of evolution and the relationship between them and historical evolutionary theory. And if both nomological (= process) and historical (= pattern) evolutionary theories are mentioned, emphasis is always placed on the latter with little consideration on how the two are connected. Even Popper (1977) focused so strongly on the historical aspect of evolutionary theory that he concluded that all evolutionary biology is historical and hence according to his approach to science, evolutionary theory is not scientific (see Hull 1999, for a detailed analysis). Caplan (1977, 1978, 1979), on the other hand, provided a convincing argument that [nomological] evolutionary theory is not circular and is deductive. A close examination of his papers reveals that Caplan's position applies only to some of the set of Darwin's theories about organic evolution (Mayr 1985) and in particular to the one dealing with evolutionary mechanisms (theory D, see below). Caplan's analysis definitely does not apply to the last of these theories (theory E, or common descent) which is the aspect of evolutionary theory foremost in most people's mind when they use the term ‘Darwinism’ or ‘evolutionary theory’. Futumya in his text books (1998:11–12; 2005:11–15) implies that a distinction exists between nomological and historical evolutionary theories, but does not develop these ideas fully.

In this essay four major points have been addressed, namely: (1) Examining the effect of horizontal versus vertical comparisons in biology on evolutionary concepts; (2) providing a definition of evolution which will serve as the foundation on which to examine the scope of evolutionary theories and to show which of these theories are nomological and which are historical; (3) arguing that Darwin proposed a set of at least five different evolutionary theories, not just one, as advocated by Mayr (1985) which can still be used today to characterize the major, independent evolutionary theories; and (4) showing which of these evolutionary theories are nomological and which are historical, following Bock (2004a), and what is the relationship between the two types of evolutionary theory.

Horizontal and vertical comparisons

Before proceeding into the main discussion on evolutionary theory, the consequences of horizontal and vertical comparisons in biology (Bock 1989a) on evolutionary thinking have to be considered. In the middle of the 19th century almost all biologists believed that only single type of comparisons existed – that is, all comparisons between all organisms are the same because at that time no general concept had yet been advocated of organisms changing over historical time. This belief was excusable in early Darwinian thinking but not thereafter, although most biologists still accept today that a single type of comparison exists and that conclusions can be readily extrapolated from any comparison to another.

It must be stressed that the idea of horizontal versus vertical comparisons is absolutely different from the concepts of horizontal versus vertical evolution. The latter set of concepts implies an absolute difference in the time needed for each type of evolutionary change, but these terms refer to speciation (or splitting of phyletic lineages = horizontal) versus phyletic evolution (= vertical); both types of evolutionary change require time, albeit different amounts. Use of the terms horizontal versus vertical evolution is misleading to the extent of being wrong and is best not used.

Horizontal comparisons are those between individual organisms of the same species taxon or between members of different species taxa (= members of different phyletic lineages) whether or not the organisms exist at the same time level. Most biological comparisons are horizontal. In horizontal comparisons, not only can homologous features be compared but also nonhomologous ones depending on the nature of the comparison. Differences observed in horizontal comparisons can never be evolutionary modifications because they do not represent changes between different stages within a phyletic lineage (see Bock 1979, 1986, 1995, 2004b, for the distinction between the species concept and the phyletic lineage concept, and between the species concept, the species category, and the species taxa.). Because, interspecific horizontal differences are between different species – between organisms of different phyletic lineages – great care must be used when extrapolating results of horizontal comparisons to represent phyletic evolutionary changes (Bock 1979).

Vertical comparisons are those between members of the same phyletic lineage existing at different times, that is, between ancestral and descendent groups. Because vertical comparisons are between organisms in different time slices of the same phyletic lineage, these comparisons are never between different species. The features being compared vertically are generally homologous ones, although it might be possible to have useful vertical comparisons between nonhomologous features. Differences noted between homologous features in vertical comparisons represent evolutionary modifications. Empirical vertical comparisons are difficult to make because one generally does not know whether the organisms being compared are members of the same phyletic lineage even when dealing with fossils. Hence such comparisons are largely hypotheses and are indicative of whether the organisms are believed to be members of the same phyletic lineage at different points in time which are almost always theoretical.

Horizontal species boundaries are between species taxa existing at the same time and same place (synchronous and sympatric) and represent the barriers to gene flow between species; these boundaries are considered to be unbridgeable, although in reality they may be somewhat ‘leaky’ but not to the extent of resulting in definite gene flow between the two species.

Vertical species boundaries are postulated to be ‘similar’ to horizontal boundaries between species taxa along the same phyletic lineage, but their exact nature has always been left rather vague. Most workers advocating vertical species boundaries have assumed that they are the same as the unbridgeable horizontal boundaries (= barriers to gene flow), and hence special evolutionary processes are required for this trans-specific evolutionary change which are different than those existing for evolution within the species limits. Yet if phyletic evolutionary change is gradual, then such vertical species boundaries cannot exist, and that there is no difference between ‘evolution within the species limits’ and ‘evolution beyond the limits of the species’ (= trans-specific evolution).

Hence, horizontal comparisons are between members of the same species taxon or between members of different species taxa (= different phyletic lineages), while vertical comparisons are between members of the same phyletic lineage at different times. Valid conclusions reached on the basis of a horizontal comparison cannot automatically be extrapolated to a vertical comparison and vice versa. In the following discussion about species, I follow the biological species concept (Mayr 1942, 1963) and not the evolutionary or the phylogenetic species concepts (see Wägele 2000 for a good modern discussion of these concepts). The biological species concept is characterized, quite correctly by Mayr as non-dimensional, as distinguished from the multi-dimensional species taxon. The evolutionary and phylogenetic species concepts do not distinguish between the species concept and the phyletic lineage concept, causing confusion. Most philosophers of biology do not make a distinction between the species concept and the phyletic lineage concept, often leading to confusion in their analyses. Furthermore, distinctions must be made between the species concept, the species category and the species taxon (Mayr 1963; Bock 1995, 2004b), to avoid the ambiguity arising when using only the term species.

For Darwin and other biologists of his time, different species taxa of organisms were considered to be separated from one another by unbridgeable gaps – a horizontal comparison. That is, definite boundaries or limits existed around each species taxon. A major problem facing Darwin and other early evolutionists was how this boundary can be overcome in the evolution of one species taxon to another. The concept of ‘the horizontal barrier’ between species taxa had been extended automatically to the idea of ‘a vertical boundary’ separating an ancestral species taxon from a descendent one – a vertical comparison. This is what Darwin had in mind when he concluded that with sufficient (phyletic) evolutionary change, a new species taxon would arise. Or to put this another way, a foremost question for biologists at that time was how much evolutionary change was necessary before a variety of a species taxon (a particular breed, such as the breeds of the domesticated dog, or a subspecies or geographic race), reached the status of a new species taxon – this question still exists for many workers. Darwin failed to realize that two distinct evolutionary processes existed – phyletic evolution and speciation – and concentrated on phyletic evolution which led to his famous disagreement with Moritz Wagner (see Mayr 1982: 562–566). Wagner had stressed the role of geographic isolation in evolution and especially in the origin (= multiplication) of new species taxa. It is impossible to be precise on how most evolutionists (including Darwin and Wagner) viewed the origin of new species taxa until the period of the Evolutionary Synthesis (1937–1948), because of vagueness in stated positions on both sides.

Yet it was clear well before 1859 biologists realized sharp and definite boundaries existed between sympatric and synchronous species taxa. In modern terms, these horizontal, unbridgeable boundaries around species taxa are formed by intrinsic genetic isolating mechanisms (Bock 1995, 2004b), preventing the flow of genetic information from species to species. With the acceptance of evolutionary ideas and the realization that species taxa had a history, biologists automatically extended this horizontal boundary around species taxa to a vertical boundary between species in the same phyletic lineage. This extension was to be expected at that time because Darwin and most other biologists did not appreciate the distinction between the species and varieties (including subspecies or geographical races) within species. By 1860 concepts about geographic races were developing in the United States and Imperial Russia (Haffer 1986, 1992, 1994), mainly in ornithology, but these ideas were rejected by most systematists and evolutionists in the United Kingdom until after the end of the nineteenth century. Further Darwin and most other evolutionists did not understand the role of external barriers in the speciation process and believed that new species arose by continuous phyletic evolution.

Although in 1859 biologists automatically extended this horizontal boundary around species taxa to a vertical boundary between ancestral and descendent species, no factual evidence exists for this vertical boundary. Nor did any need exist to continue the idea of vertical species taxa boundaries as evolutionary theory became better understood. A major source of this problem came from the exceedingly clever and catchy title of Darwin's book ‘On the Origin of Species’ which stuck in the minds of biologists, and was used in much later books such as Dobzhansky's 1937‘Genetics and the Origin of Species’ and Mayr's 1942‘Systematics and the Origin of Species.’ These latter authors borrowed the title of Darwin's book to claim proper kinship with it, but the phrase ‘the origin of species’ in these later titles should be read as ‘evolution’. Yet titles, e.g., ‘Systematics and Evolution,’ would not have been as attractive as ‘Systematics and the Origin of Species.’ And it is most difficult to think of a better title than the one chosen by Darwin for his 1859 volume although species taxa do not originate as implied in the title of his book and the text.

The concept of a vertical boundary as well as a horizontal boundary between species taxa remains until today in the thinking of almost all evolutionists and lay-persons. Until the middle of the twentieth century, this notion still implied, at least for some evolutionists (e.g., Schindewolf 1950; Herre 1951), that a distinct ‘jump’ existed – a macroevolutionary change – from one species taxon to a descendent one, the so-called evolution beyond ‘the level of a species’ or ‘trans-specific’ evolution. And they argued that a special, and usually unknown or mysterious, macroevolutionary mechanism exists that cannot be reduced to the known mechanisms of microevolutionary change – evolution within ‘the bounds of the species taxon’ (but see Bock 1979 for a reductionistic approach). Even after the period of the evolutionary synthesis, some books on evolutionary theory still used the term ‘trans-specific evolution’ or ‘evolution above the species level’ in their title (Rensch 1947), or as chapter or section heading, although without necessarily implying any special meaning to this term.

Most anti-evolutionists are mainly concerned with macroevolution which they consider being change from an ancestral species taxon to a descendent one and which violates their belief that all ‘kinds’ of organisms were created independently from one another. Most have correspondently little interest in microevolution which is regarded as small modifications within the limits of a species taxon, but never exceeding these limits. Some anti-evolutionists accept the numerous factual examples of change within species taxa, such as the development of tolerance to pesticides by many species of insects and the development of tolerance to antibiotics by bacteria. For the anti-evolutionists, these examples do not matter as they represent ‘evolution within the bounds of a species’ which differ from macroevolution (= trans-specific evolution). Based on the generally accepted belief by many evolutionists of vertical boundaries between species taxa, the anti-evolutionist can point out quite properly that ‘evolution within the bounds of a species’ cannot be extended automatically to ‘trans-specific evolution’.

Accepting the concept of gradual evolutionary change, the unbridgeable horizontal boundaries present between species taxa cannot be extended to vertical boundaries. In a phyletic lineage, no species boundaries exist between a population existing at a particular time and one existing at a later time, no matter how much phyletic evolutionary change occurs (Bock 1979). Speciation exists as a distinct evolutionary process, but one species taxon does not evolve by phyletic evolution into another species taxon. Rather in phyletic evolution, the attributes of the species taxon can change and change and change, but no matter how much these attributes modify, no species boundary is passed. A distinction simply does not exist between evolution within the bounds of a species and evolution beyond the species limit.

In speciation, two or more species taxa evolve from an ancestral species taxon. When tracing the phyletic lineage from the ancestral species taxon to each of the descendent species taxa, no species boundaries exist as evolution is a gradual change. This leads to a peculiar conundrum that no boundary exists between an ‘ancestral species taxon X’ and a ‘descendent species taxon A’ and no boundary exists between the same ‘ancestral species taxon X’ and a ‘second descendent species taxon B’, but a definite and non-bridgeable horizontal boundary exists between species taxa A and B. This is not the equivalent as saying that species X and A are the same and that species X and B are the same, but only that these two pairs of species taxa are not separated by species boundaries. Yet species taxa A and B are separated from one another by a definite, non-bridgeable horizontal boundary. Because of the phyletic evolutionary change that has taken place in one, and usually in both, of the lineages from X to A and from X to B, characteristics of the ancestral species taxon X differs from those in both species taxa A and B, yet no non-arbitrary boundaries exist between X and A or between X and B. During the phyletic evolution leading from X to A and from X to B, no species boundaries had been transgressed.

Species taxa terminate when they become extinct – that particular phyletic lineage becomes extinct – but they do not have beginnings or births. It is not possible to speak of the age of a species taxon such as the human species being 1.3 million years old or whatever age is placed on the human species. Because species taxa do not have beginnings, all existing species taxa at any point in time are equally old. The notion that a particular species has a certain age is used to indicate only that the particular phenotypic characteristics of that species appeared at that time.

Definitions of evolution

Before it is possible to talk about theories of biological evolution, it is necessary to have a definition of this term. For convenience of comparison, evolution will be defined as: Evolution is change in organisms over time with the minimum time being one generation. Hence evolutionary change is observed between organisms of one generation and their descendants. This definition does not include any condition that the changes have to be genetic or hereditary. This is a variational definition of evolution as pointed out by Lewontin (1983: 63; see also Mayr 1988: 15–16, 1991: 43–44, 1997: 176) which differs sharply from transformational evolution. I would like to compare this definition with a series of other definitions and comments taken from a number of well known sources that would be utilized by many workers interested in the meaning of biological evolution. These references are not meant to be complete or to represent the full span of possible definitions of evolution. Many important textbooks on evolution such as those by Ehrlich and Holm (1963), and by Maynard Smith (1975), do not provide a definition of biological evolution, although the former provides clear definitions of phyletic evolution (p. 327) and of speciation (p. 330) in its glossary.

Dobzhansky (1937: 11; 1951:16) in his epic book heralding the beginning of the modern evolutionary synthesis stated that: ‘As evolution is a change in the genetic composition of populations, the mechanisms of evolution constitute problems of population genetics.’ His wording in the 1951 edition is basically the same. He elaborated (Dobzhansky 1951:11) that: ‘The theory of evolution asserts that (1) the beings now living have descended from different beings which lived in the past; (2) the evolutionary changes were more or less gradual, so that if we could assemble all of the individuals which have ever inhabited the earth, a fairly continuous array of forms would emerge; (3) the changes were predominately divergent, so that the ancestors of the now living forms were on the whole less different from each other than those forms themselves are; (4) all of these changes have arisen from causes which now continue to operate, and which therefore can be studied experimentally.’ The first and, in part, the third of these statements are clearly historical evolutionary theory (see below) which the second, fourth and, in part, the third are nomological evolutionary theory (see below).

Mayr (2001: 286) in his glossary provided the definition that: ‘Evolution: The gradual process by which the living world has been developed following the origin of life.’ And in the text of his book, he (Mayr 2001: 76) wrote: ‘Evolution is best understood as the genetic turnover of the individuals of every population from generation to generation.’ (italics his). This is the standard genetic definition of evolution which excludes at least one important form of evolutionary change – that of cultural evolution. Finally Mayr (2001: 264) stated: ‘It is very questionable whether the term ‘evolutionary theory’ should be used any longer. That evolution has occurred and takes place all the time is a fact so overwhelmingly established that it has become irrational to call it a theory. To be sure, there are particular evolutionary theories such as those of common descent, origin of life, gradualism, speciation and natural selection, but scientific arguments about conflicting theories concerning these topics do not in any way affect the basic conclusion that evolution as such is a fact. It has taken place ever since the origin of life.’ Earlier Mayr (1991: 112) wrote: ‘Weismann's attitude toward evolution as such was close to that of the modern evolutionist, for whom evolution is not a theory but an accepted fact.’ Hence for Mayr, evolution is a fact (= an objective empirical observation).

Gerd von Wahlert (2002), following Professor Vernadsky of Leningrad, USSR who developed the concept of the ‘biosphere’ in the 1920s (see also Hutchinson 1965: 1), defined evolution as development (= change) in the characteristics of the biosphere over time. Whether this definition differs from that of Mayr (above) depends on whether the term ‘living world’ possesses the same meaning as ‘biosphere’; I suspect not quite. The latter includes not only living organisms but all environmental aspects (including all physical factors) of the Earth necessary to maintain these organisms. At least some of these physical factors, such as the generation of an atmosphere rich on oxygen, appear to be the consequence of the action of living organisms. The important aspect of von Wahlert's definition of evolution is that it includes a clear mention of the external environment of living organisms which is an integral factor of evolutionary causes and which is lacking in most other definitions. The biosphere, in von Wahlert's approach, includes not only the living organisms, but the physical characteristics of the Earth. Therefore his expression ‘history of the biosphere’ includes transformational evolution of the earth which is change over time of the same object and biological (= variational) evolution which is the observed change in characteristics of living organisms from one generation to a descendant one. No reason exists why these two types of change cannot be included in a single definition of evolution, but one must be careful in any further development of causes and processes involved in the historical modification of the biosphere. Von Wahlert's definition does not include any claim that evolution is factual or that it has to be genetic.

In his well-known textbook on evolution, Futuyma (1979 503; 1986: 551; 1998, Glossary; 2005: 547) provided the complex statement: ‘Evolution in a broad sense, the origin of entities possessing different states of one or more characteristics, and changes in their proportions over time. Organic evolution, or biological evolution, is a change over time of the proportions of individual organisms differing genetically in one or more traits; such as changes transpire by the origin and subsequent alteration of the frequencies of alleles or genotypes from generation to generation within populations, by the alterations of the proportions of genetically differentiated populations of a species, or by changes in the numbers of species with different characteristics, thereby altering the frequency of one or more traits within a higher taxon.’ His genetic definition of evolution also excludes cultural evolution.

In the very beginning of the text Futuyma (1986: 13) stated: ‘As is indicated above, evolutionary biology consists of two principal endeavors: inferring the history of evolution and elucidating its mechanisms.’ which clearly appreciates both the historical and the nomological theories of evolution. And he wrote further (1979: 14; 1986: 16): ‘Evolution, a fact, rather than a hypothesis, is the central unifying concept of biology.’ and hence agreed with Mayr on this point. In the next edition of this textbook, Futuyma (1998: 11) retained this position and wrote that: ‘In the light of the preceding discussion, evolution [=? historical change] is a scientific fact. But it is explained by evolutionary theory.’ He repeated this position (Futuyma 2005: 13), saying that: ‘Given these definitions, evolution is a fact. But the fact of evolution is explained by evolutionary theory.’ (italics his). He added the explanation (Futuyma 2005: 13) that: ‘What we call facts are hypotheses that have acquired so much supporting evidence that we act as if they were true.’ These are what Popper and many other philosophers of science consider as ‘very well corroborated theories’ and are still best called theories to distinguish them from objective empirical observations which can be considered as facts within the limits of observational abilities and underlying theories.

In another well-known text, Strickberger (2000: 640) defined evolution in his glossary as: ‘Evolution Genetic changes in populations of organisms through time that lead to differences among them.’ Again cultural evolution is excluded. Further Strickberger (2000: 636) defined: ‘Darwinism the concept, proposed by Charles Darwin, that biological evolution has led to many different highly adapted species through natural selection acting on hereditary variations in populations.’

Scott's (2004: 23–45) position on evolutionary ideas are presented in her Chapter 2 ‘Evolution.’ where she wrote under the major heading ‘EVOLUTION BROAD AND NARROW’: ‘In biology, evolution is the inference that living things share common ancestors and have, in Darwin's words, ‘descended with modification’ from these ancestors. The main – but not the only – mechanism of biological evolution is natural selection.’ (pp. 23–24). And just below, she stated that ‘Biological evolution is defined as the descent of living things from ancestors from which they differ.’ (p. 27).

Under the major heading ‘BIOLOGICAL EVOLUTION’, she wrote: ‘Descent’ connotes heredity, and indeed members of species pass genes from generation to generation.’ (p. 27), and at the end of this section, that: ‘Evolutionary biologists are concerned both with the history of life – the tracing of life's genealogy – and the processes and mechanisms that produced the tree of life. This distinction between the pattern of evolution and the process of evolution is relevant to the evaluation of some of the criticisms of evolution that will emerge later in this book. First let's look briefly at the history of life.’

Although Scott made a distinction between discovering the process of evolutionary change and working out the history of life, and does not claim that evolution is a fact, the order in which she presented her ideas is unclear.

Denton (1986: 36–68) in his Chapter 2 on ‘THE THEORY OF EVOLUTION’ did not provide a clear definition of evolution but he was primarily concerned with historical evolutionary theory (= the history of life) and with difficulties of major evolutionary change in the same way as was Behle (1996). Denton was concerned almost exclusively to historical evolutionary theory. And although he did not use this term, he advocated intelligent design as his primary disagreement in dealing with macroevolution.

A committee was established in the 1990s by the American Society of Naturalists and published its report under the title Evolution. Science and Society (Meager and Futuyma 2001), which is well worth reading as it covers thoroughly a broad spectrum of aspects of evolutionary biology, its teaching and its significance to many aspects of human society. I would like to present several quotes from this report.

Under, ‘II. WHAT IS EVOLUTION?’ they wrote (p. 3): ‘Biological evolution consists of change in the hereditary characteristics of groups of organisms over the course of generations. Groups of organisms, termed populations and species, are formed by the division of ancestral populations or species, and the descendant groups then change independently. Hence, from a long-term perspective, evolution is the descent, with modification, of different lineages from common ancestors. Thus, the history of evolution has two major components: the branching of lineages, and changes within lineages (including extinction). Initially similar species become ever more different, so that over the course of sufficient time, they may come to differ profoundly.’

And later in the same section (p. 4) they continued: ‘Evolutionary theory is a body of statements about the processes of evolution that are believed to have caused the history of evolutionary events. Biological (or organic) evolution occurs as the consequence of several fundamental processes. These processes are both random and nonrandom.’ (italics theirs).

And they clarified further (p. 5), saying: ‘It is important to distinguish between the history of evolution and the process held to explain this history. Most biologists regard the history of evolution– the proposition that all species have descended, with modification, from common ancestors – as a fact– that is, a claim supported by such overwhelming evidence that it is accepted as true. The body of principles that describe the causal processes of evolution, such as mutation, genetic drift, and natural selection, constitutes the theory of evolution.’

Next under, ‘III. WHAT ARE THE GOALS OF EVOLUTIONARY BIOLOGY?’ they wrote (p. 5): ‘Evolutionary biology is the discipline that describes the history of life and investigates the processes that account for this history.

Evolutionary biology has two encompassing goals:

After pursuing the comments in the scientific literature, it is always useful to consult dictionaries; as for example, referring to the Concise Oxford English Dictionary (Pearsall 2002) and the Webster's Third International Dictionary of the English Language. Unabridged (Gove 1963). Other dictionaries would provide much the same definitions.

First in Pearsall (2002), the following definitions are offered:

Next Gove (1963) presents the following definitions:

Many evolutionists and philosophers writing texts on evolution or philosophical analyses on evolution never define just what they mean by biological evolution. Dennett (1995: 21) wrote: ‘Let me lay my cards on the table. If I were to give an award for the single best idea anyone ever had, I'd give it to Darwin, ahead of Newton and Einstein and everyone else.’ As far as I can determine Dennett does not provide a definition of evolution. He did say (1995: 39) that: ‘Darwin's project in Origin can be divided in two: to prove that modern species were revised descendants of earlier species – species had evolved – and to show how this process of ‘descent with modifications’ had occurred.’ (italics his). Although numerous aspects of evolutionary theory are discussed in the excellent volume ‘The Darwinian Heritage’ (Kohn 1985), none of the many authors presented a definition of evolution.

With just this diversity of definitions, of lack of definitions of evolution and whether evolution is a theory or a fact, it is a bit surprising that any decisions could have been reached in the two most recent court cases in the USA on the teaching of the theory of evolution as advocated by most evolutionary biologists versus alternative approaches, such as scientific creationism and intelligent design.

Two points should be mentioned about this series of definitions of evolution. Although several authors (Dobzhansky, Futuyma, Scott, Meager and Futuyma, and Dennett) distinguished between the evolutionary history of organisms and the mechanisms (causes and processes) of evolution by which these organisms changed over time, they almost always placed discussion of the mechanisms after presentation of the history of the organisms, rather than the more logical reverse order. The major reason delaying Darwin until 1859 on the publication of his ideas about transformation of organisms was that he believed he could not present a valid case for the evolutionary history of organisms without first providing a strongly supported set of mechanisms. He was completely correct – nomological theory before historical theory. Darwin was well aware of the criticism and abuse piled upon the head of the then unknown author of Vestiges of a natural history of creation (Chambers 1844), largely because of the unrealistic method Chambers suggested for modification of organisms over time (see Secord 2000). Darwin resolved that he would not fall into the same pit; he wanted to present a solidly supported mechanism of natural selection when he advocated his novel concept of evolution of living organisms.

Second, some of these workers used the expression ‘the theory of evolution’ to denote the set of mechanisms of evolutionary change and considered the evolutionary history of organisms so well supported that it is factual, not a theory. (Here one could point out the position of most physicists towards the end of the nineteenth century on the absolute truthfulness of Newton's laws of motion.) These workers overlooked the point that almost all ‘anti-evolutionists’ are uninterested in evolutionary mechanisms or may even accept them for change within the limits of a species. The anti-evolutionists’ major dispute is with evolutionary history of organisms which they argue is a theory only and not a very well supported one at that. They certainly do not consider the evolutionary history of organisms so well supported that it can be regarded as factual in contrast to the discussion in Meagher and Futuyma (2001: 43).

Having presented these other definitions of evolution, I will reiterate the one advocated above:

Evolution: change in organisms over time with the minimum time being one generation. Hence evolutionary change is that observed between organisms of one generation and their descendants. This is variational or Darwinian evolution (Lewontin 1983: 63; Mayr 1988: 15–16, 1991: 43–44, 1997: 176). It differs sharply from transformational evolution, i.e., change of the same object, such a star or the earth, over time. Hence change in the characteristics of an individual organism during its life is not an evolutionary change, but an ontogenetic modification (= transformational evolution; Lewontin 1983; Mayr 1988). Aside from one very vague point in common that change over time occurs, there is nothing similar in transformational and variational evolution – the evolution of the planet earth and the evolution of life on earth differ completely. In a similar vein, there in nothing causally similar between ontogeny (embryologic development during the life of an individual organism = transformational evolution) and Haeckelian phylogeny (= variational evolution), although the latter is the basis for the former (the foundation for Mayr's concept of dual causation in biology, Mayr 2004: 30, and elsewhere).

Note that the definition of evolution advocated herein does not include any statement of heredity or genetic change. Several types of variational evolution exist depending on how information is transmitted from one generation to the next. These are:

Perhaps one could also include in this classification of types of biological evolution modifications of the phenotype that occur because of changes in the external environment and/or the internal interactions between parts of the body. The ability for these modifications in the phenotype is genetic, although the phenotypic change is not so and will reverse in the next generation if the environmental/internal interaction reverses. These modifications have been listed under a series of terms from physiological adaptation to somatic modifications (Bock and von Wahlert 1965). These changes can take place during ontogenetic development and/or during the adult stage, and may be reversible during the life of the organism; the modifications are determined by alterations in the external environment or in the internal interactions between parts of the body (such as muscles affecting the shape of the bony skeleton); no modifications occur in the genetic basis for the observed phenotypic changes. Although this type of evolutionary change is exceedingly common, I do not include it in the above list as types of evolutionary change as it does not depend on a different form of information transmission from an organism in one generation to the next. Unfortunately until recently there has been little consideration of this ‘non-genetic’ evolutionary change (Bock and von Wahlert 1965; West-Everard 2003; Starck 2005) in spite of its extreme commonness in evolutionary change.

Evolution (here I will restrict myself, only for simplicity, to genetic evolution and to sexually reproducing organisms) can be subdivided into two major processes, which are:

(A) Phyletic evolution or anagenesis is evolutionary change in a single phyletic lineage. Phyletic evolution occurs without any speciation; hence no matter how much modification occurs in phyletic evolution, no species boundaries are crossed. The major causes of phyletic evolution are: (1) the generation of individual phenotypic variation; and (2) Selective demands arising from the external environment. I use selective demands arising from the external environment because the term ‘natural selection’ is usually defined by evolutionists as an outcome, not a cause (Bock 1993). Although Darwin used natural selection as a cause in most of his 1859 book, his clearest definition of this term (Darwin 1859: 61, see below) is definitely an outcome definition which was adopted by R.A. Fisher (1930) and J.B.S. Haldane (1932) see also Huxley (1942) who stated in his Preface that his treatment of evolutionary mechanisms are based strongly on the earlier books of Fisher and Haldane). Most evolutionists (e.g., Futuyma 1986: 554; 1998: Glossary; 2005: 550) cite a definition of natural selection which is basically ‘nonrandom differential reproduction of genotypes’ (an outcome) that is adopted from the analyses of Fisher (1930) and Haldane (1932) and then use this term as a cause, leading to considerable confusion (Bock 1993: 11–15). Other workers (e.g., Lewontin 1970: 1) labeled natural selection as a ‘motive force’ (=? cause) and still others, such as Endler (1986: 4) stated that: ‘Natural selection can be defined as a process in which...’ (italics his). Interestingly the three conditions given by these two workers for natural selection are basically the same.

(B) Speciation or cladogenesis is the multiplication of species from an ancestral species (Mayr 1942, 1963, 2001) and can be thought as the splitting of a single phyletic lineage into two or more. (Speciation occurs only in sexually reproducing organisms and hence could be considered as a narrower process than cladogenesis which is general to all organisms.) Complete speciation always involves phyletic evolution in at least one of the phyletic lineages, and generally in both. No special causes exist for speciation other than those operating for phyletic evolution. But a definite initial condition is needed in the form of an external barrier separating two populations of the original species during which time intrinsic isolating mechanisms for genetic isolation evolve by phyletic evolution.

Darwin's five theories

Darwin always referred to his ideas published in his On the Origin of Species as my theory, always in the singular which has been a primary source of confusion ever since. Mayr (1982: 8) noted in the introductory chapter of his The Growth of Biological Thought: ‘Similarly, anyone whom writes about ‘Darwin's theory of evolution’ in the singular, without segregating the theories of gradual evolution, common descent, speciation, and the mechanism of natural selection, will be quite unable to discuss the subject competently.’ And in a most important, but apparently little known paper, Ernst Mayr (1985; see also 2004) elaborated on this point and demonstrated that Darwin's theory of evolution as originally advocated in his On the Origin of Species (1859) was actually a bundle of five separate but interrelated theories. Mayr showed that these theories in various combinations, but not all, were differentially advocated by diverse biologists before Darwin or by most of his contemporaries. Only after the evolutionary synthesis of 1937–1948 were all of these theories accepted by most evolutionists. But even after the evolutionary synthesis, almost all biologists and philosophers still considered evolution as an undivided, historical theory.

The five separate theories found in Darwin's 1859 book can still characterize the major areas within evolutionary biology today; they are:

Because most biologists and lay-persons are mostly interested in the last of these five theories which is clearly historical and because most people still consider evolution to be a single theory, the overwhelming conclusion by most biologists and philosophers of science is that the theory of evolution is only historical as did Mayr (2004: 32–33) and Ghiselin (2005: 133) who stated that: ‘Evolutionary biology is an historical science.’ Unfortunately this is not completely true. This belief is the source for much of the erroneous analysis in the philosophy of evolutionary biology. Mayr (2004: 33) mentioned historical narratives, but did not provide details for this approach.

Evolutionary explanations

What scientists do is to provide explanations of phenomena, but not any type of explanations. The approach used by scientists is to formulate theoretical statements that might apply to the phenomenon, generate deductions from these theories, and finally test these deductions against objective empirical observations. What is absolutely essential in the scientific methodology is not that empirical observations are made, but that these observations are objective as opposed to subjective – hence the term objective science. Objective empirical observations in the philosophy of science means that the same observations can be made by any person having the abilities to do so. Abilities means having the proper sense organs and training. People with certain types of color vision deficiencies cannot make the necessary observations depending on color in many fields of science. Subjective observations are ones that only certain persons can make but not everyone.

In an early paper commenting on whether only the choice of words separated the ideas of G.G. Simpson and H.O. Schindewolf, von Wahlert and I argued that evolutionary theory had to be considered both as nomological (= causes, mechanisms) and as historical – that a major difference existed between these two approaches (Bock and von Wahlert 1963). I continued to muse on this distinction between approaches to evolutionary biology and published my ideas in a series of papers (Bock 1973, 1978, 1988, 1991, 1994, 1999, 2000a,b, 2004a), but they were either not fully formed or they were published in rather specialized symposium volumes. Herein I would like to describe and contrast two major explanatory systems in science and to show their relationship to one another. These systems are N-D Es and H-N Es. Although the latter are probably general for science, they appear to be of most significance in fields such as biology, geology and astronomy. Almost all philosophers of science have concentrated exclusively on N-D Es as I know of no general treatment of H-N Es. An additional system of explanations exists in biology – the dichotomy of Functional Explanations versus Evolutionary Explanations (see below). Hence it is essential not only to characterize carefully the properties of N-D Es and H-N Es, but to show which of these are functional and which are evolutionary explanations.

Explanations in biology deal with phenotypic attributes of organisms and their genetic basis, including interactions between phenotypic attributes found in the same individual organisms, correlations between phenotypic attributes and diverse aspects of the external environment, and the relationships among the features in different organisms, be they conspecific or of different species. Hence, given any phenotypic attribute, a complete (= full) explanation includes resolving: (1) all existing physical-chemical properties (form, function, biological role, etc.) which are functional explanations; (2) its ontogenetic development resulting from interactions between the existing genotype and the external environment (programmed systems, Mayr 1974, 1988, 1997) which are also functional explanations; and (3) its evolutionary origin and, therewith, the evolutionary origin of the genotype (constituting the programmed systems), which constitutes evolutionary explanations (Bock and von Wahlert 1963). Although the topic of full versus partial explanations in biology is an important one, I will not consider it further herein except to say that partial biological explanations can be most important and are all that can be achieved and/or desired in most cases.

Mayr (1961, 2004) has argued strongly that the formation of all biological phenotypic attributes depends on two different set of causes working simultaneously which he has termed proximal and ultimate causes and which form the basis of his seminal conclusions about dual causation in biology and the autonomy of biology from the physical sciences. His proximal and ultimate causes are better called functional (= proximal) and evolutionary (genetic; = ultimate) causes with the latter result from the evolutionary history of the organism.

Throughout, I will use functional explanations in the general sense of functional analyses in biology (Mayr 1982), not in the sense of functional explanations in philosophy (see Nagel 1961) as the latter do not appear to be of any value in scientific explanations and are best omitted from scientific explanations. In addition, almost all discussions of function in biology by philosophers of science are presented in a historical evolutionary sense which places them in sharp contrast to explanations by most functional biologists; this distinction should be noted carefully when comparing the conclusions of papers on function and functional explanations by most biologists versus those by most philosophers. A further problem is that many biologists in the mid-nineteenth century and some even today, equated functional explanations with teleological explanations. When many biologists realized that Darwinian evolution eliminated teleology from biology, they also eliminated discussions about functional properties of biological features (as well as functional explanations) from their evolutionary analyses – a good example of throwing the baby out with the bath water. This thinking resulted in serious problems for evolutionary biology as most, if not all, evolutionary explanations depend on well established functional explanations and on a full understanding of the interactions of organismic features with the external environment (Hutchinson 1965; Bock 2002, 2003; von Wahlert 2002).

Nomological-deductive explanations

Nomological-deductive explanations are the ‘standard’ form of explanation in science – covering law explanation – and deal with general explanations of a class of phenomena, asking how has each occurred? This is performed by formulating a deduction from an appropriate set of laws (be they causes, processes, or outcomes) and a set of initial and boundary conditions (observations of the conditions in which the phenomenon exists), both of which form the explanatory sentence, or explanans, and from these reach a particular conclusion or deduction, the explanandum. (Hempel and Oppenheim 1948; Hempel 1965: 335–338). The deduction is then compared with objective empirical observations – factual observations of the phenomenon. If the observed phenomenon agrees with the deduction (the degree of error is dependent on what would be considered admissible), the explanation is accepted. If an explanandum, resulting from the conjunction of the set of facts invoked (initial and boundary conditions) and the set of general laws, disagrees with objective empirical observations, then that N-D E is not valid (has been falsified), and one must search for the reasons underlying this falsification. Falsification means only that the explanandum does not agree with independent, objective, empirical observations. If the explanation is not accepted, it is necessary to investigate the source of the error – be it the set of law-like statements, or the set of initial and boundary conditions used to formulate the deduction, or the objective empirical observations used to test the deduction. Falsification of an explanation does not automatically imply that the general laws used in the explanation are in error, although this is a possibility. Possibly, the initial or boundary conditions used in the empirical test were wrong, or the empirical observations were incorrect.

Because they are general, one nomological scientific theory can be used to test another, and two or more nomological theories can be included under the umbrella of an overarching theory. And as will be discussed below, nomological scientific theories are required in the testing of any historical scientific theories.

Nomological-deductive explanations answer the question: how has a particular phenomenon [explanandum] occurred? N-D Es apply to universals (non-limited number of phenomena), do not depend on the past history of the objects or the phenomena being explained, and their premises (the nomological statements) are assumed to be always true. In saying that N-D Es apply to universals, these explanations are not temporally-spatially restricted within the proper region of the phenomena, which for biology is the earth and more specifically the ‘surface’ (= the upper part of the crust) of the earth. Examples of N-D Es include clarification of oceanic tides using gravity and of phyletic evolution evoking natural selection (nonrandom, differential survival and reproduction of organisms). Explanation of how a certain feature of an organism is an adaptation to selective demands arising from the external environment is a N-D E, not a H-N E; the origin of the adaptation, which is an entirely different question, is a historical-narrative explanation (see below).

For almost all philosophers of science (e.g., Popper 1959; Nagel 1961) scientific methods apply only to N-D E. This is interesting because Popper's approach has been accepted by many systematists (e.g. cladists) as the foundation for their analyses of the relationships among organisms; this cannot be not correct because any explanations concerning the tree of life, such as phylogenetic classification, deal with singulars (the existing phylogeny of organisms) and not with universals (see below, and Bock 2000a, 2004a).

Many philosophers of science and many biologists have rejected the idea that law-like statements exist in science (e.g., Mayr 1997: 60–63; but see Elgin 2006 for an argument for laws in biology), sometimes substituting a vague idea of concepts in place of law-like statements. The reasons for this rejection of law-like statements by biologists and philosophers of biology is not entirely clear, but may stem from the conclusion that variation of phenomenon in biological organisms precludes the existence of laws in biology rather than that this variation arises from the nature of the initial and boundary conditions which generally vary, often considerably, among biological objects such as individual members of the same species and will, of course, result in variation in the deductions. In this connection, determinism is often raised as a critical factor without the realization that the scientist must set the acceptable limits of determinism in every case. Further, many of these workers accept that only a single type of explanations exists in science and hence confuse H-N Es with N-D Es. If this is done, then the conclusion follows that no laws exist in biological explanations, which include historical narratives, because historical laws do not exist. At this point the argument becomes circular.

Historical-narrative explanations

Contrary to the beliefs of many philosophers, not all science is nomological. Some sciences, such as biology and geology, are very largely historical. But little or no mention of these explanations is found in the literature of the philosophy of science (e.g., Feigl and Brodbeck 1953; Boyd, Gasper and Trout 1991; Cornwell 2004). Indeed, considering numbers of scientists and amounts of funding (which includes medicine and agriculture as well as biology, geology, and some astronomy), sciences with a historical aspect are now and have been for a long time in the large majority. Why philosophers failed to realize the importance of historical science may be because the philosophy of science is little more than one and a half centuries old and has been concentrated very largely on physics as the ideal science. And physicists have developed their science as a strictly non-historical inquiry, which is certainly a completely valid thing to do and is certainly the prerogative of physicists. But others do not have to accept physics as the science on which to base all philosophy of science. In view of the overwhelming acceptance that there are no historical laws, the major difficulty for philosophers in dealing with these historical aspects of science would be how to extend the nomological-deductive approach to them (see Hempel 2001, especially chapters 14 and 15). This was clearly the reason why Popper (1977; see Hull 1999 for an excellent analysis of this matter) said, or implied, that evolutionary theory was not scientific. The difficulties of dealing with N-D versus H-N Explanations and assigning different explanations in evolutionary biology to one or the other of these two types are well illustrate in the interesting analysis of adaptation by Amundson (1996).

Historical-Narrative Explanations provide an understanding of the existing attributes of a particular set of objects or phenomena at specified points in time; these explanations absolutely depend on the past history of these objects and to be scientific they must use pertinent N-D Es. The latter point is essential! Any explanation of historical events that is not based on pertinent N-D Es is not scientific. This includes not only approaches such as scientific creationism and intelligent design, but also claims by some scientists of the existence of pure order and/or design in nature (including some approaches to biological classification, see Brower 2000), self-determinism and structuralism. Phenomena and objects explained by a H–N E are singulars, not universals, and have definite spatial-temporal positions. H–N Es are considered in a non-deductive and probabilistic basis with the hope of reaching the most reasonable and probable explanation for the objects studied.

Several aspects of H-N Es are stressed, the, first being the most important:

Historical-narrative explanations in biology include the evolution, phylogeny and classification of organisms or the evolutionary history of their genetic characteristics or of their phenotypic attributes – that is, anything related to the history of life, such as historical biogeography. All full explanations in biology would include a historical-narrative portion which is why full biological explanations are so difficult to formulate and test.

Because they deal with singular events, particular historical scientific theories cannot be used to test any other scientific theories, be they nomological or historical.

Both nomological-deductive and H-N Es are scientific under the criterion of demarcation for scientific explanations advocated by almost all philosophers of science in that they are both available for testing against objective, empirical observations. N-D and H-N Explanations differ in the many ways of how they are expressed, tested, and used to test other theoretical statements, and must not be confused. The accuracy of most tests of H-N Es may be weak, and a distinction must be made between valid tests and weak or unconvincing tests (see Bock 1989b: 339–342, discussing the concept of homology). Many of the tests available for H–N Es are valid, but are relatively poor or non-robust and should not be rejected as invalid tests. A robust, valid test is one that has a high ability to distinguish between correct and an incorrect hypotheses.

Being theoretical scientific statements, H-N Es are available to tests against empirical observations, but such tests are often difficult and inconclusive. Generally H-N Es are not tested by falsification (in spite of numerous statements in the literature) but usually by confirmation with the addition of more and more corroborating support. This procedure is closely akin, if not identical, to induction in the strict sense of that concept. Objections cannot be raised to inductive testing of H-N Es because they are theoretical statements about a singular, containing a finite number of objects, in contrast to N-D Es which cover universals or an unlimited number of objects. Testing of H-N Es depends on argument chains involving pertinent N-D Es and often on a large number of background assumptions (hypotheses, many being initial and boundary conditions), and they must be finally tested against objective empirical observations. One should proceed to the empirical observations as directly as possible, although the argument chain is often complex. The empirical observations and their roles as tests, whether falsifying or confirming, should be designated clearly.

What makes H-N Es scientific is point 1 (above) – that ‘H-N Es must be based on pertinent and well-tested N-D Es, and these N-D Es, together with the pertinent empirical observations, form part of the chain of arguments used in testing the H-N E.’ If no N-D Es exist or if these N-D Es have not been well corroborated, then that H-N E lies outside of science. The argument that scientific order exists in nature by itself and in the absence of any N-DE is simply invalid as a scientific statement.

Theories of evolution

As some sciences contain very different types of explanations, it seems reasonable to propose that in such sciences as biology, geology and astronomy, two different types of theory exist– these being nomological theories and historical theories. Therefore we should speak of nomological evolutionary theory and historical evolutionary theory. Further, for historical evolutionary theory, it is best to consider both general theory and special theories. One can discuss general historical evolutionary theory such as the Haeckelian phylogeny of organisms and special theories such as the evolutionary history of birds, of insects, of aquatic carnivores. etc. In geology, the movement of continental plates over the earth's surface would be a general historical theory and the splitting, including the time, of North and South America from Europe and Africa to form the Atlantic Ocean would be a special theory.

Let's reconsider the five theories of Darwin as outlined by Mayr (1985). The first four theories are clearly nomological evolutionary theories, and are:

Recall that there are no special nomological causes restricted to speciation. Although the process of speciation is nomological, it depends on the nomological causes of phyletic evolution plus the important initial condition of an external barrier separating two populations for a sufficiently long period during which Intrinsic Isolating Mechanisms for Genetic Isolation evolve (Mayr 1963; Bock 1995, 2004b).

These four theories are all nomological albeit of different types. Some could be further subdivided, especially 3 and 4, but I wished to keep this list the same as the five theories of Darwin from Mayr (1985).

These theories are law-like because they apply to all living organisms on the Earth although they have been tested only for a very small number of species. To my knowledge, none of these nomological theories of evolution failed testing against the appropriate objective, empirical observations. In a reasonably large number of cases, direct observations have been made of all of these aspects of nomological evolutionary change, including phyletic evolution in domesticated animals and plants, development of tolerance to pesticides by many insects, and development of tolerance to antibiotics by many bacteria. Phyletic evolution has been observed in the House Sparrow, Passer domesticus, after its introduction into North America. This species has spread rapidly over the continent and differentiated into numerous local populations or races (Johnston and Selander 1964; Selander and Johnston 1967). Speciation has been observed by the re-creation of naturally occurring as well as novel polyploid species of plants [Appalachian ferns, Asplenium, Wagner 1954; Spartina townsendii (the only naturally occurring species of plants whose origin has been ‘observed’ in the wild), Huskins 1931; the artificial radish-cabbage hybrid RaphanobrassicaStebbins 1950; and many others, Grant 1963, 1981]. Moreover some breeds of domesticated animals that have become so different that they would act as different species under natural conditions. If it was possible to release a population of giant Irish Wolfhounds and a population of dwarf Chihuahuas (these breeds were chosen as the largest and smallest breeds of dogs, but the same would happen with many other combinations of dog breeds) in a region where both could survive and reproduce, one will find only Irish Wolfhounds and Chihuahuas generation after generation responding to one another as good species. The large difference in size between these two groups of dogs serves as the intrinsic isolating mechanism for genetic isolation between these two taxa regardless of whether they are technically placed in the same species, Canis familiaris, and regardless of the fact that, with series of intermediate steps involving different breeds of dogs, genetic material could be transferred from Irish Wolfhounds to Chihuahuas. Moreover large breeds of dogs and Northern Hemisphere wolves, Canis lupus, readily interbreed with viable and fertile offspring although the large and toy breeds of dogs cannot because of their size difference, The same would be true if populations of dwarf horses and the largest dwarf horses were released in a region where both would survive and breed. If one wishes to speak of any aspect of evolutionary theory being factual, it is these direct observations of different aspects of nomological evolutionary theory, not the history of life which remains theoretical, but exceeding well corroborated with very extensive testing without any falsification.

Only the last of Darwin's five theories as listed by Mayr is a historical evolutionary theory. This is:

Historical biogeography and any other biological theory, that fits the characteristics of H-N Es outlined above, would also fall into the class of historical theories. Most important is that the historical evolutionary theory of common descent, as well as any other historical biological theory, depends firmly on a set of appropriate nomological theories to be scientific. In the case of the historical evolutionary theory of common descent, it is based on the preceding four nomological evolutionary theories. Nomological evolutionary theory is primary and historical evolutionary theory is secondary, and must be considered and discussed in that order. It is incorrect to speak first of the pattern of evolutionary change (= historical theory) and then the process of evolutionary change (= nomological theory).

The fifth of Darwin's theories – that of common descent – is a general theory and deductions from it can be tested against a number of objective empirical observations. If an evolutionary history of living organisms (both fossil and recent) is obtained from the nomological theory of evolution, then one can reach three further deductions, namely:

These deductions had been tested successfully against a exceedingly large number of empirical observations of the spatial and chronological distributions of organisms and the occurrence of well developed features in ancestral groups versus vestigial features in descendent groups. Hence one can conclude that general historical evolutionary theory is extremely well corroborated, and that ultra-massive counter observations would be needed to disprove Darwin's historical theory of common descent. But this well-tested and supported historical evolutionary theory is still a theory as noted by almost all philosophers of science and is not a fact or factual as many evolutionists like to characterize it. The general historical theory of evolution, although exceedingly well corroborated, cannot be used for testing any special historical evolutionary theories and especially for testing any nomological theories because historical theories cover singular events; hence they cannot be used to test other historical events or to test nomological theories which cover general events.

In addition to the general historical theory of evolution, endless special theories exist which deal with the evolutionary history and classification of all groups of organisms at all hierarchal levels from subspecies to kingdoms. Hence the theory of whether the Rodentia and the Lagomorpha are closely related to one another within the Eutheria forming the taxon Glires, or the theory of whether pinnipeds (seals and other aquatic carnivores) had a single or a double origin from the terrestrial carnivores, or whether birds descended from an early group within the archosaurian reptiles (Martin 2004) or from a later and more specialized group within the theropod dinosaurs (Sereno 2004) or perhaps that some of the late ‘dinosaurs,’ the maniraptoran radiation, are actually not dinosaurs but flightless descendants of an early bird such as Archaeopteryx (Paul 2001, 2002; Martin 2004) are all special historical evolutionary theories.

Special historical evolutionary theories deal with singular events and each must be tested independently against objective empirical observations, which generally include an argument chain of well corroborated nomological theories and the observations supporting them. To be scientific, historical evolutionary theories must also be tested against relevant, well corroborated nomological evolutionary theories. Any historical evolutionary theory must also be tested using other well-tested, non-evolutionary nomological theories. A historical evolutionary theory on the origin of avian flight must also be tested with nomological aerodynamic theories as well as those concerning functional properties of vertebrate muscle-bone systems, metabolism, respiration, etc. If, finally, a well corroborated and convincing historical theory has been reached to explain the evolution of avian flight, this historical theory works for birds only and cannot be applied to explain the evolution of flight in bats or in pterosaurs, which are different singular events and may well have very different historical explanations (Bock and Bühler 1995).

Functional explanations

Although this essay deals with diverse forms of evolutionary explanations, a brief word should be said about functional explanations. Two different, but interrelated systems of explanations exist in biology, which are (1) the dichotomy of N-D E versus H-DEs and (2) the dichotomy of Functional Explanations versus Evolutionary Explanations. The latter system stems from the useful division of biology into the major areas of functional and evolutionary biology as noted by Mayr in his Growth of Biological Thought (1982). He did not use the terms Functional and Evolutionary Explanations, but his division of biology into these two major areas can be taken as the basis to consider this dichotomy of explanations. It should be noted that the use of functional explanation in biology is sharply different from the use of this term by many philosophers (Nagel 1961); I will use it strictly in the biological meaning. It should also be noted that many or most philosophers of biology use the term function in a historical evolutionary sense which I do not advocate because I believe that it leads only to confusion.

These two systems of explanations in biology do not have a simple relationship to one another. All functional explanations are N-D Es and all H-N Es are evolutionary, but evolutionary explanations can be either N-DEs or H-N Es. Hence it is essential not only to characterize carefully the properties of N-D Es and H-N Es, but to show which parts of biology, and especially of evolutionary biology, are nomological-deductive and which are historical-narrative. Nomological evolutionary theory is primary to historical evolutionary theory. Moreover, although I cannot provide a complete argument at this time, functional explanations appear to be primary to evolutionary explanations. Part of this problem is that good definitions of the areas of functional and of evolutionary explanations in biology are not available. These two areas of biology cannot be characterized simply by the commonly used distinction of ‘How’ questions versus ‘Why’ questions which is too vague to be of real use.

Conclusions

In discussions inside and outside of science, it is not possible to talk simply about the theory of evolution and about the proofs supporting this theory. Nor is it possible to take the position that the theory of evolution has been so well corroborated and supported that it can be considered to be factual. Such approaches simply lead to confusion and failure to reach any real agreement. Most definitions of biological evolution are at the same time not sufficiently broad in that they exclude several valid forms of change (social evolution and template evolution) and include an unnecessary criterion that evolutionary change has to be genetic. Further, the theory of evolution as proposed originally by Darwin and still advocated today by most people is not a single theory but a bundle of five independent theories as originally mentioned by Mayr in 1982 and further elaborated by him in 1985 and repeated in at least four of his later books (Mayr 1991: 35–39; 1997: 177–189; 2001: 86–87; 2004: 97–115). These theories are not of the same nature in that four are clearly nomological and one – common descent – is definitely historical. Philosophers of science have concentrated almost exclusively on nomological science which has further confused matters in that biologists have tried to force all evolutionary explanations into this mold or that, using the opposite tack, some philosophers of science under the belief that evolution is only historical, have declared evolution theory not scientific. Nomological and historical scientific theories are markedly different in their expression, what they cover and how they are tested; all most be tested against objective empirical observations. None can be proved and none are factual.

Historical evolutionary theories must be based on the appropriate nomological evolutionary theory to be scientific. Classifications, phylogenies, dendrograms, clarifications and all other explanations of the evolutionary history of groups of organisms and of their particular attributes, including distribution both in time and space, are historical theories; these explanations must be presented and tested in a very different way than the standard nomological theories in science. As for all historical theories, evolutionary historical theories and explanations must be presented and tested realistically with a sensible estimate of the degree of confidence that one can have in the particular explanation. The testing of historical theories against empirical explanations is a far more difficult task than realized by most biologists, and more historical theories enjoy a far lower degree of confidence than most biologists estimate (Bock 2004a).

Dialogues between people accepting evolution and those objecting to evolutionary explanations are almost entirely limited to historical evolutionary theory. Most anti-evolutionists simply do not concern themselves with nomological evolutionary theory. Many are quite comfortable with possible evolutionary change within the species limit with their objections being largely to the idea of ‘evolution beyond the species’. No resolution is possible using many of the traditional arguments supporting evolution theory as advanced by most biologists, and quite possibility no resolution will ever be possible. But if evolutionary biologists and philosophers of science present clearly and completely evolutionary theory in all of its fascinating diversity, then at least we know where the arguments are and which views are based on science and which on belief.