View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Bowling Green State University: ScholarWorks@BGSU

Bowling Green State University ScholarWorks@BGSU

Biological Sciences Faculty Publications Biological Sciences

3-2014

Darwin’s Diagram of Divergence of Taxa as a Causal Model for the Origin of

Juan Luis Bouzat Bowling Green State University, [email protected]

Follow this and additional works at: https://scholarworks.bgsu.edu/bio_sci_pub

Part of the Commons

Repository Citation Bouzat, Juan Luis, "Darwin’s Diagram of Divergence of Taxa as a Causal Model for the Origin of Species" (2014). Biological Sciences Faculty Publications. 45. https://scholarworks.bgsu.edu/bio_sci_pub/45

This Article is brought to you for free and open access by the Biological Sciences at ScholarWorks@BGSU. It has been accepted for inclusion in Biological Sciences Faculty Publications by an authorized administrator of ScholarWorks@BGSU. Volume 89, No. 1 THE QUARTERLY REVIEW OF BIOLOGY March 2014

DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA AS A CAUSAL MODEL FOR THE ORIGIN OF SPECIES

Juan L. Bouzat Department of Biological Sciences, Bowling Green State University Bowling Green, Ohio 43402 USA e-mail: [email protected]

keywords , , , , phylogeny

abstract On the basis that Darwin’s theory of evolution encompasses two logically independent processes (common descent and natural selection), the only figure in (the Diagram of Divergence of Taxa) is often interpreted as illustrative of only one of these processes: the branching patterns representing common ancestry. Here, I argue that Darwin’s Diagram of Divergence of Taxa represents a broad conceptual model of Darwin’s theory, illustrating the causal efficacy of natural selection in producing well-defined varieties and ultimately species. The Tree Diagram encompasses the idea that natural selection explains common descent and the origin of organic diversity, thus representing a comprehensive model of Darwin’s theory on the origin of species. I describe Darwin’s Tree Diagram in relation to his argumentative strategy under the vera causa principle, and suggest that the testing of his theory based on the evidence from the geological record, the geographical distribution of organisms, and the mutual affinities of organic beings can be framed under the hypothetico-deductive method. Darwin’s Diagram of Divergence of Taxa therefore represents a broad conceptual model that helps understanding the causal construction of Darwin’s theory of evolution, the structure of his argumentative strategy, and the nature of his scientific methodology. Now let us see how this principle of great benefit being derived from divergence of character, combined with the principles of natural selection and will tend to act. The accompa- nying diagram will aid us in understanding this rather perplexing subject (Darwin 1859:116).

Introduction actual structure of what Darwin himself de- INCE the publication of the first edition scribed as “one long argument” (Darwin Sof On the Origin of Species by Means of Nat- 1859:459; Hodge 1977; Mayr 1991; Waters ural Selection by Charles Darwin in 1859, 2003). Ernst Mayr (1991) argued that “Dar- there have been numerous debates not only win’s theory” involved at least five indepen- about the nature of Darwin’s theory (or theo- dent theories, which included Evolution as ries) and its significance (Kohn 1985; Hodge such (i.e., descent with modification in con- and Radick 2003; Ruse 2009), but also on the trast to Special Creation), Common Descent

The Quarterly Review of Biology, March 2014, Vol. 89, No. 1 Copyright © 2014 by The University of Chicago Press. All rights reserved. 0033-5770/2014/8901-0002$15.00

21

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 22 THE QUARTERLY REVIEW OF BIOLOGY Volume 89

(i.e., the relationship among species by an- Here, I will argue that Darwin’s one long cestry), the Multiplication of Species (to ex- argument assumes that there is indeed a plain the origin of diversity), Gradualism causal relationship between the two pro- (regarding the mode and tempo of evolution- cesses; and that Darwin’s Diagram of Diver- ary change), and Natural Selection (as a gence of Taxa (Darwin’s Tree Diagram), the mechanism to explain ). Argu- only figure illustrated in the Origin of Species, ably, in a historical context, these supposedly represents a causal model of his theory on independent theories were considered by the origin of species by means of natural Darwin as part of a single, unified theory of selection (Figure 1). I will argue that the evolution. That is, Darwin viewed these as a structure of Darwin’s argument, as repre- “logically inseparable package” (Mayr 1991: sented by the Tree Diagram, suggests that 37), which would justify the use of a common Darwin did not consider common descent argumentative strategy (i.e., one long argu- and natural selection to be logically indepen- ment) throughout the Origin. dent processes. Under Darwin’s perspective, Today, most biology textbooks describe natural selection explained the branching pat- common descent and natural selection as terns of species; i.e., natural selection pro- the two major components of Darwin’s the- vided the mechanism to understand how the ory of evolution (e.g., Reece et al. 2011; Free- patterns of common descent originated. The man et al. 2014; Urry et al. 2014). These causal relationship between natural selection elements are considered conceptually as well and common descent is clearly described by as logically independent hypotheses (e.g., M. J. S. Hodge’s (1977) interpretation of Mayr 1982:505–510; Sober and Orzack 2003: Darwin’s argumentative strategy following the 426), given that the truth or falsity of these vera causa principle (Kavaloski 1974; Hodge hypotheses is independent from each other. 1977). Hodge specified the three necessary This interpretation is consistent with the elements underlying the principle of vera thinking of some of Darwin’s contempo- causa: i.e., the existence of a cause, its compe- raries as well as modern biologists. For exam- tence to produce an effect, and its putative ple, (1868) accepted natural responsibility for the phenomenon to be ex- selection, but believed major groups of con- plained. Hodge argues that, in the Origin, temporary organisms originated indepen- Darwin structured his argument following dently. Motto Kimura (1968) accepted the John Herschel’s (1830) argumentative ideals common ancestry of genes and species, but for the testing of theories, which required believed evolutionary changes at the molec- independent evidence for causal existence, ular level were mostly neutral (i.e., driven by competence, and responsibility. The argumenta- rather than natural selection; tive structure of the Origin thus makes the Sober and Orzack 2003). Throughout this case for the existence of natural selection paper “Common Descent” refers to the (Chapters I–III), its competence to produce branching patterns of divergent species new species (Chapter IV), and its responsibil- originating from common ancestral ity for the production of extant and extinct forms. Other authors (e.g., Sober, personal species (Chapters X–XIII; Hodge 1977). The communication) interpret common ancestry Tree Diagram provides therefore a concep- as a claim that all forms can be traced tual model of Darwin’s thesis, centered on back to one or a few ancestral lineages. Com- the mechanism of natural selection as the mon ancestry is also often assumed to be part vera causa for the origin of species. of Darwin’s idea of “descent with modifica- The ideas presented here are consistent tion.” However, the concept of “descent with with Sober’s (2009) characterization of Dar- modification” may be applied narrowly to win’s argumentative strategy in writing the the idea of evolutionary change within a sin- Origin in the specified order in which it was gle lineage. One thus can argue that the written, with an initial description of artificial concept of common descent comprises the selection, followed by natural selection, and idea of descent with modification; however, finally common ancestry. According to So- the opposite is not necessarily true. ber, Darwin assigns “causal priority” to natu-

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 23

Figure 1. Darwin’s Diagram of Divergence of Taxa Presented in the ORIGIN The diagram integrates the Principle of Divergence and the Principle of Natural Selection together with the process of extinction into a conceptual model of evolution. ral selection, since it is selection that leads to competence under the principle of vera causa. (i.e., explains) branching. In fact, in his “Big Finally, I suggest that Darwin’s Tree Diagram Species Book” (the manuscript from which may be interpreted as a working hypothesis the Origin represented only an abstract of the (the case for competence), as one can infer a original book Darwin intended to complete) series of predictions that can be tested Darwin stated that “[n]o theory of the deri- through experimentation/observation in a vation of groups of species from a common hypothetico-deductive framework (providing parent can be thought satisfactory until it the case for responsibility). Darwin’s evidential can be shown how these wondrous correla- observations (presented in Chapters X–XIII of tions of structure can arise” (Stauffer 1975: the Origin) can, thus, be directly related to spe- 175). The theory of natural selection thus cific predictions that emerged from the Tree solved this question, providing a mechanistic Diagram as a conceptual model of evolution. explanation for the correlations of structure, both between traits in an organism and Darwin’s Theory among distinct species. In this paper, I first describe the major Although Darwin refers to “my theory” 57 components of Darwin’s theory, establishing times throughout the Origin, he mostly uses the causal efficacy of natural selection as the this term in relation to expectations, difficul- explanatory mechanism driving divergence ties, and objections of the theory, or to de- of character and common ancestry. I will scribe evidence that would be consistent then argue that Darwin’s Diagram of Diver- with, or which would allow rejection of, his gence of Taxa represents a causal model for theory. Darwin never formally defined what his theory on the origin of species by means he calls “my theory.” However, at the begin- of natural selection, establishing the case for ning of Chapter XIV of the Origin (Recapit-

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 24 THE QUARTERLY REVIEW OF BIOLOGY Volume 89 ulation and Conclusion) he refers to the served, by the term of Natural Selection” (p. “theory of descent with modification through 61). However, this chapter does not develop, natural selection” (p. 459), which implies as many would argue, what Darwin calls “my that his was a unified theory that combined theory.” common descent and natural selection. This It is not until Chapter IV (entitled Natural idea is consistent with one of the passages in Selection) that Darwin puts forward the idea which Darwin describes the two great laws of of natural selection in the context of the nature, Unity of Type and the Conditions of origin of species. That is, Chapter IV formal- Existence, in relation to what he calls “my izes what Darwin calls “my theory,” the causal theory” (p. 206). It is there where he ex- link between natural selection as a creative plicitly provides an argument for a uni- force leading to adaptive divergence into the fied theory of common descent and natural origin of new species, thus generating pat- selection, as he emphasizes that the Law of terns of descent, ancestry, and extinction. In Conditions of Existence (which embraces the first 35 pages of this chapter, Darwin natural selection) is a higher law that encom- provides multiple examples of natural selec- passes that of Unity of Type (which is ex- tion operating in nature, describes the pro- plained by unity of descent; see below). I will cess of , and emphasizes the argue that a concise description of Darwin’s importance of interbreeding, reproductive theory is summarized in the title of his book isolation, and gradual change. Darwin’s goal On the Origin of Species by Means of Natural in this chapter was, however, to present a Selection, or the Preservation of Favoured Races in unified theory for the origin of species the Struggle For Life. Although the title does through the process of natural selection. not make any explicit allusion to common Darwin uses the only figure in the Origin, the descent, Darwin’s theory was intended to Diagram of Divergence of Taxa (Figure 1), provide an evolutionary mechanism (natural to illustrate the logical connection between selection) as a causal explanation for the or- natural selection, adaptive divergence, and igin of species and thus the observed pat- extinction in explaining descent with modi- terns of common ancestry, in contrast to the fication and common ancestry. As first de- prevalent idea that species were created in- scribed on page 116 of the Origin, the Tree dependently. Diagram illustrates the principles of diver- The overall structure of Darwin’s “one gence and natural selection (conditions of long argument” is shown in Figure 2. Darwin existence), together with the process of ex- starts with the description of established facts tinction, into a single conceptual model of on the variation of domestic and wild species evolution. as an essential feature of populations (Chap- The Tree Diagram is therefore not a sim- ters I and II). These chapters provided the ple schematic of the pattern of common an- foundations (arguing from analogy; Hodge cestry, as presented by many (Ragan 2009; 1977) to infer an evolutionary mechanism Tassy 2011). The Tree Diagram represents a for adaptive change, i.e., the process of natural conceptual model of Darwin’s theory, by selection. In Chapter III (entitled Struggle for which he assigns causal efficacy to the pro- Existence), Darwin provides a detailed expla- cess of natural selection as an explanatory nation of the major elements involved in the mechanism for the origin of species. That is, process of natural selection (i.e., variation, in this model, natural selection is the mech- differential fitness, inheritance, and the Mal- anism that explains adaptive divergence and thusian principle), which characterize the so- the branching tree. In this sense, I would called “.” Chapter III of argue that the “adaptive end” of natural se- the Origin provides a precise description of lection, which explains biological design the Principle of Natural Selection and the (Chapter III), is not the most important as- first explicit definition (since the term was pect of Darwin’s theory as presented in the mentioned only briefly on page 5 of the In- Origin. It is natural selection, when presented troduction): “I have called this principle, by as a creative force leading to adaptive diver- which each slight variation, if useful, is pre- gence, which provides causal efficacy in Dar-

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 25

Figure 2. The Structure of Darwin’s “One Long Argument” in the ORIGIN OF SPECIES The schematic highlights the three major elements of the Principle of vera causa in Darwin’s argument: the case for existence, competence, and responsibility of natural selection on the origin of species. win’s theory on the origin of species. In a putable that Darwin assigned causal priority to way, the Origin was written as one long argu- natural selection over common ancestry, as he ment to provide a mechanistic explanation stated that “the law of the Conditions of Exis- (natural selection) as the vera causa for the tence is the higher law; as it includes, through origin of species. the inheritance of former adaptations, that of It is unquestionable that in the Origin Dar- Unity of Type” (Chapter VI; p. 206). In a way, win described both Unity of Type (due to com- one could argue that Darwin’s thesis in the mon descent) and Conditions of Existence Origin may be better interpreted not as a direct (natural selection) as the “two great laws” that response to ’s have formed all natural beings. It is also indis- (Paley 1802) arguments on Divine Design (the

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 26 THE QUARTERLY REVIEW OF BIOLOGY Volume 89

“adaptive end” of natural selection), but to of Species (1837; Figure 3). In fact, ’s (1817) ideas on the indepen- Darwin thought that Wallace’s (1855) paper dent creation of species (i.e., natural selection did not add anything new to the question of as a mechanism for adaptive diversification). how species originated (Kottler 1985). Al- The idea that Darwin’s theory should not be though in his paper Wallace described the interpreted as common ancestry plus natural phenomenon of branching divergence, he selection, but rather natural selection as the did not actually explain it (Kotler 1985:381); causal mechanism to explain common ances- i.e., he did not provide an evolutionary try and the origin of organic diversity, is also mechanism to account for it. However, Dar- supported by the chronological development win felt indeed forced to demonstrate prece- of some historical events leading to the pub- dence when he received Wallace’s (1858) lication of the Origin. The idea that living letter describing the independent formula- organisms may have descended from com- tion of the principle of natural selection. It mon ancestral forms predated Charles Dar- was the independent development of the win. Early naturalists, including Buffon principle of natural selection by Alfred Rus- (1749), (1794), and to sel Wallace, and not the idea of common some extent Lamarck (1809), had previously descent, which led and John D. suggested that species may change into dif- Hooker to rush a joint presentation of ferent organisms and that some groups may Darwin and Wallace’s ideas to the Lin- have descended from a single ancestral nean Society, less than two weeks after form. However, the actual formalization of receiving Wallace’s letter on June 18, the idea of common descent into a tree-like 1858 (Darwin and Wallace 1858). structure of species ancestral/descent rela- tionships was first published by Alfred Russel Darwin’s Diagram of Divergence Wallace in his 1855 paper, “On the law which of Taxa has regulated the introduction of new spe- The first description of Darwin’s Diagram cies.” Here, Wallace introduced for the first of Divergence of Taxa appears in Chapter IV time “the analogy of a branching tree, as the of the Origin. The diagram is a schematic best mode of representing the natural ar- representation of three major processes (p. rangement of species and their successive 116): the principle of divergence; the princi- creation” (Wallace 1855:191). In fact, although ple of natural selection; and the process of no formal figure was represented in this ar- extinction. The Tree Diagram is not just a ticle, Darwin’s own personal copy of Wal- descriptive model of the patterns of com- lace’s (1855) paper revealed a hand-drawn mon descent. As indicated above, evidential sketch of a tree on its margins and the an- priority of common ancestry had been notation “[u]ses my simile of tree” (Brooks shown by Darwin himself in his Notebook B 1984:245; Kottler 1985:382). on the transmutation of species many years What was still missing, however, was a the- before the publication of the Origin (Barret ory that could provide a mechanistic expla- et al. 1987). nation for the tree-like patterns of ancestral I argue that, through the Diagram of Di- descent, i.e., the vera causa for the origin of vergence of Taxa, Darwin’s intention was to species. And Darwin thought that natural se- draw a logical connection between natural lection was the key element to solve “the selection and the divergence of species into mystery of mysteries.” After all, Darwin did varieties and newly formed species. Natural not panic when he read Wallace’s (1855) selection was presented as a creative force paper describing precisely the idea of com- leading to the adaptive divergence of species mon ancestry and a tree-like structure for the into varieties and subsequently into new spe- organization of diversity, even though he cies, thus generating the observed patterns had independently had this exact same idea of descent and common ancestry. The struc- about 18 years earlier, when he described a ture of the argument is formalized under the “-like tree” and the “I think” tree in his conceptual frame of two competing theories: Notebook B (pp. 26 and 36) on the Trans- evolution or the transmutation of species

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 27

Figure 3. Darwin’s “Coral-Like” Tree and the “I Think” Tree Described in 1837 These trees were sketched on pages 26 and 36, respectively, of his Notebook B on the Transmutation of Species (Barret et al. 1987). (Reproduced with permission of Cambridge University Library.) and independent creation. It was this “causal on the horizontal axes), and their patterns theory” that Darwin thought “banish[es] the of ancestral descent. The schematic of the belief of the continued creation of new or- “little fan of diverging dotted lines” (p. 117) ganic beings” (pp. 95–96). Interestingly, in is perhaps the key element in the Tree Dia- his “Big Species Book” Darwin framed his gram since this represents the causal effi- theory as an alternative to fortuitous chance cacy of natural selection in leading to rather than creation, as he stated that “no adaptive divergence and ultimately the ori- one I should think could extend this doc- gin of new species. In Darwin’s own words, trine of chance to the whole structure of an the “little fan of diverging dotted lines of animal, in which there is the clearest relation unequal lengths proceeding from (A), may of part to part, & at the same time to other represent its varying offspring” (p.117). It is wholly distinct beings” (Chapter V of Dar- not clear if he is referring specifically to the win’s Natural Selection; Stauffer 1975:174). individuals’ offspring or to species/varieties’ Darwin’s Diagram of Divergence of Taxa offspring (i.e., each dotted line representing (Figure 1) is a theoretical tree that represents a group of individuals). Most would inter- a truncated section of a group of species (A–L) pret these dotted lines as incipient varieties with varying degrees of morphological sim- analogous to those selected by breeders, ilarities (based on their relative proximity which Darwin described in Chapter I. In any

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 28 THE QUARTERLY REVIEW OF BIOLOGY Volume 89 case, his statements emphasizing that the originate from a common parent; and “variations are supposed to be extremely that both the number and divergence of slight” and that “[o]nly those variations which varieties will generally increase over time. are in some way profitable will be preserved or Through the Diagram of Divergence of naturally selected” (p. 117), clearly refer to the Taxa, Darwin assigns therefore “causal prior- population-level process of natural selection ity” to natural selection as an explanatory based on individual variations described in process for the origin of varieties and, ulti- Chapters I through III of the Origin. mately, the origin of new species. As described Perhaps Darwin’s Tree Diagram may be above, after many generations populations better interpreted as a “zoom-in/zoom-out” can be transformed through natural selec- picture, representing different processes at tion into well-marked varieties, subsequently multiple scales. That is, at some point, we into the “doubtful category of sub-species” can expand the scope of the diagram to rep- (p. 120) and, ultimately, into well-defined resent a larger temporal scale, denoted by species. In Darwin’s own words, “the diagram the numbered horizontal lines (I–XIV). illustrates the steps by which the small differ- According to the diagram, each horizontal ences distinguishing varieties are increased line represents 1000 generations (or even into the larger differences distinguishing 10,000). At that scale, the dotted lines repre- species” (p. 120). The causal effect of natural sent varieties and their modified descen- selection in generating descent with mod- dants. “When a dotted line reaches one of ification and common ancestry is also ev- the horizontal lines, and is there marked by idenced in the description of Darwin’s original a small numbered letter, a sufficient amount diagrams represented in Chapter VI of his of variation is supposed to have been accu- “Big Species Book” manuscript (Stauffer mulated to have formed a fairly well-marked 1975). The Tree Diagram in the Origin was variety” (p. 117). derived as a composite figure of Diagrams I, The causal efficacy of natural selection in III, and IV in the “Big Species Book,” repre- the formation of well-marked varieties is senting the expected patterns generated by then extended into the origin of new species the principle of natural selection, and the by depicting two general processes: the ten- principle of divergence and the process of dency of variability to be in itself hereditary extinction as bearing on natural selection. and the principle of divergence in character, In contrast, Diagram II reveals the evolu- both represented in the Tree Diagram. In- tionary patterns of the same ancestral spe- heritance is broadly represented by the con- cies but expected under the alternative nection of the small numbered letters (a1–a2, explanation of chance, where the preserva- a2–a3, etc.) and the continued variation (rep- tion of varieties is not driven by the process resented by the fan of dotted lines) that is of natural selection, but is left to random retained across generations. That is, varieties survivorship (Figure 4). Darwin’s descriptions “will tend to vary, and generally to vary in of the theoretical patterns derived from Dia- nearly the same manner as their parents var- grams I and II emphasize the role of natural ied” (p. 118). The principle of divergence, by selection as a causal effector of directional which extreme varieties or varieties with ex- change, increased divergence, and increased treme characters will have the best chance of diversification, as these changes are not pre- surviving, is represented by the increased dif- dicted under the doctrine of chance, i.e., in ferentiation of modified descendants in each absentia of natural selection (Figure 4). of the lineages (e.g., from a1–a10 or m1–m10), The Tree Diagram illustrated in the Origin as indicated by the fact that, e.g., variety a2 also embodies the gradual nature of natural differs more from A than does variety a1 (as selection and evolutionary change, as natural a1 is more closely located to A in the selection acts slowly over long periods of horizontal axis of morphological varia- time, producing slightly different varieties, tion). These two processes (inheritance which will become well-defined varieties and, and character divergence) lead to two eventually, species. As described in Chapter major inferences: that modified varieties IV, at one level, variation of diverging dotted

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 29

Figure 4. Diagrams I and II from Darwin’s “Big Species Book” These diagrams were part of Darwin’s Natural Selection manuscript being the second part of his “Big Species Book” written from 1856 to 1858 (Stauffer 1975). The diagrams illustrate the effects of natural selection on species divergence and extinction (Diagram I) and the effects of chance (Diagram II). Under the doctrine of chance, Darwin predicts no change within a lineage (M–m10), no increased divergence (M–L distance similar to m10–L), and no increased diversification (a1–l1 do not differ much more than a10–l10). lines in the Tree Diagram is supposed to be Thus, in the Tree Diagram, the concept of extremely slight (p. 117). Gradual change is “natura non facit saltum” emerges as a conse- characterized by increased divergence in quence of the principle of divergence through character, as the taxa represented by the dot- the process of natural selection. ted lines become well-marked varieties when Darwin’s Diagram of Divergence of Taxa reaching successive generation intervals, and also represents causal efficacy for the hierar- varieties are intermediate between ancestral chical organization of emerging lineages and and descendant forms (i.e., a3 Ͼ a2 Ͼ a1). the origin of higher taxonomic groups. This

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 30 THE QUARTERLY REVIEW OF BIOLOGY Volume 89 is represented by the condensed version of scription of the process of modification of the Tree Diagram from generation intervals higher taxonomic groups (p. 125) has led XI to XIV (Figure 1). The origin of subgen- some researchers to suggest that Darwin may era or even distinct genera is represented by have also provided group selection argu- the relatively recent divergence of closely re- ments for the origin of diversity (Ruse 1980). lated species (e.g., a14, q14, and p14) from spe- As Darwin stated, “the struggle for the pro- cies that have diverged at an earlier period duction of new and modified descendants, (e.g., b14 and f 14). The uneven distribution of will mainly lie between the larger groups, the species in the horizontal axes also shows which are all trying to increase in number. the morphological similarities of closely re- One large group will slowly conquer another lated species. The eight descendant species large group, reduce its numbers, and thus from A can thus be grouped into three sub- lessen its chance of further variation and genera or genera of closely related species. improvement” (p. 125). This statement sug- The complete diagram reflects therefore the gests that we may be able to predict the hierarchical organization of nature, with indi- success of highly diversified groups as they viduals being grouped into varieties, varieties have higher “fitness” (i.e., they produce grouped into species, and species grouped more varieties/species) compared to those into higher taxa (genera, families, and or- that produced a reduced number of new ders); this resulting from the combined effects groups. I believe, however, that this is just a of natural selection and common ancestry. consequence of Darwin’s emphasizing on The evolutionary foundation of this hierar- the causal priority of natural selection for chical organization is what provided the the origin of diversity, rather than a group bases for modern , which defines selection argument for the success of diver- natural taxa as monophyletic groups (Mayr sifying taxa, as for predicting the future suc- 1942). cess of specific groups based on their past A comprehensive look at the Diagram of history (reflected in the Tree Diagram), Divergence of Taxa shows that Darwin also Darwin stated that “which groups will ulti- incorporated unaltered lineages and the mately prevail, no man can predict” (p. process of extinction within his conceptual 126). model of evolution. In the case of unaltered Darwin’s Tree Diagram as a model of evo- lineages (e.g., F–F 14), causal efficacy can also lution has two major consequences for the be attributed to natural selection, in this case temporal dynamics of diversity, which not as a diversifying process, but as a stabiliz- provide causal efficacy for explaining the ing force weeding out the unfit. This process genealogical relationship by which all liv- of negative selection was originally described ing beings are united, as well as the hierar- by in 1835 and has been de- chical organization and natural classification scribed by Dawkins as the first conceptual of organic diversity. The process of descent bridge to the evolutionary understanding of with modification through natural selection natural selection (Dawkins 2010:219). results in an increase in the number of spe- The process of extinction results, however, cies through time and a tendency of these from the replacement of predecessors and species to increasingly diverge in characters. the original parent by improved descen- This is represented in the tree by the subor- dants. The intermediate forms (e.g., a6, d5, dination of groups; e.g., of closely related k8) and their parent species (A) will generally species into genera, closely related genera become extinct. Darwin’s explanation con- into families, and families into higher taxo- sistently refers to inherited adaptive advan- nomic groups. The Tree Diagram is there- tages of diversified varieties in “the natural fore used by Darwin as a model to represent economy of their country” (p. 122). That is, evolutionary patterns at multiple scales since, less improved states of a species will tend to at different points throughout the Origin, become extinct, as they are replaced by more capital letters (A–L) and numbered let- improved states (p. 121). The application of ters (an–zn) in the diagram refer some- the principle of natural selection to the de- times to well-defined varieties and species

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 31

(e.g., Chapter IV), and other times to sub- TABLE 1 genera and genera (e.g., Chapters X and Structure of the evidence described in the Origin XIII). The subordination of groups is then (with corresponding chapter references) in support represented by the hierarchical organization of Darwin’s theory of evolution by means of of genera (a14–z14) grouped into subfamilies natural selection (e.g., a14–q14–p14), subfamilies grouped into Evidence in support of the theory 14 14 14 14 14 families (a –q –p –b –f ), and families Evidence from the Geological Record (Chapters IX–X) into two orders (defined by all extant descen- 1. The process of modification is gradual (Gradualism) dants from A and I, respectively). At the largest 2. The amount of change does not correspond with the scale, all numbered letters at the generation succession of geological formations (Variable Rates of interval XIV (a14–z14; representing current Change) time) of the Tree Diagram are united into 3. Ubiquitous nature of Extinction one class by an ancient but unseen common 4. Parallelism in successional forms 5. Affinities between successional forms ancestor of genera A–L (pp. 420–421). Evidence from the Geographical Distributions of Organisms (Chapters XI–XII) Evidence in Support of the Theory General Patterns (driven by migration/dispersal and The last section of Darwin’s “one long ar- common ancestry) gument” consists of evidential support for 1. Regional affinities of organic beings the theory from his observations on the dis- 2. Affinities of groups from different continents (single tribution of species in the geological record, centers of creation) on the geographical distribution of varieties, Patterns on Oceanic Islands (driven by migration/isolation and natural selection) species and higher taxonomic groups, and 1. Depauperate diversity of islands compared to on the hierarchical organization of nature continents (Table 1). 2. Increased number of endemic species Hodge (1977) has argued that, in doing 3. Deficiency of certain classes so, Darwin followed the argumentative strat- 4. Affinity of island species with those of the nearest egies of Herschel and Whewell, by present- mainland ing evidence favoring responsibility under the Evidence from the Affinities of Organic Beings (Chapter XIII) vera causa principle, i.e., showing that the 1. Hierarchical organization of Nature proposed theory on the origin of species by 2. The natural classification system natural selection is indeed responsible for 3. Single ancestral group the observed patterns in nature. I would ar- gue, however, that Darwin’s theory, as repre- sented in the Diagram of Divergence of regarding the temporal and spatial distri- Taxa, may be framed under a hypothetico- bution of species and the hierarchical or- deductive framework in which the theory can ganization of nature. The explanatory power be tested either experimentally or through ob- of the theory results, therefore, from the fact servation. That is, the Tree Diagram does that the evidence from apparently indepen- not represent a simple descriptive model dent phenomena is consistent with the pre- (of what has happened) but a hypo- dictions deduced from Darwin’s Diagram of thetico-deductive model that generates test- Divergence of Taxa. Other authors (e.g., able predictions regarding the evolutionary Harman 1965; Thagard 1978), however, process. Perhaps the best evidence for this have argued that this represents an inductive idea comes from Darwin’s “Big Species strategy by inference to the best explanation, Book” (Stauffer 1975), in which he contrasts in the sense that a variety of different phe- the evolutionary patterns predicted by two nomena can be explained by the same causal alternative hypotheses: natural selection (Di- process. agram I) versus chance (Diagram II; see Fig- A summary of Darwin’s evidence in sup- ure 4). Through the last few chapters of the port of the theory is presented in Table 1. Origin (Chapters IX, X, and XII), Darwin Following Darwin’s structure on the presen- consistently refers back to the Tree Diagram tation of evidence, one can assess the specific as he presents multiple evidential facts predictions of the model, as represented in

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 32 THE QUARTERLY REVIEW OF BIOLOGY Volume 89 the Diagram of Divergence of Taxa, in rela- common ancestors A and I). It is apparent tion to the temporal patterns of successional that, over the same time period (i.e., the full forms (evidence from the geological record), temporal scale of the diagram), A the spatial patterns of organisms (evidence changed at a higher rate than clade I, since from the geographical distribution of organ- the former resulted in eight descendant spe- isms), and the hierarchical organization of cies (a14–q14) whereas the latter only led to six nature (based on evidence regarding the af- species (n14–z14). Ultimately, one can argue finities of organic beings). that an extreme example of variability in the rate of change is represented by lineage F, evidence from the geological record which shows no divergence in character at all The temporal scale of Darwin’s Tree Dia- (i.e., no change). Darwin deliberately incor- gram, represented by ancestral/descent taxa porated variation in the rate of change in his (small numbered letters) in successive gen- conceptual model of evolution, as he was eration intervals (intervals I–XIV; Figure 1) familiar with many of the contingencies enables one to make predictions regarding (e.g., variability in fecundity, variation in the the rate of change and the affinities of spe- degree of intercrossing, rate of breeding, cies that existed in the past (Chapters IX and etc.) that different species exhibit in nature, X). Two major predictions become apparent as well as the existence of ancient “living when looking at the Tree Diagram. First, the fossils.” process of modification through time must A series of predictions from Darwin’s Tree be gradual (Gradualism). This is inferred Diagram were assessed by comparing species from the Principle of Divergence, repre- patterns in the fossil record. These include sented in the diagram as a steady and grad- predictions regarding the ubiquitous nature ual divergence in character while following of the extinction process, affinities of extinct the genealogical sequence of varieties/spe- species, and the relationships between ex- cies throughout a lineage (e.g., a1, a2, a3, an). tinct and extant species in the same geo- We must keep in mind that the horizontal graphical region (Table 1). The process of axes in the diagram are not meaningless; extinction played a central role in Darwin’s they represent the dimension of morpholog- theory. As mentioned above, the Tree Dia- ical variation such that species in close prox- gram illustrates the process of extinction as imity (e.g., A, B, C, or i3, m3) are more similar bearing on natural selection. The diagram to each other than to other species in the shows multiple examples of , rep- same axis (e.g., A, G or i3, x3). The change in resented by lineages that were not able to a lineage is therefore gradual since, as pre- reach generation interval XIV (Figure 1). dicted by the Principle of Divergence, at any That is, only few species will have descen- time a taxa would have intermediate mor- dants; in Darwin’s Tree Diagram only species phology between ancestral and descendent A, F, and I from the 11 ancestral species forms (e.g., a3 Ͼ a2 Ͼ a1). (A–L) have left contemporary descendants. The second prediction relates to the rate Disregarding the condensed diagram from of change between different groups. Darwin generation intervals XI to XIV, which does noted that “The amount of organic change... not illustrate extinction events, a minimum does not strictly correspond with the succes- of 14 extinctions can be identified between sion of our geological formations” (p. 313). generation intervals I and X. Scaling down That is, many times the same geological for- into the representation of different varieties mations showed evidence of massive changes by all dotted lines drawn, the diagram shows (e.g., adaptive radiations of some groups) at a total of 133 extinction events. It is particu- some locations and limited change at other larly at this level in which extinction becomes locations. Thus, different groups must have an essential part of Darwin’s explanatory changed at different rates. This prediction model since, under his theory, extinction is a can be inferred by comparing the two ma- direct consequence of the replacement of jor represented throughout the less successful varieties by more successful Tree Diagram (i.e., descendant species from ones through the process of natural selec-

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 33 tion. One could argue that under the Tree (e.g., a1–a2) are more closely related than Diagram one may also predict a greater rate those from remote formations (a1–a7). of extinctions in those lineages that do not These predictions led Darwin to formu- tend to change (i.e., lineages that may not be late a generalized law, the Law of Succes- able to become adapted to potential changes sional Types, which determined the in the environment). This is shown by the “relationship in the same continent be- fact that only one of the nine nondivergent tween the dead and the living” (p. 339). lineages (lineage F) was able to persist until That is, the expected association by com- generation XIV, while those lineages that mon descent between the extant fauna showed divergence (i.e., from ancestral and their ancestral forms. species A and I) are represented by 14 descendant species (a14–z14). The affini- evidence from the geographical ties of species in the geological record were distributions of organisms therefore consistent with three major predic- The Diagram of Divergence of Taxa can tions associated with Darwin’s Diagram of also be interpreted in relation to Darwin’s Divergence of Taxa on the gradual nature of observations on the geographical distribu- change, the variable rates of change, and the tion of species (Chapters XI and XII). How- ubiquitous nature of extinction (Table 1). ever, in this case, predictions from the model Darwin also emphasized a parallelism in require linking the Tree Diagram to the pro- the successive forms of life throughout the cess of migration/dispersal as one of the key world, with the same families, genera, and elements driving the geographical distribu- sections of genera being represented in dif- tion of organisms. Darwin emphasized that ferent regions, but not necessarily by the the major causes driving patterns on the af- same species (p. 323). However, this predic- finities of groups from local areas include tion does not follow logically from Darwin’s phylogeny (as depicted by the Tree Diagram) Tree Diagram, as this pattern is explained on and barriers to migration. As described the basis of new species having formed from above, the Tree Diagram has both a tempo- dominant species that spread widely and var- ral dimension represented by the generation ied in relation to the different environments intervals and a morphological dimension rep- they encounter. resenting character divergence on the hor- Predictions on the affinities of extinct and izontal axes. However, a “spatial scale” of extant species can be clearly inferred from geographic distribution is not apparent in Darwin’s Tree Diagram, as Darwin refers the diagram. One could argue that predic- back to the figure (in Chapter X) to assess tions regarding the geographical distribu- the evidence of extinct taxa being generally tion of organisms can easily be inferred intermediate in character with their modi- from the Tree Diagram, as the temporal fied descendants (p. 333), on the fauna of continuity of lineages through inheritance each geological period being intermediate and common descent would lead to species in character between preceding and suc- inhabiting local regions being more closely ceeding faunas (p. 334), and on the fossils related to one another than they are to spe- from consecutive formations being more re- cies in other local regions. However, migra- lated than those from remote formations (p. tion, or lack of thereof as a consequence of 335). These patterns are also consistent with geographical barriers, must be accounted specific predictions that can be inferred from for to provide a causal explanation for the the Diagram of Divergence of Taxa (Figure observed patterns. 1). For example, ancestral taxa A tend to be For Darwin, two general patterns of spe- intermediate in morphology with their de- cies relationships are presented as evidence scendants (e.g., a2 and m2,ora10 and m10), from the geographical distribution of organ- fossils from past formations (e.g., a1, m1) isms (Table 1). The first one refers to the tend to be more closely related than their regional affinities of organic forms. As shown descendants from later formations (e.g., a10, previously (see the section Evidence from m10), and fossils from successive formations the Geological Record), the Tree Diagram re-

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 34 THE QUARTERLY REVIEW OF BIOLOGY Volume 89

flects the grouping of species by common of Taxa in combination with the dispersal/ ancestry (e.g., a14–q14–p14 versus o14–e14–m14). migratory tendencies of species. Although no explicit geographic dimension In summary, the general patterns of the is included in the diagram, one can infer geographic distribution of organisms are that geographically isolated groups with re- explained by Darwin through the causal stricted migration would tend to be more efficacy of natural selection leading to the di- closely related to each other than to species versification of species into new forms (re- from other locations. For example, if the flected by the patterns of ancestral/descent two major diverging lineages from the dia- and multiplication of new forms in the Tree gram (i.e., lineages originating from ances- Diagram) in combination with a theory of tors A and I) would represent diversifying migration/isolation as a creative force. taxa from different continents, then one would predict that organisms on each of the evidence from the affinities continents would be more closely related by of organic beings ancestry, as reflected by the fact that a14 The conceptual model of Darwin’s theory through m14 are all descendants from A, and on the origin of species, as underscored in n14 through z14 are all descendants from I. the Diagram of Divergence of Taxa, also pro- Similar patterns can be identified at differ- vides a causal explanation for the hierarchical ent scales, as long as a scale of geographic organization of nature, a natural classification distribution is overlaid on top of the Tree system, and the relationships by which all Diagram. living forms are united (Chapters IX and X; The second general pattern regarding the Table 1). These three features are explained geographic distribution of organisms refers by Darwin’s theory, since they “all naturally to the affinities of groups from different con- follow on the view of the common parentage tinents as evidence for the presence of a of those forms which are considered by nat- single center of creation (as a contrasting uralists as allied, together with their modifi- hypothesis to the predominant view of mul- cation through natural selection, with its tiple centers of creation). This is also exem- contingencies of extinction and divergence plified in the Tree Diagram by patterns of of character” (p. 456). divergence followed from dispersal and dif- The hierarchical organization of groups is ferentiation from common ancestral forms. the natural outcome of three major features Darwin is puzzled by the simplicity of this depicted in the Diagram of Divergence of view, in which each species has originated Taxa: common ancestry, in combination with within a single region, as he stated: “He who a general tendency of species to increase in rejects it [this view], rejects the vera causa of number (i.e., multiplication of species), and ordinary generation with subsequent migra- divergence in character (as a result of natu- tion, and calls in the agency of a miracle” (p. ral selection). “[T]he inevitable result is that 352). the modified descendants proceeding from The causal agency of phylogeny (i.e., com- one progenitor become broken up into mon ancestry and diversification by natural groups subordinate to groups” (p. 412). This selection) in combination with migration/ is exemplified by Darwin’s referring to the isolation is then used by Darwin to explain numbered letters in the last generation in- the geographic patterns of species distribu- terval of the Tree Diagram (a14–z14; Figure 1). tions on oceanic islands (Chapter XII). De- At this level, each letter may represent a ge- creased levels of diversity on islands, nus composed of multiple species. At the increased number of endemic species, same time, these genera may be grouped deficiency of certain taxonomic groups, into subfamilies (e.g., a14–p14) and subse- and the affinity of island species to those of quently into families (a14–f 14) based on their nearest species in the mainland are consid- descent from ancestral species a10 and a5, ered evidential support for his theory, as they respectively. At the largest scale, all descen- are consistent with the conceptual model dants from A and I may represent distinct represented by the Diagram of Divergence orders of a large class (encompassing a14–z14)

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 35 that have descended from a single common Conclusions progenitor not represented in the tree. The characterization of nature’s organiza- It was the recognition of this subordina- tion in tree-like schemas preceded Darwin tion of groups resulting from the mutual and the Origin (Ragan 2009). Before Darwin, affinities of organic beings that provided a most tree-like structures aimed at describing justification for a natural system of clas- relationships among organisms represented sification and the foundations for mod- classifications and keys organized hierarchi- ern evolutionary taxonomy (Mayr 1942). cally, but with no phylogenetic meaning After Darwin, taxonomic groups could (Ragan 2009; Tassy 2011). That is, these only be considered “natural” as long as schemas were conceived to show affinities their grouping was consistent with this pat- between organisms, not evolutionary trans- tern of common ancestry and subordination formations. Lamarck (1809) published the (monophyletic groups). The natural system first to illustrate the rela- of classification, therefore, emphasized that tionships between major groups of animals on the arrangement of groups must be genea- the basis of origin. Lamarck’s tree thinking logical but the degree of modification must was, however, framed under his theory of be expressed by ranking groups into higher evolution, which focused on phyletic trans- taxonomic categories. This provides justifica- formation. Thus, his trees tended to be tion for considering higher taxa as arbitrary more linear (rather than bifurcating pat- groups (pp. 421–422); that is, as conceptual terns) with extant groups depicted as ances- constructs based on subjective interpretations tral species. Lamarck’s trees may therefore of the “degree of modification” required to be considered prephylogenetic, since they define a particular taxonomic category. were conceived using “classified taxa based Finally, the Diagram of Divergence of on a non-transformational concept of life to Taxa provides the basis to explain the nature illustrate transformation” (Tassy 2011:91). Fur- of the relationship by which all living beings thermore, Lamarck’s trees were intended as are united. Going backward in time, a logical factual representations of evolutionary trans- inference from the Tree Diagram suggests formations of specific taxonomic groups (e.g., that all living forms coalesce into a single for the origin of animals and the origin of humans) rather than as theoretical models for common ancestral form, i.e., the unseen pro- the evolutionary process as a whole. genitor of species A–L depicted in the dia- It was not until Darwin that true phyloge- gram. The idea of a single origin of life is netic trees illustrating evolutionary transfor- only lightly addressed in the Origin (p. 484). mations and genealogical relationships by Most naturalists that believed in the transmu- common descent were conceived. Darwin’s tation of species suggested that most organ- Tree Diagram in the Origin was indeed the isms originated from multiple independent first published theoretical tree that illus- progenitors. Darwin himself expressed in the trated both a schema of pattern and a schema Origin that animals and plants may each have of process (Tassy 2011:93). The pattern re- descended from four or five ancestral proto- ferred to the relationships between varieties types. However, on the basis of his proposed and species on the basis of common descent theory and intuitive speculations on com- and the process to the actual principle of di- mon features of the chemical composition, versification driven by natural selection. cellular structure, and laws of growth and Throughout this paper, I argue that Dar- reproduction, he conceded to the idea win’s Diagram of Divergence of Taxa pre- of a single primordial form, as he con- sented in the Origin goes beyond the de- cludes: “Therefore I should infer from anal- scriptive aspects of the principle of diver- ogy that probably all the organic beings gence and common descent. In fact, I which have ever lived on this earth have de- showed that Darwin used the Tree Dia- scended from some one primordial form, gram as a conceptual model for his theory of into which life was first breathed” (p. 484). evolution, to establish a causal relationship

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 36 THE QUARTERLY REVIEW OF BIOLOGY Volume 89 between natural selection leading to diver- most naturalists entertain, and which I for- sification and common descent. merly entertained–namely, that each species Darwin’s Diagram of Divergence of Taxa is has been independently created–is errone- not just a theoretical representation of the ous” (p. 6). relationships among living organisms and Arguably, one could consider that under the way organic diversity evolved (Darwin’s the vera causa principle Darwin followed in- theory). I believe the Tree Diagram also il- ductive reasoning in establishing the case for lustrates Darwin’s argumentative strategy for the existence of natural selection, as he arrived the origin of species by means of natural to the generalization of the struggle for ex- selection under the vera causa principle. As istence (Chapter III) through the accumula- previously shown, the characterization of tion of empirical observations on artificial natural selection as the evolutionary mecha- selection of domestic species (Chapter I), nism that explains divergence and common and on variation, reproductive growth, and descent enabled Darwin to make the case for competition under nature (Chapters II and the competence of natural selection to produce new species, one of the necessary conditions III). In contrast, on the case for competence of the principle of vera causa (Hodge 1977). and responsibility, I believe Darwin’s argu- In addition, the characterization of Darwin’s ment represents a classical example of the Tree Diagram as a conceptual model of evo- hypothetico-deductive method. That is, Dar- lution, i.e., as a working hypothesis, allowed win’s theory, as depicted in the Diagram of Darwin confirmation of his theory through Divergence of Taxa (Chapter IV), emerges predicted patterns observed in the geologi- as a classical hypothesis with multiple test- cal record, the geographical distributions of able predictions. This is clearly evidenced in organisms, and the affinities of organic be- Darwin’s original notes from his “Big Species ings. This diverse number of observations by Book” manuscript (Stauffer 1975), in which Darwin made the case for the responsibility of he presents natural selection as an alterna- natural selection in the production of extant tive hypothesis to the doctrine of chance and extinct species, the final condition to (Figure 4). The actual testing of these hypoth- ascribe vera causa to the phenomenon of the eses was accomplished through Darwin’s evi- origin of species. dential observations on the geological record, Darwin’s argumentative strategy also illus- the geographic distribution of organisms, and trates that his research program followed the affinities of organic beings (Chapters IX– both inductive and deductive principles (Ayala XIII), making the case for responsibility (see Fig- 2009). Although Darwin himself claimed that ure 2). his research followed “true Baconian princi- My intention throughout the paper was to ples,” he also emphasized the importance of capture Darwin’s reasoning in developing making observations in relation to support- the Diagram of Divergence of Taxa as a con- ing or rejecting a particular view (i.e., a hy- ceptual representation of his theory of evo- pothesis), what we now consider as the core lution. It is true that the idea of common element of the hypothetico-deductive method. In Darwin’s view, the best explanation for a descent is logically independent from Dar- wide range of biological and geological data win’s idea of natural selection as the main was the hypothesis that species originated as mechanism of species transformation. This is a result of decent by modification through evidenced by the fact that many of Darwin’s the working of natural selection, as exempli- contemporary readers accepted the idea of fied in the Diagram of Divergence of Taxa. common descent but rejected natural selec- This view was presented by Darwin in the tion as the dominant evolutionary mechanism context of the competing theory of Special (Mayr 1991). The argument in the Origin was Creation, the dominant pre-Darwinian par- however not about natural selection indepen- adigm. This is evidenced at the end of Dar- dently of the splitting of one species into two; win’s introduction to the Origin, in which he the argument was about natural selection driv- specifically indicated that “the view which ing the actual splitting. That is, under Darwin’s

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions March 2014 DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA 37 view, without natural selection there was in- win’s argument follows the inference to deed no vera causa for transmutation and com- the best explanation; that is, the criteria by mon descent. which a hypothesis can be judged to provide Waters (2003) suggests that Darwin’s argu- a better explanation than a competing hy- ment was flexible enough to provide compel- pothesis. In the case of Darwin, the alterna- ling arguments for evolution independently of tive hypothesis to which he consistently natural selection. Under his perspective, the referred throughout the Origin was Special Origin offers a strong case for transmutation Creation. and common descent, regardless of natural Regardless of anyone’s views about the na- selection being the dominant evolutionary ture of Darwin’s one long argument, one mechanism. It would be difficult, however, to thing seems clear: that the only figure in the argue that transmutation and common de- Origin of Species, the Diagram of Divergence scent were the central elements of Darwin’s of Taxa, is not just a simple tree illustrating one long argument, since both ideas were genealogical relationships between organ- published prior to the publication of the Or- isms, but a conceptual model that provides igin, by Lamarck (1809) on the transmuta- significant insights into the causal construc- tion of species and by Wallace (1855) on tion of Darwin’s theory of evolution, the common descent. It is true, though, that structure of his argumentative strategy, and prior to the Origin there had not been much the nature of his scientific methodology. evidence presented formally on the issue of causal responsibility for these two processes. acknowledgments Darwin’s one long argument has also been I would like to specially thank Michael Bradie (Depart- framed under William Whewell’s idea that ment of Philosophy, Bowling Green State University) causality can only be established on the basis as well as all of the students who have participated in of consilience (Waters 2003), the process by my seminars on Darwin and Wallace for their construc- tive comments and fruitful discussions on the ideas pre- which a wide variety of apparently indepen- sented in this paper. I would also like to express my dent phenomena can be explained as a re- gratitude to Elliot Sober, Francisco Ayala, Richard sult of the same cause (Whewell 1840; Hull Burkhardt, and Ronny Woodruff, who provided con- 2003). This is consistent with the idea structive comments on earlier versions of the manu- proposed by Thagard (1978) that Dar- script.

REFERENCES

Ayala F. J. 2009. Darwin and the scientific method. animaux et d’introduction a` l’anatomie compare´e. Paris Proceedings of the National Academy of Sciences of the (France): A. Belin. United States of America 106:10033–10039. Darwin C. 1859. On the Origin of Species by Means of Barret P. H., Gautrey P. J., Herbert S., Kohn D., Smith Natural Selection, or the Preservation of Favoured Races S. 1987. Charles Darwin’s Notebooks, 1836–1844: Ge- in the Struggle For Life. London (United Kingdom): ology, Transmutation of Species, Metaphysical Enqui- John Murray. ries. Ithaca (New York): Cornell University Press. Darwin C., Wallace A. R. 1858. On the tendency of Blyth E. 1835. An attempt to classify the “varieties” of species to form varieties; and on the perpetuation animals, with observations on the marked seasonal of varieties and species by natural means of selec- and other changes which naturally take place in tion. Journal of the Proceedings of the Linnean Society of various British species, and which do not consti- London 3:45–62. tute varieties. Magazine of Natural History 8:40–53. Darwin E. 1794. Zoonomia, or, the Laws of Organic Life. Brooks J. L. 1984. Just Before the Origin: Alfred Russel London (United Kingdom): J. Johnson. Wallace’s Theory of Evolution. New York: Columbia Dawkins R. 2010. Darwin’s five bridges: the way to University Press. natural selection. Pages 203–227 in Seeing Further: Buffon G. L. L. 1749. Histoire Ge´ne´rale des Animaux. His- The Story of Science, Discovery, and the Genius of the toire Naturelle de l’Homme, Tome II. Paris (France): Royal Society, edited by B. Bryson. New York: Harper L’imprimerie Royale. Perennial. Cuvier G. 1817. Le Re`gne animal distribue´ d’apre`s son Freeman S., Allison L., Black M., Podgorski G., Quil- organisation, pour servir de base a` l’histoire naturelle des lin K., Monroe J., Taylor E. 2014. Biological Science.

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions 38 THE QUARTERLY REVIEW OF BIOLOGY Volume 89

Fifth Edition. Glenview (Illinois): Pearson Educa- after Darwin. Biology Direct 4:43. doi:10.1186/1745- tion Inc. 6150-4-43. Harman G. H. 1965. The inference to the best expla- Reece J. B., Urry L. A., Cain M. L., Wasserman S. A., nation. Philosophical Review 74:88–95. Minorsky P. A., Jackson R. B., Campbell N. A. Haeckel E. 1868. The History of Creation. Translated by 2011. Campbell Biology. Ninth edition. Boston (Mas- E. Ray Lankester, 1880. New York: D. Appleton sachusetts): Pearson Education Inc. and Company. Ruse M. 1980. Charles Darwin and group selection. Herschel J. F. W. 1830. A Preliminary Discourse on the Annals of Science 37:615–630. Study of Natural Philosophy. London (United King- Ruse M. 2009. The Darwinian revolution: rethinking dom): Longman, Rees, Orme, Brown and Green its meaning and significance. Proceedings of the Na- and John Taylor. tional Academy of Sciences of the United States of Amer- Hodge J., Radick G. 2003. The Cambridge Companion to ica 106:10040–10047. Darwin. Cambridge (United Kingdom): Cambridge Sober E. 2009. Did Darwin write the Origin back- University Press. wards? Proceedings of the National Academy of Sciences Hodge M. J. S. 1977. The structure and strategy of of the United States of America 106:10048–10055. Darwin’s “Long Argument.” British Journal for the Sober E., Orzack S. H. 2003. Common ancestry and History of Science 10:237–246. natural selection. British Journal of Philosophy of Sci- Hull D. 2003. Darwin’s science and Victorian philos- ence 54:423–437. ophy of sciences. Pages 173–196 in The Cambridge Stauffer R. C. 1975. Charles Darwin’s Natural Selection: Companion to Darwin, edited by M. Hodge and G. Being the Second Part of His Big Species Book Written Radick. Cambridge (United Kingdom): Cam- From 1856 to 1858. Cambridge (United Kingdom): bridge University Press. Cambridge University Press. Kavaloski V. C. 1974. The Vera Causa principle: a Tassy P. 2011. Trees before and after Darwin. Journal historico-philosophical study of a metatheoretical of Zoological and Evolutionary Research 49: concept from Newton through Darwin. PhD diss., 89–101. University of Chicago. Thagard P. R. 1978. The best explanation: criteria for Kohn D. 1985. The Darwinian Heritage. Princeton theory choice. Journal of Philosophy 75:76–92. (New Jersey): Princeton University Press. Urry L. A., Cain M. L., Wasserman S. A., Minorsky Kimura M. 1968. Evolutionary rate at the molecular P. V., Jackson R. B., Reece J. B. 2014. Campbell level. Nature 217:624–626. Biology in Focus. Boston (Massachusetts): Pearson Kottler M. J. 1985. Charles Darwin and Alfred Russel Education Inc. Wallace: two decades of debates over natural se- Wallace A. R. 1855. On the law which has regulated lection. Pages 367–432 in The Darwinian Heritage, the introduction of new species. Annals and Mag- edited by D. Kohn. Princeton (New Jersey): azine of Natural History 16:184–196. Princeton University Press. Wallace A. R. 1858. On the tendency of varieties to Lamarck J.-B. 1809. . Paris depart indefinitely from the original type. Paper (France): Dentu et l’Auteur. sent by Wallace to Darwin. Reprinted in Chapter Mayr E. 1942. Systematics and the Origin of Species from II of A. R. Wallace. 1870. Contributions to the Theory the Viewpoint of a Zoologist. New York: Columbia of Natural Selection. London (United Kingdom): University Press. McMillan and Company. Mayr E. 1982. The Growth of Biological Thought: Diver- sity, Evolution, and Inheritance. Cambridge (Massa- Waters K. 2003. The arguments in the Origin of Species. chusetts): Harvard University Press. Pages 120–146 in The Cambridge Companion to Dar- Mayr E. 1991. One Long Argument: Charles Darwin and win, edited by M. Hodge and G. Radick. Cam- the Genesis of Modern Evolutionary Thought. Cam- bridge (United Kingdom): Cambridge University bridge (Massachusetts): Harvard University Press. Press. Paley W. 1802. Natural Theology. London (United Whewell W. 1840. The Philosophy of Inductive Sciences, Kingdom): Printed for R. Faulder by Wilks and Founded Upon Their History. London (United King- Taylor. dom): John W. Parker. Ragan M. A. 2009. Trees and networks before and Handling Editor: Michael Ruse

This content downloaded from 129.1.59.69 on Wed, 15 Oct 2014 12:21:08 PM All use subject to JSTOR Terms and Conditions