Note: This is Part 6 in a series reviewing Jerry Coyne’s Why Evolution Is True. Read Part 1 here, Part 2 here, Part 3 here, Part 4 here, and Part 5 here.
Theological arguments are also prominent in The Origin of Species. For example, Darwin argued that the geographic distribution of living things made no sense if species had been separately created, but it did make sense in the context of his theory. Cases such as “the presence of peculiar species of bats on oceanic islands and the absence of all other terrestrial mammals,” Darwin wrote, “are facts utterly inexplicable on the theory of independent acts of creation.” In particular: “Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands?” According to Darwin, “on my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across.”34
But Darwin knew that migration cannot account for all patterns of geographic distribution. He wrote in The Origin of Species that “the identity of many plants and animals, on mountain-summits, separated from each other by hundreds of miles of lowlands, where Alpine species could not possibly exist, is one of the most striking cases known of the same species living at distant points without the apparent possibility of their having migrated from one point to the other.” Darwin argued that the recent ice age “affords a simple explanation of these facts.” Arctic plants and animals that were “nearly the same” could have flourished everywhere in Europe and North America, but “when the warmth had fully returned, the same species, which had lately lived together on the European and North American lowlands, would again be found in the arctic regions of the Old and New Worlds, and on many isolated mountain-summits far distant from each other.”35
So some cases of geographic distribution may not be due to migration, but to the splitting of a formerly large, widespread population into small, isolated populations–what modern biologists call “vicariance.” Darwin argued that all modern distributions of species could be explained by these two possibilities. Yet there are many cases of geographic distribution that neither migration nor vicariance seem able to explain.
One example is the worldwide distribution of flightless birds, or “ratites.” These include ostriches in Africa, rheas in South America, emus and cassowaries in Australia, and kiwis in New Zealand. Since the birds are flightless, explanations based on migration over vast oceanic distances are implausible. After continental drift was discovered in the twentieth century, it was thought that the various populations might have separated with the landmasses. But ostriches and kiwis are much too recent; the continents had already drifted apart when these species originated. So neither migration nor vicariance explain ratite biogeography.36
Another example is freshwater crabs. Studied intensively by Italian biologist Giuseppe Colosi in the 1920s, these animals complete their life cycles exclusively in freshwater habitats and are incapable of surviving prolonged exposure to salt water. Today, very similar species are found in widely separated lakes and rivers in Central and South America, Africa, Madagascar, southern Europe, India, Asia and Australia. Fossil and molecular evidence indicates that these animals originated long after the continents separated, so their distribution is inconsistent with the vicariance hypothesis. Some biologists speculate that the crabs may have migrated by “transoceanic rafting” in hollow logs, but this seems unlikely given their inability to tolerate salt water. So neither vicariance nor migration provides a convincing explanation for the biogeography of these animals.37
An alternative explanation was suggested in the mid-twentieth century by Léon Croizat, a French biologist raised in Italy. Croizat found that Darwin’s theory did “not seem to agree at all with certain important facts of nature,” especially the facts of biogeography. Indeed, he concluded, “Darwinism is by now only a straitjacket… a thoroughly decrepit skin to hold new wine.” Croizat did not argue for independent acts of creation; instead, he proposed that in many cases a widespread primitive species was split into fragments, then its remnants evolved in parallel, in separate locations, into new species that were remarkably similar. Croizat called this process of parallel evolution “orthogenesis.” Neo-Darwinists such as Ernst Mayr, however, pointed out that there is no mechanism for orthogenesis, which implies–contrary to Darwinism–that evolution is guided in certain directions; so they rejected Croizat’s hypothesis.38
In Why Evolution Is True, Coyne (like Darwin) attributes the biogeography of oceanic islands to migration, and certain other distributions to vicariance. But Coyne (unlike Darwin) acknowledges that these two processes cannot explain everything. For example, the internal anatomy of marsupial mammals is so different from the internal anatomy of placental mammals that the two groups are thought to have split a long time ago. Yet there are marsupial flying squirrels, anteaters and moles in Australia that strikingly resemble placental flying squirrels, anteaters and moles on other continents, and these forms originated long after the continents had separated.
Coyne attributes the similarities to “a well-known process called convergent evolution.” According to Coyne. “It’s really quite simple. Species that live in similar habitats will experience similar selection pressures from their environment, so they may evolve similar adaptations, or converge, coming to look and behave very much alike even though they are unrelated.” Put together common ancestry, natural selection, and the origin of species (“speciation”), “add in the fact that distant areas of the world can have similar habitats, and you get convergent evolution–and a simple explanation of a major geographic pattern.”39
This is not the same as Croizat’s “orthogenesis,” according to which populations of a single species, after becoming separated from each other, evolve in parallel due to some internal directive force. According to Coyne’s “convergent evolution,” organisms that are fundamentally different from each other evolve through natural selection to become superficially similar because they inhabit similar environments. The mechanism for orthogenesis is internal, whereas the mechanism for convergence is external. In both cases, however, mechanism is crucial: Without it, orthogenesis and convergence are simply words describing biogeographical patterns, not explanations of how those patterns originated.
So the same question can be asked of convergence that was asked of orthogenesis: What is the evidence for the proposed mechanism? According to Coyne, the mechanism of convergence involves natural selection and speciation.
34 Darwin, The Origin of Species, Chapters XIII (pp. 347-352) and XV (p. 419). Available online (2009) here.
35 Darwin, The Origin of Species, Chapters XII (pp. 330-332). Available online (2009) here.
36 Alan Cooper, et al., C. Mourer-Chauviré, C.K. Chambers, A. von Haeseler, A.C. Wilson & S. Paabo, “Independent origins of New Zealand moas and kiwis,” Proceedings of the National Academy of Sciences USA 89 (1992): 8741-8744. Available online (2008) here.
Oliver Haddrath & Allan J. Baker, “Complete mitochondrial DNA genome sequences of extinct birds: ratite phylogenetics and the vicariance biogeography hypothesis,” Proceedings of the Royal Society of London B 268 (2001): 939-945.
John Harshman, E.L. Braun, M.J. Braun, C.J. Huddleston, R.C.K. Bowie,
J.L. Chojnowski, S.J. Hackett, K.-L. Han, R.T. Kimball, B.D. Marks, K.J. Miglia,
W.S. Moore, S. Reddy, F.H. Sheldon, D.W. Steadman, S.J. Steppan, C.C. Witt & T. Yuri, “Phylogenomic evidence for multiple losses of flight in ratite birds,” Proceedings of the National Academy of Sciences USA 105 (2008): 13462-13467. Abstract available online (2008) here.
Giuseppe Sermonti, “L’evoluzione in Italia – La via torinese / How Evolution Came to Italy – The Turin Connection,” Rivista di Biologia/Biology Forum 94 (2001): 5-12. Available online (2008) here.
37 Giuseppe Colosi, “La distribuzione geografica dei Potamonidae,” Rivista di Biologia 3 (1921): 294-301. Available online (2009) here.
Savel R. Daniels, N. Cumberlidge, M. Pérez-Losada, S.A.E. Marijnissen &
K.A. Crandall, “Evolution of Afrotropical freshwater crab lineages obscured by morphological convergence,” Molecular Phylogenetics and Evolution 40 (2006): 227–235. Available online (2009) here.
R. von Sternberg, N. Cumberlidge & G. Rodriguez. “On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura),” Journal of Zoological Systematics and Evolutionary Research 37 (1999): 19–38.
Darren C.J. Yeo, et al., “Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater,” Hydrobiologia 595 (2008): 275-286.
38 Léon Croizat, Space, Time, Form: The Biological Synthesis. Published by the author (Deventer, Netherlands: N. V. Drukkerij Salland, 1962), p. iii.
Robin C. Craw, “Léon Croizat’s Biogeographic Work: A Personal Appreciation,” Tuatara 27:1 (August 1984): 8-13. Available online (2009) here.
John R. Grehan, “Evolution By Law: Croizat’s ‘Orthogeny’ and Darwin’s ‘Laws of Growth’,” Tuatara 27:1 (August 1984): 14-19. Available online (2009) here.
Carmen Colacino, “Léon Croizat’s Biogeography and Macroevolution, or… ‘Out of Nothing, Nothing Comes’,” The Philippine Scientist 34 (1997): 73-88.
Ernst Mayr, The Growth of Biological Thought (Cambridge, MA: Harvard University Press, 1982), pp. 529-530.
39 Coyne, Why Evolution Is True, pp. 92-94.