In parts one and two, I showed that suggestions by some intelligent design advocates that evolutionary biologists have only recently become aware of widespread convergence are false. At least one ID proponent, though, has gone further, suggesting that convergence is a post hoc rationalization invented by ‘Darwinists’ to hide their dirty little secret that common descent is not supported by evidence.
Physicist Lee M. Spetner makes this argument in his book The Evolution Revolution. I don’t own The Evolution Revolution, but Casey Luskin has helpfully, and approvingly, quoted some critical passages:
Convergent evolution is the Darwinists’ lollapalooza. They made it up to keep their phylogenetic tree from falling apart, but they can’t say how convergence happens. — As quoted by Casey Luskin, 2014-10-19
Convergent evolution is the appearance of the same trait or character in independent lineages. It is, however, an invention. It was invented solely to avoid addressing the failure of phylogenetic trees to support Common Descent. –As quoted by Casey Luskin, 2014-10-19
…the concept of convergence was invented to explain away many discrepancies in the phylogenetic trees, examples of convergence are inherently contradictions to the M[odern] S[ynthesis]. —2016-09-26
Here’s the thing about post hoc rationalizations, though: to properly qualify, they should really be post hoc. As with Günter Bechly’s revisionist history, this argument has a temporal continuity problem. In the macro world at least, causes precede effects. To the best of our knowledge, this is always true. If Darwinists ‘invented’ convergence to shore up their phylogenies, it stands to reason that Darwinists must have existed when convergence was invented. I think it’s fair to say that there were no ‘Darwinists’ before 1858, when Darwin and Alfred Russel Wallace jointly presented the theory they had each arrived at independently to the Linnaean Society. Unfortunately for Dr. Spetner’s argument, the concept of convergence (as he defines it) substantially preceded the publication of Darwin’s theory.
I said in part one that I want to use the same definition of convergence as the authors I’m criticizing, and Dr. Spetner helpfully provides one, “the appearance of the same trait or character in independent lineages.” The idea that organisms can be organized into ‘natural’ groupings dates back at least to Aristotle, and was formalized by Linnaeus over a hundred years before the publication of the Origin. Linnaeus’ and Aristotle’s classifications were not evolution-based, and so it would be a stretch to talk about their groupings as “lineages.” Both did recognize, though, that similarities exist between species from different groups. For example, whales share with fishes an aquatic habit, fins, and an (often) fusiform body, but both Aristotle and Linneaus recognized that they share deeper similarities with terrestrial mammals. Many other examples of pre-Darwinian biologists seeking ‘natural’ groupings could no doubt be cited, but I don’t pretend to be a historian of biology.
Some of the groupings of those pre-Darwinian biologists were explicitly evolutionary, though. Lamarck, for example, wrote of ‘affinities’ among groups of organisms as indicative of their position in the ‘order of nature’. Although his view of evolution was mostly linear rather than branching, the proximity of two groups was a result of their evolutionary relationships. He discussed in some detail the appearance of the trait of flight (including gliding) in four independent lineages:
The faculty of flight would seem to be quite foreign to [mammals]; yet I can show how nature has gradually produced extensions of the animal’s skin, starting from those animals which can simply make very long jumps and leading up to those which fly perfectly ; so that ultimately they possess the same faculty of flight as birds, though without having any affinities with them in their organisation.
Flying squirrels (Sciurus volans, aerobates, petaurista, sagitta, volucella) have more recently acquired this habit of extending their wings when leaping, so as to convert their body into a kind of parachute ; they can do no more than make a very long jump by throwing themselves to the bottom of a tree, or leaping from one tree on to another at a moderate distance. Now by frequent repetition of such leaps in the individuals of these races, the skin of their flanks is dilated on each side into a loose membrane, which unites the hind-legs to the fore-legs and embraces a large volume of air ; thus saving them from a sudden fall. These animals still have no membranes between the digits.
The galeopithecus (Lemurvolans) doubtless acquired this habit earlier than the flying squirrels (Pteromis, Geoffr.) ; the skin of their flanks is still larger and more developed ; it unites not only the hind-legs with the fore-legs but also the tail with the hind-legs and the digits with each other. Now these creatures make longer leaps than the preceding, and even perform a sort of flight.
Lastly, the various bats are mammals which probably acquired still earlier than the galeopithecus the habit of extending their limbs and even their digits to embrace a great volume of air, and sustain themselves when they launch forth into the atmosphere.
Lamarck clearly recognized flying squirrels, flying lemurs, and bats as separate lineages (he classified them in three separate families), and certainly he considered all three separate from birds. Remember, Dr. Spetner’s own definition of convergence is “the appearance of the same trait or character in independent lineages.” Fifty years before the Origin, Lamarck was writing about convergence.
I’m not trying to burn Dr. Spetner on a technicality here (well, I am, but that’s not the point). Sure, there’s a contradiction between saying convergence was ‘invented’ by Darwinists and the fact that convergence was recognized before Darwinists existed. He used the wrong term; who cares. But if we replace ‘Darwinists’ with ‘biologists’, Dr. Spetner’s claims are still false. This isn’t about pedantry; it’s about calling out stories that are at odds with the real history of biology.
The truth is that biologists long before Darwin recognized the existence of natural groupings of organisms connected by deep similarities. Not all interpreted those similarities to indicate evolutionary relationships, but what of it? Whales and fishes are separate lineages whether they descended from a common ancestor or were separately created. These early biologists also recognized that superficial similarities often occur between species from disparate groups. This is the reason Linnaeus classified whales among the mammals, in spite of a superficial similarity to fish. It’s the reason he classified eels with the fishes instead of the snakes, and manatees with elephants, instead of, say, walruses.
The idea that convergence was ‘invented’ by Darwinists to ‘explain away’ discrepancies in phylogenetic trees is false. The phenomenon of superficial similarities between distantly related species (“the same trait or character in independent lineages”) was well known to Darwin, and to biologists working long before he published the Origin. Convergent evolution is an inference from phylogenetic trees, not an invention to hide their alleged ‘failure’. Dr. Spetner’s claims to the contrary are simply wrong.
Lamarck, J. B. 1809 (translation 1914). Zoological Philosophy: An Exposition with Regard to the Natural History of Animals. English translation by H. Elliot. McMillan and Co., London.
McBirney, A. and Cook, S. 2009. The Philosophy of Zoology before Darwin: a translated and annotated version of the original French text by Edmond Perrier. Rotterdam, Netherlands: Springer Netherlands.