I promised in part one of this series that I would show why the argument that convergence is a problem for evolution is daft, and I haven’t really done that. What I’ve done so far is show that the argument includes a false premise, namely that evolutionary biologists have only recently become aware that convergence is widespread.
In parts one, two, and three, I showed that some intelligent design proponents misrepresent the history of biological thought regarding convergence. They have created an alternate history in which biologists from Darwin to Dawkins were barely aware of convergent evolution, and have only in the last few decades been forced to confront it. Whether this is dishonesty or just bad scholarship, I can’t say, but it is a big, stinking pile of wrong.
But I haven’t really engaged their core argument, a fair paraphrase of which is that convergence, the appearance of similar phenotypes in distantly related species, is evidence against (or even falsifies) common descent. For example, Cornelius Hunter says convergence
…violates the evolutionary pattern. Regardless of adaptation versus constraint explanations, and any other mechanisms evolutionists can or will imagine, the basic fact remains: a fundamental evidence and prediction of evolution is falsified. —2017-05-25
Casey Luskin says it
…Challenges Darwinism and Destroys the Logic Behind Common Ancestry —2015-02-09
…just what evolutionary theory should not expect. —2017-06-26
…it is an epic myth, willingly perpetuated by evolutionary biologists, that the similarities between organisms mostly fall in a hierarchic pattern of nested groups and thus suggest common ancestry and indicate phylogenetic relationship. In reality this claim is contradicted by a flood of incongruences and reticulate patterns that shed doubt on fundamental paradigms of evolutionary biology like the notions of homology and common descent. —2018-04-23
This argument is daft. That the similarities between organisms mostly fall in a hierarchic pattern of nested groups is not a myth; it is exactly right. This pattern is so well established that we can fairly call it a fact, and it has been known for hundreds of years. The pattern of nested similarities is the basis of the Linnaean classification system established nearly 300 years ago, and subsequent discoveries have provided no cause to doubt it.
But wait, didn’t I say in part one that “biologists have long known that convergence is widespread and common”? That’s right, I did. Doesn’t that contradict the pattern of nested similarities? No, not at all. That’s because I’m not claiming that all traits fall into a nested hierarchy. What I said is exactly right is that most traits fit a hierarchical pattern. And they do.
So when Cornelius Hunter points out the similarities in general appearance between marsupial sugar gliders and placental flying squirrels, he is right that they have several traits in common: flaps of skin used for gliding, large eyes that allow them to see in low light, and general adorableness that makes people want them as pets (don’t keep them as pets; they make bad pets). What he is ignoring is that sugar gliders have many more traits in common with other marsupials, and flying squirrels with other placental mammals, than they have with each other.Sugar gliders share with other marsupials a pouch (marsupium) in which they carry their young, epipubic bones, no corpus callosum, two uteri and two vaginas in females, a split penis in males, a cloaca, altricial young, and several diagnostic characters of the skull. In addition, they share with other members of the order Diprotodontia large lower incisors, a short jaw, no lower canines, and partially fused second and third digits. Along with all of those traits, they share with the other members of the family Petauridae obvious facial markings; a well-defined dorsal stripe; rear-opening pouch; opposable first and second digits, a 3/2, 1/0, 3/3, 4/4 dental formula; and four-cusped molars.
Flying squirrels share with other placental mammals a well-developed placenta (of course), wide pelvis, no epipubic bones, mortis and tenon ankle joints, a bony prominence (malleolus) at the bottom of the calf bone, corpus callosum, and relatively precocial offspring. They share with other rodents continuously growing, chisel-like incisors; with other squirrels a diastema (gap between incisors and molars) and a 1/1, 0/0, 1/1, 3/3 dental formula; and with other flying squirrels a cartilaginous projection from the wrist, several unique features of the humerus, prominent distal process of the radius, distinct pisiform bone, and several other skeletal features (Thorington et al. 2005).
In summary, flying squirrels share many characters with other placental mammals, still more with other rodents, still more with other squirrels, and still more with other flying squirrels. Sugar gliders share many characters with other marsupials, still more with other members of their order, and still more with other members of their family. The similarities are, in other words, nested in a hierarchical fashion. Are there traits that don’t fit that pattern? Certainly. They are outnumbered many-fold by those that do. To suggest that the similarities between sugar gliders and flying squirrels contradict the expectations of common descent is to ignore a massive pile of data from comparative anatomy, skeletal morphology, and genomics.
I could, of course, give many other examples. Yes, cephalopods and vertebrates independently evolved camera-type eyes, but cephalopods share many more characters with other molluscs than they do with any vertebrates. Yes, toothed whales and insect-eating bats independently evolved echolocation, but again each group has many more traits in common with its close relatives than with any member of the other group. Yes, birds, bats, insects, and pterodactyls independently evolved powered flight, but anyone who doesn’t realize that many more traits are shared within those groups than among them has never had a course in comparative anatomy.
It will always be possible to cherry pick one or a few traits shared between distantly related groups and declare that the similarities falsify common descent. In every case, though, you have to ignore the vast majority of traits, nearly all of which fit the expectations of common descent, i.e. successively fewer traits shared with more distant relatives. In fact, I defy anyone to provide an example of two species from different phyla, classes, or orders that share more traits with each other than with other members of their own taxon.
The similarities between organisms do mostly fall in a hierarchic pattern of nested groups. This hierarchic patter does suggest common ancestry and indicate phylogenetic relationship. Bechly’s “epic myth” is exactly true.
Thorington, R. W. J., C. E. Schennum, L. A. Pappas, and D. Pitassy. 2005. The difficulties of identifying flying squirrels (Sciuridae: Pteromyini) in the fossil record. J. Vertebr. Paleontol. 25:950–961. DOI: 10.1671/0272-4634(2005)025[0950:TDOIFS]2.0.CO;2