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 glider (left) and southern flying squirrel (right). By Alessandro Di Grazia (http://www.petsugargliders.eu CC BY-SA 4.0) and Ken Thomas (KenThomas.us [Public domain]), respectively, both via Wikimedia Commons.
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.
Stable links:
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
Good clear summary. However even if those asserting convergence falsifies evolution had taken a comparative annotomy class I don’t think it would make any difference as their reasoning is too motivated by the need to disprove evolution. Yo see exactly the same process in the way they dismiss the evidence from fossils. Never the less it is useful to have the evidence laid out for those who are uncertain.
Maybe I should have said anyone who doesn’t realize that many more traits are shared within those groups than among them failed to understand comparative anatomy.
Above and beyond the points you make, it seems highly disingenuous to claim that convergence somehow falsifies the evolutionary model.
The whole point of evolution is that it tends to produce organisms that are well-adapted to their environment. So when we see organisms with different evolutionary histories occupying similar niches, it’s absolutely a prediction of evolution that they can and will have phenotypic similarities.
(Forgive me if you made this point in your earlier posts in this series.)
You’re right, and I haven’t made that point, just to maintain focus on this particular line of argument.
I’ve read somewhere (sorry, can’t recall where) that early geologists tried to develop a hierarchic classification of rocks and minerals on the model of the Linnean taxonomy. A reasonable thing to try, since they had no idea why this approach worked in biology, but – of course – the attempts all failed, because rocks and minerals are not phylogenetically related.
Linnaeus included minerals in his classification scheme. As you say, it didn’t exactly catch on.
I’ve been thinking about ways of classifying similarities and differences between organisms as being “deep” or “superficial”. I would have thought this was a solved issue, but I guess it can be difficult to express. So, something like this:
We would expect the reproductive system — which means the gonads, the genitals, the karyotype; everything that organisms reproduce with — to be deeply similar between related organisms, and less so with more distantly related organisms. So canids, for example, can have wildly different superficial differences (like limb length, muzzle length, coat type, etc), but have reproductive systems so similar that even more distantly related species, like dogs and coyotes, can mate and have offspring. The reproductive systems of canids and felids are not similar enough for mating, but they are more similar to each other as placental mammals than they are to those of marsupials. Given that context, the similarities between dogs/wolves and thylacines are utterly superficial (limb and muzzle conformation); the differences between reproductive systems are very deep.
Of course, the deepest similarities and differences turn out to be in the genes; the units of inheritance.
So maybe the way to phrase the matter as a rebuttal is something along the lines of “There is nothing about evolution that says that distantly related species cannot evolve superficially similar traits.”
Your thoughts have converged on those of many important thinkers. Biologists were arguing about which traits most reliably indicated ‘affinity’ in ‘natural’ classifications long before these were interpreted as phylogenetic relationships. Here’s Darwin, pp. 365-366 of the 6th edition of the Origin:
A distinction between reproductive and other traits as reliable indicators is not universal. While among plants flower and fruit characters tend to be more reliable than vegetative characters, on the other hand there’s some convergence in flower morphology (pollination syndromes – shared adaptations for particular pollinators), and divergence in orchid floral morphology in pursuit of reduced interspecies cross-pollination, and on the other hand leaf-venation patterns are a lot more conserved than leaf shape. (American lindens were grossly oversplit based on variability in leaf form, but if you look closely enough you find that you get much of that variation even on a single plant.)
Darwin comments somewhere that the characters which are taxonomically significant are different in different groups – if I recall correctly to the effect that what is a generic character (suitable for distinguishing genera) in one group in another group may be variable within a single species.
On the one hand the early cladistic practice of treating each trait as having equal weight is obviously wrong (an approximation) if you intend a cladistic analysis to reproduce relationships, but on the other hand how do you avoid the risk of bias if you try to identify better weightings.
While I am glad to see that I am not completely out to lunch in my thinking, I am bothered by the fact that I don’t recall it being emphasized in education and outreach. Have I just missed seeing it?
Or is it actually the case that the question of superficial vs deeply fundamental similarity is so complex and controversial that other biological educators prefer to avoid the matter for the sake of simplicity, and my paragraph above is too oversimplified?
One way to look at it is that if you classify traits coarsely you find convergence, but if you classify traits finely the convergence goes away. For example bat, pterosaurs and birds all elongate limb bones to support wings, but the way elongation is partitioned among bones is different, and the wing surfaces are skin in the first two groups and feathers in birds. At the coarse level of wings insect wings are also convergent, but they have very different structures and origins.
For morphology you can address the problem of distinguishing superficial versus fundamental similarities by chopping traits more finely in both quality and quantity. This doesn’t work so well with DNA, but you can suppress the problems by hitting them with the double hammer of more loci and more taxa. (More loci alone doesn’t always work – if I recall correctly this is known as the Felsenstein zone.)
I’m afraid a reliable answer would be too far outside of my expertise. When I first started learning about phylogenetics (in the early 2000s), genetic data were already the norm, though trait-based phylogenies would still show up now and then. My impression is that DNA- or amino acid-based phylogenies require fewer arbitrary decisions (of the sort that another Stewart refers to): how finely should one dice up the available traits? In addition, the question of how different traits should be weighted is usually dealt with quantitatively and empirically in likelihood/Bayesian phylogenetic inference, in the form of the inferred substitution rate model; this might partly explain why this debate isn’t as prominent today.
I was thinking about what I wrote, and it occurred to me that what I was calling “deep” or “fundamental” traits are what are more usually called “primitive” in zoological jargon, and “superficial” traits are more usually called “derived”. And looking back at your post, I think your phrasing of traits that individual members “share” (between different species, or different members of other orders) also tries to capture that same sense of complexes or clusters of traits that are highly similar within subgroups, and highly different between members in outgroups. another Stewart’s “fine” and “coarse” also tries to emphasize that, but I think may be trickier wording.
Another idea that occurred to me for communicating the overall concept more easily, maybe, is to emphasize how at least some of the convergent traits are the result of fairly simple patterns of development and growth. Cells receiving growth signals mean that the tissues those cells are in can just grow more, so limb bones can just grow longer with more growth hormone signaling, or skin can continue growing into flaps. A simple demonstration might be something like showing pictures of humans with congenitally short limbs, and with very long limbs, with the obvious question of whether the trait of short limbs actually makes those more related to animals with short limbs than to the other humans with longer limbs. Or something like that.
Just thinking out loud about science communication.