Convergence part 7: convergence in Volvox

Last year I wrote a series of posts on convergent evolution and misrepresentations of the history of the concept by proponents of intelligent design including Günter Bechly, Lee M. Spetner, Granville Sewell, and others. I didn’t intend for there to be a two-month gap before the final installment (nor am I sure this is the final installment), but here we are.

To quickly recap, I showed in the first three installments that

The Discovery Institute is producing a revisionist history. To hear them tell it, convergence is something that evolutionary biologists have either barely heard of or that they “invented” or “made up” to hide problems with the tree of life. Convergence “destroys the tree of life,” it “contradict[s] the [modern synthesis],” and it is “quite unexpected” to evolutionary biologists. All of that is a big, stinking pile of wrong. In reality, biologists since Darwin, and including Darwin, have always appreciated the importance of convergence, have written thousands of papers about it, and have included it in every evolutionary biology textbook I’m aware of.

I explained why the argument that convergence is evidence against common descent is daft, and I gave a spectacular example of convergent (or parallel) recruitment of life cycle genes in plants and brown algae. I also promised that I would write about convergent evolution in Volvox, which I have so far failed to do.

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Convergence part 6: the deepest of deep homologies

You are more closely related to a mushroom than a kelp is to a plant. It’s strange to think about, but it’s true. Kelps seem very plant-like, with their root-like holdfasts, stalk-like stipes, and leaf-like blades. But kelps are brown algae, part of the stramenopile (or heterokont) lineage of eukaryotes, which are very distant from the land plants and their green algal relatives, all of which are within the archaeplastida (the direct descendants of the primary origin of chloroplasts). Mushrooms (fungi) and humans (animals), on the other hand, are both opisthokonts, practically cousins at the scale we’re talking about.

Pawlowski 2013 Fig. 1

Figure 1 from Pawlowski 2013. Deep phylogeny of eukaryotes showing the position of small eukaryotic lineages that branch outside the seven supergroups (modified after Burki et al. 2009; drawings S Chraiti). You are represented by a fish, which at this scale you might as well be.

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Convergence part 4: “an epic myth”

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 onetwo, 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

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Convergence part 3: “the Darwinists’ lollapalooza”

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.

Phylogenetic tree from The Origin of Species

Not the first tree of life. The one figure from The Origin of Species. By Charles Darwin [Public domain], via Wikimedia Commons.

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

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Convergence part 2: “exceptions to the rule”

In part 1, I argued that some advocates of intelligent design give a misleading picture of the history of evolutionary thought on the topic of convergence. To hear them tell it, convergence, or at least convergence as a widespread phenomenon is a recent discovery, unknown to Darwin and to the architects of the modern synthesis. For example, Günter Bechly says,

One of the most essential doctrines of Darwinian evolution, apart from universal common descent with modification, is the notion that complex similarities indicate homology and are ordered in a congruent nested pattern that facilitates the hierarchical classification of life. When this pattern is disrupted by incongruent evidence, such conflicting evidence is readily explained away as homoplasies with ad hoc explanations like underlying apomorphies (parallelisms), secondary reductions, evolutionary convergences, long branch attraction, and incomplete lineage sorting.

When I studied in the 1980s at the University of Tübingen, where the founder of phylogenetic systematics, Professor Willi Hennig, was teaching a first generation of cladists, we still all thought that such homoplasies are the exceptions to the rule, usually restricted to simple or poorly known characters. Since then the situation has profoundly changed. Homoplasy is now recognized as a ubiquitous phenomenon (e.g., eyes evolved 45 times independently, and bioluminiscence 27 times; hundreds of more examples can be found at Cambridge University’s “Map of Life” website).

I don’t know who gave Dr. Bechly the idea that homoplasies are rare, but I’m pretty sure it wasn’t Willi Hennig. Dr. Bechly was there, and I wasn’t, but I’m going to go out on a limb here anyway and say that Willi Hennig wasn’t even at the University of Tübingen in the 1980s. I can be fairly confident that this is the case, because Willi Hennig died in 1976.

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Convergence part 1: “quite unexpected”

A number of advocates of intelligent design have written variations on the theme that convergence is a problem for evolution. I aim to show why this argument is daft.

First of all, what is convergence? Definitions differ, and I’m not going to get into an extended discussion of the differences. A definition that will serve well enough is Anurag Agrawal’s, “the independent evolution of similar phenotypes.” A phenotype, and this will be important, can refer to a single trait, multiple traits, or the entire set of traits expressed by an organism. Green-eyed is a phenotype. A calico pattern of fur color is a phenotype. All of the traits that make up a particular cat are also a phenotype. A phenotype can describe a trait (or set of traits) of an individual or of a species, so just as being 5’10” tall is a phenotype, so is being bipedal.

Convergence typically refers to the latter kind of phenotype, those that characterize a species. So if, for example, seasonal changes in coat color have independently evolved in a bird, a lagomorph, a mustelid, and a canid, that’s an example of convergence of a single trait.

Zimova 2018 Fig 1

Figure 1 from Zimova et al. 2018. Seasonal coat colour species in their winter (top row) and summer (bottom row) coats. (A) Rock ptarmigan; (B) mountain hare; (C) stoat; (D) Arctic fox. None of these species share a common ancestor with seasonal coat polymorphism; they evolved it convergently. Photos by stock.adobe.com: Pilipenko D, Paul Carpenter, Stephan Morris, Diego Cottino; Mills lab research photo, and Seoyun Choi.

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