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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Jonathan Wells debunks something nobody believes

Black bear

Black bear, Glacier National Park, September 2014.

Charles Darwin speculated that whales might have evolved from bears. He was wrong, but then he didn’t have the benefit of molecular sequence data, detailed morphological comparisons, and sophisticated methods of phylogenetic inference. We’ve known for at least 50 years that cetaceans (whales, dolphins, and porpoises) are most closely related to ungulates, specifically even-toed ungulates (artiodactyls). The current consensus is that the closest living relatives of cetaceans are hippopotamuses. Not everyone agrees with this specific relationship, but no one really doubts that whales are closely related to ungulates.

You wouldn’t learn that from reading Discovery Institute Senior Fellow Jonathan Wells’ recent post, “From Bears to Whales: A Difficult Transition.”

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Uncommon Descent on Elizabeth Pennisi’s Science article

Two-headed quarter

Image from www.twoheadedquarter.net.

Yesterday, I ran a bit long about Elizabeth Pennisi’s new article in Science, “The momentous transition to multicellular life may not have been so hard after all.” I’m not the only one who noticed it, though; Uncommon Descent also commented (“At Science: Maybe the transition from single cells to multicellular life wasn’t that hard?“). There’s not much to it, just a longish quote from the article followed by this:

So at the basic level, there is a program that adapts single cells to multicellularity? Yes, that certainly makes multicellularity easier and even swifter but it also make traditional Darwinian explanations sound ever more stretched.

So if the evolution of multicellularity is easy, that’s evidence against “traditional Darwinian explanations.” Remember “Heads I win, tails you lose“?

…if multicellularity is really complicated, that’s evidence for intelligent design. But if multicellularity is really simple, that’s evidence for intelligent design.

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Multicellularity in Science

I spent the last week of June backpacking in Baxter State Park, Maine. When I finally emerged from the woods, my first stop was Shin Pond Village for a pay shower, a non-rehydrated breakfast, and free internet access. Among the week’s worth of unread emails were a nice surprise and a not-so-nice surprise. The not-so-nice surprise was a manuscript rejected without review; the nice surprise was a new article by Elizabeth Pennisi in Science, which came out when I was somewhere between Upper South Branch Pond and Webster Outlet.

Upper South Branch Pond

Upper South Branch Pond, Baxter State Park, Maine. I spent two nights here.

The article, for which I was interviewed before Baxter, synthesizes recent work across a wide range of organisms that suggests that the evolution of multicellularity may not be as difficult a step as we often assume:

The evolutionary histories of some groups of organisms record repeated transitions from single-celled to multicellular forms, suggesting the hurdles could not have been so high. Genetic comparisons between simple multicellular organisms and their single-celled relatives have revealed that much of the molecular equipment needed for cells to band together and coordinate their activities may have been in place well before multicellularity evolved. And clever experiments have shown that in the test tube, single-celled life can evolve the beginnings of multicellularity in just a few hundred generations—an evolutionary instant.

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The Essential Tension

The Essential Tension

When I ran across The Essential Tension by Sonya Bahar, my first thought was that it sounded very much like something my PhD advisor could have written:

‘The Essential Tension’ explores how agents that naturally compete come to act together as a group. The author argues that the controversial concept of multilevel selection is essential to biological evolution, a proposition set to stimulate new debate.

The subtitle is Competition, Cooperation and Multilevel Selection in Evolution, which is more than vaguely reminiscent of the ‘cooperation and conflict’ framework Rick Michod has built over the last twenty years.

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Evolution of outcrossing and selfing

Sex is costly. You could die trying to find a mate. Your mate could kill you, or give you a disease. You could be unable to find a mate in the first place, in which case you’d be better off if you could reproduce asexually. Even without those risks, though, even in a simple genetic simulation, sexual reproduction means you only pass on half of your genes to your offspring.

So why do it? We know that it’s possible to reproduce without sex; lots of things do. It’s not just bacteria and protists, either: asexual reproduction occurs in some plants, insects, snails, amphibians, and reptiles, among many others. The logic of natural selection suggests that sex must confer some benefit that outweighs all the costs, at least in some situations. Essentially all of the proposed benefits of sex have to do with outcrossing, or mixing your genes with those of another, genetically distinct, individual.

Nevertheless, a lot of things that reproduce sexually do so without outcrossing. This is especially common in plants, where it’s called “self-pollination” or just “selfing.” Selfing is thought to provide short-term advantages relative to outcrossing–basically by avoiding the costs I’ve listed above. However, selfing also doesn’t provide most of the benefits associated with sex, so it’s thought to be a bad strategy in the long term. This leads to selfing being thought of as a “dead-end” strategy: the short-term advantages make it unlikely that a selfing species will return to outcrossing, and the reduced genetic variation produced by selfing make diversification less likely.

Erik Hanschen and colleagues have tested these predictions in the volvocine algae (I’m among the “colleagues,” as are John Wiens, Hisayoshi Nozaki, and Rick Michod): do selfing species ever return to outcrossing, and do they have a lower rate of diversification than outcrossing species? Both mating systems exist within the volvocine algae, and so they make a good test case. Roughly speaking, the term heterothallic refers to outcrossing species and homothallic to selfing species:

Hanschen et al. Fig. 1

Figure 1 from Hanschen et al. 2017. Diversity of mating systems in the volvocine green algae and their respective life cycles. (A) In outcrossing (heterothallic) species, distinct genotypes (male on left and female on right) sexually differentiate producing either eggs or sperm. A diploid zygospore (red) is produced after fertilization. Sexual offspring hatch and enter the haploid, asexual phase of the life cycle. (B) In selfing (homothallic) monoecious species, a single genotype is capable of producing both gamete types. Upon sexual differentiation, each sexual colony produces both sperm and eggs. (C) In selfing (homothallic) dioecious species, a single genotype sexually differentiates, producing either eggs or sperm, but not both within the same colony. Cartoons in panels (A–C) are shown with anisogamous, Volvox-like morphology for illustrative purposes only.

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Nice Aeon article on biological individuality

Siphonophores by Ernst Haeckel

By Ernst Haeckel – Kunstformen der Natur (1904), plate 17: Siphonophorae (see here, here and here), Public Domain, Link

Derek Skillings from University of Bordeaux/CNRS has a new article at Aeon about biological individuality:

For millennia, naturalists and philosophers have struggled to define the most fundamental units of living systems and to delimit the precise boundaries of the organisms that inhabit our planet. This difficulty is partly a product of the search for a singular theory that can be used to carve up all of the living world at its joints.

Skillings reviews the deep historical roots of the question, touching on the views of Charles Darwin and his grandfather, both Huxleys (T. H. and Julian), Herbert Spencer, and other 19th and early 20th century thinkers, as well as some more recent authors, including Daniel Janzen and Peter Godfrey Smith.

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