But evolution converges!

Stephen Jay Gould became famous in part for his book Wonderful Life: The Burgess Shale and the Nature of History. By examining the strange creatures in the Burgess Shale formation Gould makes the case that evolution is a highly contingent process, and that if you reran the experiment of life what we’d see might be very different from what we have now.

But the scientist whose study of the formation that inspired Gould’s interpretation, Simon Conway Morris, had very different views. Though it can sometimes be churlish, his rebuttal can be found in The Crucible of Creation: The Burgess Shale and the Rise of Animals. Simony Conway Morris does not believe that contingency is nearly as powerful a force as Gould would have you believe. And his viewpoints are influential. Richard Dawkins leaned on him to make the case for convergence in evolution in The Ancestor’s Tale.

This crossed my mind when reading Carl Zimmer’s new column, When Dinosaurs Ruled the Earth, Mammals Took to the Skies:

Today, placental mammals like flying squirrels and marsupials like sugar gliders travel through the air from tree to tree. But Volaticotherium belonged to a different lineage and independently evolved the ability to glide.

They were not the only mammals to do so, it turns out. Dr. Luo and his colleagues have now discovered at least two other species of gliding mammals from China, which they described in the journal Nature.

Dr. Meng said that the growing number of fossil gliders showed that many different kinds of mammals followed the same evolutionary path. “They did their own experiments,” he said.

This ultimately comes down to physics. There are only so many ways you can make an organize that flies or glides. Mammals come to the table with a general body plan, and that can be modified only so many different ways.

This is not a foolproof point of datum in favor of convergence as opposed to contingency. Frankly these are often vague verbal arguments which are hard to refute or confirm. And even molecular evolutionary analyses come to different conclusions. It may be that we are asking the wrong question. But, it does suggest that evolution may work in a much narrower range of parameters as time progresses because of the winnowing power of selection.

9 thoughts on “But evolution converges!

  1. I have a vague memory that a few years after Gould’s book came out, it was discovered (learned? determined? recognized?) that one of the key interpretations of the Burgess Shale fossils was based on a misunderstanding of one of them, that it had been analyzed upside down. further, that Gould had leaned heavily on the that interpretation so that this bit of revisionist natural history undermined the basis of the argument he had constructed. That’s not to say that the argument could not, perhaps, be reconstructed on other evidence but that the evidence Gould used turned out to be faulty.

    Is this (my recollection) wrong? Not that it would be the first time …

  2. You’re probably thinking of Huallucigenia. From wikipedia on H.:

    Hallucigenia is a genus of Cambrian xenusiids known from articulated fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world.[3] Its quirky name reflects its unusual appearance and eccentric history of study; when it was erected as a genus, the animal was reconstructed upside down and back to front. Hallucigenia is now recognized as a “lobopodian worm”. It is considered by some to represent an early ancestor of the living velvet worms, although other researchers favour a relationship closer to arthropods.

    From wikipedia on Wonderful Life (book):

    Some of the anatomical reconstructions cited by Gould were soon challenged as being incorrect, most notably Simon Conway Morris’ 1977 reconstruction of Hallucigenia.[16] Conway Morris’ reconstruction was, “so peculiar, so hard to imagine as an efficiently working beast” Gould speculated that Hallucigenia might be “a complex appendage of a larger creature, still undiscovered.”[17] It was later brought to light by paleontologists Lars Ramskold[18] and Hou Xianguang[19] that Conway Morris’ reconstruction was inverted upside down, and likely belonged to the modern phylum Onychophora.[20]

    There’s been some work since:


  3. I got a sugar glider in the house last year. Thought it was a chipmunk at first but, boy, those critters can jump as well as fly. He was way high on the walls.

  4. WRT the typical examples given, I only ever see three different types of convergent evolution:

    1) Simple and clear engineering solutions (eg. the camera eye design, coiled shells)

    2) Streamlining (dolphins/tuna/sharks)

    3) Convergence within closely related taxa (the flying squirrel example mentioned above).

    It seems that whenever you have distantly related creatures filling the same niche, they always have rather different forms (excluding types 1 and 2). So, eg. a praying mantis and a chameleon arguably occupy the same niche (arboreal, visually-oriented predators that rely on camouflage to ambush their prey), yet they look nothing alike. Are there any examples of convergence between radically unrelated creatures (eg. arthropod vs vertebrate, but again, excluding 1 and 2)?

  5. Jamie,

    I think it’s a mistake to narrowly define convergent evolution solely on bauplan, although that is of course the most obvious. Convergent evolution can also encompass behavioral traits, chemical pathways, and trophic changes. To give some general examples, venom, bioluminescence, eusociality, biomineralization, parental care, and even multi-cellularity are examples of multiple lineages developing convergent traits.

  6. “Are there any examples of convergence between radically unrelated creatures (eg. arthropod vs vertebrate, but again, excluding 1 and 2)?” Yes. 1) coral reef fishes that swim with their pectoral fins have fins that have converged on the shape and movement pattern of hawk moths and hummingbird, 2) many very small animals that swim or fly with oscillating appendages in a certain fluid-dyanmic window (Reynolds number of around .1-10) have hairy limbs/wings including aquatic beetles, flying beetles, flying wasps, aquatic crustaceans like Daphnia, 3) eel-like convergence in many groups of fishes but also several groups of aquatic salamanders, 4) worm/snake like convergence in many groups of annelids, amphibians (caecillians), reptiles (snakes, glass-lizards), 5) succulents, etc. etc.

  7. Karl: I would tend to think that some of the examples you give fall into my type 1) (Simple and clear engineering solutions). But I do take your point. It just seems to me that talk of convergent evolution is often used to imply that aliens (or a replay of the tape of life on Earth) would result in mammals and birds etc.

    Jeff: Wing/fin shapes seem to fall under my type 2 (Streamlining), while vermiform and eel-like body plans might be both type 1 and 2.

    Thank you both for the responses. I’m now off to see the eclipse!

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