(Not) all moles are created equal

Most people could probably identify the animals below:

Short stubby bodies, long powerful claws, regressed eyes, living underground: these are all moles. Aren’t they?

Counterintuitively, their DNA tells a different story. The mole at the top left is a true mole (Talpidae), a group of moles found in Eurasia and North America. Their DNA is not particularly similar to the other two moles. In fact, their DNA is much more similar to mammals like bats, whales and pandas.

The mole at the top right is a golden mole (Chrysochloridae), a group of mole-like mammals found exclusively in southern Africa. These “moles” are genetically more similar to a variety of mammals that hail from Africa, like tenrecs, aardvarks and elephants.

The “mole” at the bottom, munching on a centipede, is an Australian marsupial mole (Notoryctidae). Both species, like other marsupials, have a pouch in which they raise their young. You can probably tell where I’m going with this: they are more genetically similar to other marsupials than they are to the other “moles”.

So why is it that animals with striking physical similarities have such incredibly dissimilar DNA? Evolutionary theory predicts that unrelated species can appear very similar to one another if they adapt to very similar ways of life, a phenomenon called convergent evolution. If an animal becomes adapted for living underground, you can imagine that their eyes are likely to regress and their arms will become very good at digging. So even though their anatomy looks very mole-like, their DNA appears to tell the story of their ancestry.

Questions for Creationists

If God created all of these different types of moles, why is it that their DNA is so dissimilar? If DNA is the ‘blueprint’ to form an animal, and God created these ‘blueprints’ in all of these moles, shouldn’t their DNA be more similar to each other than to wildly different things like bats, elephants, and kangaroos?

References

1. Stanhope, M. J., Waddell, V. G., Madsen, O., De Jong, W., Hedges, S. B., Cleven, G. C., … & Springer, M. S. (1998). Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals. Proceedings of the National Academy of Sciences95(17), 9967-9972.

Photo credit

true molegolden mole, marsupial mole, bat, whale, panda, tenrec, aardvark, elephant, dunnart, koala, wallaby

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3 thoughts on “(Not) all moles are created equal

  1. Kyle K

    That’s a good question. But I have a question as well. Homologous structures are normally used as evidence of common descent correct? Indeed it has been one of the foundational arguments for evolution since the time of Darwin. Yet if convergent evolution is a real thing, and, therefore, common ancestry isn’t necessary in order to account for homologous structures, then shouldn’t homology no longer be considered an evidence of evolution? It doesn’t seem to me that you can have it both ways. If evolution can converge on homologous forms as specific as a mole on multiple occasions, then why assume common ancestry for, say, a whale and a bat simply because they each have five digits? Why not rather assume convergent evolution?

    1. Hi Kyle K: excellent point! This very issue has been the cause of many debates between anatomists/paleontologists and molecular phylogeneticists. What I think it says is that individuals from the former camp have overextended their assumptions for what must be homologous structures, and through the study of DNA we’ve begun to realize how many of those alleged examples of homology are incorrect. I have numerous examples posted here: https://evolutionforskeptics.wordpress.com/category/molecular-phylogenetics-2/

      However, many other putatively homologous structures have withstood the test of time and this is based on 1) their reaffirmation from molecular phylogenetics, 2) in-depth anatomical analyses, and 3) developmental studies.

      For category 1, I’ll use your example of the presence of digits in vertebrates. If the five digits in a human and a bat and a whale are homologous, we should expect that molecular phylogenetic analyses would corroborate this. In fact, every vertebrate that has digits, including humans, whales, bats as well as crocodiles, frogs and lizards, is more genetically similar to each other than they are to fishes, which of course have no digits. This is consistent with the fossil record, which shows that when fishy-froggy-looking things that had digits appeared in the fossil record (e.g., Ichthyostega, Tiktaalik), it suddenly gave way to numerous lineages of vertebrates appearing in the fossil record that also had digits. This is suggestive that an early animal acquired the evolutionarily novel trait of digits that was inherited by its descendants. However, these digits appear to have been modified in many ways throughout history. Many things have kept the five digits that we think were ancestral, but some have reduced their number (e.g., horses, deer, birds, sloths), a few increased their number (e.g., the fossil Acanthostega), and some have lost them all together (e.g., snakes, other legless lizards, caecilians). It’s in these examples that homology gets tricky. For example, does the backwards facing toe on birds correspond to the big toe or pinky toe of a human? What about the single digit hoof of a horse? This is where the fossil record and especially developmental biology can illuminate us with a probable answer, but in some cases there’s a good chance that the homology will be impossible to determine.

      For category 2, I’ll give the example of wings in vertebrates. Three groups of vertebrates (though a possible fourth was recently found) have or had wings: birds, bats and pterosaurs. One might say that these wings are homologous because, well, they’re wings and few things can fly! However, upon closer inspection, one sees many differences that separate these groups. Birds have extremely reduced arms and digits (lots of bone loss) that make the bony support for the wing, with posteriorly-facing feathers providing the flight surface. Bats have intact arm bones and digits, but they appear to have stretched out their digits and use a skin membrane (patagium) as a flight surface. The membrane also extends to the hindlimbs, which does not occur in birds. In pterosaurs, it appears they stretched out their ring finger (how do we know it’s that particular finger? Another homology question!) and similarly had a skin membrane from their arms to their hindlimbs. So just by looking at these structures in detail it seems apparent that these “wings” do not have the same underlying structure (i.e., not likely homologous). Additionally, looking at estimates of phylogeny based on anatomy (for pterosaurs) and molecules (birds and bats), it appears quite clear that these three groups are not closely related and therefore it is most simple to assume that they acquired the novelty of flight independently. But when looking at birds as a whole, and comparing, say, the wing of a sparrow and that of a flightless penguin or ostrich, the issue of homology comes up once again. While there are indeed differences, the general structure of these birds’ wings are very similar to one another and not so similar to bats or pterosaurs. Add in the fact that DNA and various other aspects of anatomy unite birds, it suggests that the bird flavor of flight, and therefore wings, was present in their common ancestor but modified to suit the various ecologies of different birds.

      For time’s sake, I won’t give a specific example from developmental biology, but I could gladly produce one if needed. Otherwise, I have a few examples here: https://evolutionforskeptics.wordpress.com/category/ontogeny-recapitulates-phylogeny-2/

      In short, the confluence of evidence provides weight to many structures in vastly different species as being homologous. However, I concede your point: convergent evolution weakens the concept of homology alone as an argumentative tool, and I can’t imagine that it would convince many skeptics. I still think that it is indeed evidence of evolution, or that at the very least it is consistent with evolution, but this weakness is partly why homology is not one of the concepts I discuss on this blog. Despite it being useful for introducing someone to the evidence for evolution, it is not, in and of itself, a thoroughly convincing topic (though I think developmental biology strongly augments homology-based arguments).

      I should add, however, that none of the classical examples of homology used to teach evolution, to my knowledge, has been rejected by evolutionary biologists. Though it would be very interesting to find out if there is!

  2. Pingback: Using DNA to determine your ultimate paternity: Why molecular phylogenetics matters – Evolution For Skeptics

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