Walking, galloping, and sauntering towards genetic differentiation

“This validates, at a major scale (across all vertebrates), what a handful of studies have found within narrow taxonomic groups…”

My citation manager has a special folder—elegantly named “TEACHING??”—where papers get stored for eventual use in a classroom. These papers tend to have a similar set of characteristics that unite them, mainly that they illustrate broad concepts simply and interestingly. Depending on how much foundational knowledge you have on a subject, your reaction to reading these papers can/should fall between “Oh, that makes sense” to “Ah, I get it now” to “Well, yeah, of course”.
A new addition to my “TEACHING?”** folder was this paper by Iliana Medina and colleagues, “Walk, swim or fly? Locomotor mode predicts genetic differentiation in vertebrates.”
The idea? The number of species and the rate at which they’ve appeared is far greater for organisms on land compared to those in water. But here on Earth, there is a much larger amount of water in which to diversify compared to terrestrial environments. So what gives?
One of the primary hypotheses that explain this phenomenon relies on how much resistance terrestrial and aquatic species undergo when moving around. Picture the classic model of allopatric speciation: a species exists within a certain range, a barrier (picture a long, transparent wall made by humans) appears that separates what was one range into two or more, and the separated populations diverge from one another. These barriers are more likely on land (mountains, rivers, etc) compared to the sky and ocean.
Medina et al. tested this hypothesis by gathering two key pieces of data from almost 500 published studies on geographic patterns of vertebrates: a common metric of genetic variance among populations of each species (Fst) and the mode of locomotion of each species. They used these data to look for statistical associations between locomotion type and genetic differentiation while taking into account phylogenetic relationships, molecular data type, and geographic scale.
As you might expect, locomotion is significantly associated with genetic differentiation. Vertebrates that move terrestrially (mostly mammals and reptiles in this study) show greater differentiation between populations at smaller scales than vertebrates that fly or swim. This pattern even holds when looking at mammals alone, where all three modes of reproduction are well-represented.

Figure 1 from Medina et al. (2018). On the left, Phylogenetic relationships between 327 vertebrate species, colored by their mode of locomotion (green = walk, blue = swim, red = fly). On the right, a visualization of the association between genetic structure (measured with either microsatellite or mitochondrial data) and geographic distance across the three type of locomotion. 

Importantly, one big caveat here is that Fst is not a direct genetic measurement of dispersal. Therefore, there is no doubt that other life history traits are influencing the patterns of genetic variation among populations of all these diverse vertebrates. However, on this scale, locomotion is certainly a powerful part of explaining why diversification happens differently by land:

This link implicitly underlies the assumption that a greater variety of geographical features can act as barriers to terrestrial dispersal, resulting in a higher incidence of reproductive isolation among populations on land. This idea has been used to explain why speciation rates are higher on land than in the sea…..Our study also clarifies the idea that it is not gross environment type per se – land vs. sea – that is key in affecting dispersal ability, rather it is the mode of locomotion used by species.

You might also notice a major group of vertebrates are absent from that tree, the amphibians. Because amphibians often split their lives between aquatic and terrestrial locomotion, I can imagine how difficult it would be to classify them in an analysis like this. However, it would have still been interesting to see how they compare to their vertebrate relatives. We’ll just have to see if future students catch that omission as well.
Medina, Iliana, Georgina M. Cooke, and Terry J. Ord. “Walk, swim or fly? Locomotor mode predicts genetic differentiation in vertebrates.” Ecology Letters (2018). https://doi.org/10.1111/ele.12930
**Maybe I should ask Kathryn to provide a sequel to her sampling naming post, “Best practices in folder management”?

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