Evolutionary transitions

tiktaalik-transitional-fossil

It’s very hard to find pictures of evolutionary transitions! Taken from transitional-fossil.com, which made me smile.

I’ve been thinking a little about transitions between discrete characters, in part because of the excellent corHMM package. corHMM is great for measuring the rate at which discrete characters transition from one to the other, but you can’t tell whether certain transitions tend to happen under certain conditions. For example, it’s one thing to know how frequently plants transition between dormant and non-dormant seeds, but another to know under what global temperatures that tends to happen.

In theory, that’s where transition.calc comes in. It takes a discrete (e.g., seed dormancy) and continuous (e.g., cold tolerance) character, reconstructs ancestral states and transitions, and then plots one against the other. I prefix all of this with “in theory” because:

  • It’s a very, very dirty hack, and I’m certain there’s an analytical solution for this. Moreover, I’m not convinced that taking the modal transition that happens along a branch and working with that will actually work.
  • I pulled the code together from some other scripts and I’m not convinced that it matches up the discrete and continuous reconstructions properly (seems important, right?…)
  • I haven’t validated the script with simulated data. To do that, I need to write a flexible way of defining models of evolution across a phylogeny (hopefully, that’s next fortnight’s post…)

…however, I do have some hope that this works because of the scripts I pilfered it from (…which have proven remarkably hard to generalise, thank you very much Past Will). I found what appear to be significant results in a seed dataset that weren’t there when I randomly evolved two separate traits across the phylogeny. That’s not to say I haven’t introduced another error somewhere, but it’s a good sign. I’m currently having a debate with co-authors as to whether there’s a difference between this and just regressing the continuous variable against the discrete; I’d be grateful for feedback!

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