Spatially varying selection shapes life history clines among populations of Drosophila melanogaster from sub-Saharan Africa

Fabian et al. (2015). Journal of Evolutionary Biology. Spatially varying selection shapes life history clines among populations of Drosophila melanogaster from sub-Saharan Africa

Various kinds of Drosophila melanogaster mutants. I don't think any of these show clinal variation across Africa!

Various kinds of Drosophila melanogaster mutants. I don’t think any of these show clinal variation across Africa… Taken from D’Avila et al. (2008)

Lynsey McInnes

Lynsey Bunnefeld

I picked this paper on a whim as it looked like it dealt in genetics and ecology, but not in the phylogeographic sense that I am usually drawn to. I liked it a lot, mostly for the approach it outlined rather than any specific results.

In brief, the authors are looking to see if they find evidence for adaptive differentiation in life history traits among tropical populations of Drosophila melanogaster as a function of altitude or longitude, arguing that such clines are seen as a function of latitude, and longitude, and particularly altitude, could be considered parallel gradients in environmental conditions. Indeed, they purport to find evidence for this differentiation.

I had some doubts about aspects of their methodology, particularly the mismatch in the genetic resources they assign to each population (i.e., they do not come from the populations they sampled for life history variation), but I am also happy to believe that irrespective of some potentially dodgy leaps of faith, the results they uncover do reflect reality.

And so, I really enjoyed this paper. My brain is still wired in as a macroecologist and any paper I read that tried to tackle some of the many assumptions of macroecological analyses is an impressive one to me. Here, the authors have taken a small(ish) dataset from a band of tropical populations and measured a ton of stuff in order to test a specific hypothesis on differentiation expected as a function of a geographic cline. Sounds so simple, but is not really that common in macroecology despite its concern with spatial diversity patterns. Oops.

And then you come to the next macroecological qualm. OK, Drosophila melanogaster is a wide-ranging species, so intraspecific variation is to be expected, but so are many other species and it is not so often (although things are definitely improving) that macro scale studies consider this intraspecific variation in life history or ecological or behavioural traits. We need more of this!

One could argue that the scale that macroecology operates on means that this kind of variation is not important, that it gets swamped by interspecific variation, but I doubt it. Because, on the flip side, there IS a general consensus that processes acting at multiple scales matter to understand species diversity patterns, so finer (and conversely broader) scales than species-as-unit analyses are relevant.
Don’t worry, I don’t think I am the only person to think this way, I am just still in the beating myself up about how late these things have dawned on me phase.
So, what to do? We probably need more sampling, more genetic resources, more models and better formulation of testable hypotheses. But the hardest thing will be (at least for me) accepting that really interesting insights can be made using model systems or subsets of a taxonomic group, gone is the possibility to use ‘all mammals’, ‘all birds’, etc. (Yes, I am that kind of dirty charismatic vertebrate macro person).  Perhaps general patterns/rules of thumb will emerge quite quickly, along the lines of you need a range of this size to show X and a body size of this size to show Y, or you need to live in environment A to show Z. And then the really interesting part will be piecing together how well intraspecific diversity patterns might predict speciation or extinction probabilities.
Interesting times.

Will Pearse

It’s hard to argue with a paper that does exactly what it says on the tin. This is a nice demonstration of variation within a species across environmental gradients, and an excellent demonstration of how to set up a question and then just go right ahead and answer it.

I was struck by the lack of variation in viability despite the variation in what many people would call life history traits. It’s sobering to consider that there can be this much variation in how a species operates, and yet no general variation in something that’s quite an important component of fitness. Traits play out in their environmental context, and if there can be this much variation within a species we should all be a little more careful when interpreting the importance of very slight differences across species. Of course the authors get at this with their trade-off analysis, but for me (at any rate) it was a nice reminder. I liked that the authors linked all of this variation to climatic variables, but unless I missed something I didn’t see where they explicitly tested climatic factors vs. geographical summaries. I do buy their argument from parsimony that temperature (not altitude + latitude + longitude), and I imagine co-linearities made testing things difficult, but somehow I wanted to see it.

Speaking of variation, staring at the regressions and their oddly high r2 values (I’m becoming a pastiche of myself), I noticed that mixed effects models they used detect a lot of within-line variation. I’m not saying this as a criticism; rather, I think it’s incredible how they were able to partition this out so neatly. It really drives home the importance of variation within species (and populations!), and definitely got me thinking about how biased our measures of trait values are going to be if we can’t grow species in culture like the authors were able to do. I really do just take gene-environment interactions for granted, and completely ignore the micro-processes that Lynsey is now studying. Maybe we do need to start more explicitly linking micro-processes to the macro-ones that I tend to think about.


About will.pearse
Ecology / evolutionary biologist

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