Tuesday, July 17, 2012

Thoughts and reflections about the nature of selection from a NESCent-meeting in North Carolina

Posted by Erik Svensson

I just returned from a three-day working group meeting at "National Evolutionary Synthesis Centre" (NESCent) in Durham (North Carolina). The working group's topic is "Environmental and demographic determinants of natural selection", and there were 18 participants from the Australia, Canada, Sweden, the US and UK, i. e. a very multantional and scientifically diverse crowd, some of which you see on the pictures above and who you might recognize already. The organizers of this working group are Andrew MacColl, Adam Siepielski, Stephanie Carlson and Tim Coulson, four colleagues whose work I did know very well since before, but this was actually the first time we met IRL.

This working group will perform a meta-analysis and litterature synthesis on a favourite topic of mine: the ecological causes of natural and sexual selection on phenotypic traits in natural populations. I have been interested in this topic since my postdoc, especially the role of density-dependent natural selection on egg size, but also in later years when Tom Gosden and I studied spatial variation in sexual selection and its ecological consequences in the damselfly Ischnura elegans

 In spite of several decades of excellent selection studies in natural populations (summarized in a large meta-analysis ten years ago by Joel Kingsolver and colleagues), our knowledge about ecological agents (predators? competitors? parasites? climate?) are still very limited and far from quantitative. This is unfortunate, as natural selection is an ecological process, that might (or might not!) lead to evolutionary change. In fact, and to cite the great population geneticist Ronald Fisher in the famous opening sentence of his landmark volume The Genetical Theory of Natural Selection (published in 1930): "Natural selection is not evolution". Natural selection might lead to evolution - if the traits under selection are heritable and some other conditions are met - but need not to do so, and frequently does not.

Natural selection can for instance also reduce population size and cause extinction - and how often this is the case in nature we do not know. Nor is all evolutionary change due to natural selection - only extreme adaptationists would claim this. Much of evolutionary change can be due to - for instance - genetic drift, gene flow, recombination or simply neutral changes (for non-coding DNA). Thus, evolution and natural selection should not be confused - and one should not forget that one can study natural selection without any knowledge about genetics or the underlying genetic basis of traits under selection (Darwin is a good example of an evolutionist who knew more about natural selection than many molecular geneticists do today).

In this working group, we will search the litterature and add to already existing databases information about selective agents to those published studies where selection on phenotypic traits have been quantified already. Questions that can then be adressed include: How much of the variation in selection gradients can be explained by various environmental factors? What is the role of climatic factors and other abiotic factors vs. biotic factors like predation, competition, food supply or parasites? Do abiotic factors like climate and biotic factors interact - and could they even reinforce each other? We take up the challenge put forward by Michael Wade and Susan Kalisz in their important conceptual paper "The causes of natural selection" from 1990.

The questions are endless - and we know very little about the answers, but they are likely to be important. It is interesting that there is so much to do in this field and in the study of the ecology of selection in the "postgenomic" era, when we realize that the molecular revolution did not, and will not, solve all problems in evolutionary biology, and certainly not these ecological questions about the nature of selection on quantitative traits (whose genetic background is often due to many loci of small effects, resulting in the "missing heritability" problem).

The radical conclusion remains however: we do not need to know anything about neither quantative genetics or molecular genetics to study the ecological causes of selection - as this a question at an entirely different level of organization and  the ecological process of selection does not have anything to do with genetics, as so elegantly clarified and formalized by Lande and Arnold in their classical 1983-paper.

I am happy to have the opportunity to participate in a working group like this, and also impressed by a facility as NESCent - where synthetic work of this kind is funded - to the benefit of not only US scientists but also scientists from other countries. Hopefully, this working group will find some interesting new patterns and move the field forward - and perhaps help us to improve the empirical study of selection. I have already learned a lot - and hope to learn even more in the future. And that, after all, is the goal of science, not only publishing papers - which is of course also nice - but which is only one way of many others to communicate science. This blog post has hopefully also inspired and informed somewhat in that respect.

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