Pitfalls in modeling the effects of climate change on genetic diversity

by Luigi Guarino on April 22, 2012

Don’t you just hate it when a striking message from an elegant model is complicated by, well, facts? I may have Nibbled a press release on a recent modeling study from Wageningen University. The crux of the results was that as species migrate north due to climate change, they shed diversity from the central, most diverse part of their distribution, which is bad for their ability to adapt.

Plant and animal species can lose their ability to adapt as a result of climate change. This is shown by research performed by Marleen Cobben with which she hopes to obtain her doctorate at Wageningen University (part of Wageningen UR) on 17 April 2012. Cobben used computer calculations to illustrate how the genetic base of plants and animals is seriously deteriorating due to climate change. The smaller genetic base makes species more vulnerable to problems such as diseases. Moreover, the fragmentation of landscapes and the loss of wildlife areas is accelerating this decline.

This was interesting to me because we routinely, and perhaps somewhat blindingly, these days say that climate change will lead to shifts in the distributions of species. Crop wild relatives, say. Shift that will absolutely require germplasm collecting and ex situ conservation. Nothing else will do. Forget about in situ, ex situ it must be. That’s because, when added up, these shifts in the distributions of individual species will result in profound alterations in the geographic patterns of species diversity. Some hotspots will disappear, some diversity-poor areas will be enriched. Difficult to plan in situ conservation under these conditions. Ergo, need to collect. Also, the distributional shifts required for a species to track the climate will in most cases surely be faster than the rate of migration of the species, leading inexorably to its extinction. Need to collect, and quick. I mean, what can a poor species do under climate change besides move or perish? Need to collect, I tell you.

Well, adapt, of course, that’s what it can do. And collecting is not going to help with that. Need to do in situ, maybe assisted migration, you clod.

So a study which suggests that climate change is likely to also result in a decrease in genetic diversity within species would seem to push the pendulum further towards ex situ. Without being able to delve into the particularities of the model, the results seemed plausible to me, assuming that the highest diversity was indeed found in the central part of the distribution. Genetic erosion ensues. Won’t be able to adapt. Need to collect!

I can’t remember if I did nibble it, but I certainly sent the link to the Crop Wild Relatives mailing list. And it elicited an interesting, skeptical reply from Prof. Jonathan Gressel of the Weizmann Institute of Science in Israel. The professor pointed to a possible mechanism by which climate change could conceivably increase genetic diversity.

Unfortunately it is common for modelers to to say that their research “shows” (in this case), demonstrates or even proves something. As a sometime modeler (first model on herbicide resistance published in the Journal of Theoretical Biology in 1978), the best models can do is suggest priorities for experimentation to validate them. Ignoring (or not knowing) one important parameter can skew the model. My mathematician colleague always kept mumbling at me: “Garbage in, Garbage Out”. I would hazard a guess that one parameter was left out of the simulations: the fact that sub-lethal stresses increase mutation rates. Thus, climate change stress will increase mutational diversity in pre-existing genes. For a discussion of this, see: Pest Management Science 67:253-257, 2011.

Oh no, you mean we have to do both ex situ and in situ? Well that won’t do at all. While I naturally hope Marleen Cobb successfully defended her PhD last week, I hope that when she comes round she’ll tweak her model and help us decide once and for all.

{ 3 comments… read them below or add one }

Jacob April 23, 2012 at 2:45 pm

There is a small but growing literature that says genetic diversity can increase as a result of range expansion. It is not so much about mutation as it is about drift.

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Marleen Cobben April 24, 2012 at 11:00 am

Thanks for this interesting discussion. Maybe it’s good to put some things in perspective. The results from our model suggest that the spatial process of range shifting under climate change can have a big impact on the genetic composition of new populations. This is because the individuals arriving first in a new climatically suitable natural area, have a competitive advantage (in numbers) over individuals arriving later. So even if these latter individuals are better adapted to the local conditions, adaptation of the population takes a while because there’s initially so little to select for. No news thus far I’d say. The problem is, that this ‘while’ is fairly long compared to the predicted rate of temperature increase. The range shift therefore pushes on, while a substantial part of the species’ genetic diversity is lagging behind, in the original populations. Of course, when climate is deteriorating in these original populations, and they go extinct, the genetic diversity in these populations is lost, simply because it has not made it to the new populations yet. So far for the model. This process has been shown in many empirical studies as well. In Europe, populations of wild species in the ice age refugial areas in the Mediterranean are still genetically more diverse than populations in the recolonised areas (where the ice retreated) in the north. And it has been a while since we had ice there…
What this tells us is that it may be a good idea to keep an eye on populations in the range centre and the retracting range margins: they are likely more genetically diverse than more recently established populations, and they are vulnerable to extinction because climate will locally deteriorate in the future. Regarding the possibilities for adaptation in new populations, things are more fuzzy. Here I call upon empiricists to check whether any evolutionary changes they find (and many have already been observed) could (partly) be the result of spatial processes (founder effects or drift as Jacob is suggesting) as opposed to selection and adaptation. You can’t select for what’s not there, and this may have consequences for the population’s fitness in the future.
The time and spatial scales involved in these complex interactions of ecology and evolution make life hard for researchers working in the field. I try to contribute by suggesting what to look for and where to do that. In any case we can safely assume that range shifts have genetic consequences.

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Jacob April 25, 2012 at 7:52 pm

Nice explanaiton, Marleen.

The issue is, like always, in part the question: what is diversity? If we don’t use allelic richness but phylogenetic diversity (PD) as our yardstick, the picture becomes a bit more complicated. Models show that due to drift (founder effects) rare alleles can become frequent in the frontier population and can show their effects in the population without getting swamped.

So you get more PD on that front, even though you may loose alleles at the level of the whole species. This gain in PD would at least partly compensate the loss of PD due to the disappearance of populations in the warmer parts of the geographic distribution.

Also, you might be selecting the quicker migrants on the frontier, which might be useful if climate change is going to accelerate.

I think it might not be such a good idea to enrich frontier populations too quickly and if it is done it should be done selectively, with different seed sources in different areas, ad be closely monitored.

On the issue raised by Prof. Gressel, I think that a higher mutation rate won’t be a very important factor — far less important than issues of drift.

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