Agricultural Biodiversity Weblog

Are populations on the edge of the geographic range of a species not so important to conserve as more central ones? That’s the provocative question tackled by a recent meta-analysis. (( ECKERT, C.G., SAMIS, K.E., LOUGHEED, S.C. (2008). Genetic variation across species’ geographical ranges: the central-marginal hypothesis and beyond. Molecular Ecology, 17 (5), 1170-1188. DOI: 10.1111/j.1365-294X.2007.03659.x)) Theory would suggest that marginal populations should be less diverse, and therefore possibly of less conservation value. But the theory has never really been properly tested, so the common assumption that marginal populations are less diverse is just that — an assumption.

Its theoretical underpinning is that individuals and populations are likely to be fewer and more widely spaced on the periphery compared to the centre of the range of a species. That means that effective population sizes are likely to be lower and isolation more pronounced, which suggests that genetic diversity within populations should be lower and among populations (differentiation) higher in marginal areas.

That turns out to be more or less the case for the 134 population genetic studies (of both plants and animals) reviewed by the authors: “any given species is more than twice as likely to show the predicted pattern as not, and usually a change in diversity is accompanied by a parallel change in differentiation.”

There are some caveats, however. The differences were generally pretty small. The actual mechanisms producing them not clear (were the differences the legacy of historical environmental changes or the result of ongoing evolution?). The sampling of species was biased taxonomically and geographically. Plus all of the studies looked at (supposedly) neutral variation rather than traits which might actually have adaptive importance.

But the results are nevertheless intriguing. Especially when you think about how they might be different for crops (I don’t think any of the 134 studies reviewed were of domesticated species). If anything, one would predict geneflow from the center to the periphery, and indeed among peripheral populations, to be stronger in crops than in wild species. That means that differences in genetic diversity and differentiation between centre and periphery are likely to be even smaller, maybe non-existent. Sounds like something worth checking.