Sunday, July 6, 2008

Why do range expansions run out of steam?

I've not posted for a while as I've been away at conferences and I'm been trying to catch up with the work I've missed. I'll try to keep my posts more regular and I hope I've not dropped off people's lists (if I was on them to begin with).

What stops a species from extending its range indefinitely, why does a species not adapt and spread to every environment it encounters? This question has strong evolutionary and ecological importance, and it is not just an academic question as shown by the changing range of mosquitoes due to climate change.

A number of factors are likely to play a role in stopping the spread of a species (and I strongly suspect that I've missed a few). One of the major reasons is that the populations at the species margin can be prevented from adapting to new environments because gene flow into the low density species margin from the rest of the species range can swamp/hinder adaptation to a new environment (see Kirkpatrick and Barton). Likewise, if the species is excluded from new a new range by a different organism filling the ecological niche that the species would occupy, gene flow with each species can stop the two species from co-occurring by preventing character displacement (see Case and Taper).

Finally, populations at the margins of a species range/expansion can have low levels of genetic diversity, meaning that they lack standing genetic diversity which would allow them to adapt to new environments. This last point has received support from observations in many species that 'neutral' (i.e. at random loci) genetic diversity is lower in the margins of a species range than in the centre, but few of these studies have shown that this compromises the ability of the populations at the species margin to adapt (Eckert et al.).

A new study by Pujol and Pannel starts to correct this shortcoming by showing that in a species of plant, populations at the edge of a species range have (compared to plants from the main species range) lower neutral diversity, lower variability in a potentially important adaptive trait, and that they have a slower respond to artificial selection on this trait. Obviously, this result is for just one plant species so more work is needed to show that this is a general effect, but it certainly indicates that low diversity at the edge of a species range can be important in slow the rate of spread of a species.

I wonder if the lack of genetic variation in the marginal populations could be self re-enforcing (this may have been pointed out before). The reduced response to selection could leave these marginal populations more vulnerable to demographic fluctuations due to environmental change. If so these marginal populations might frequently undergo bottlenecks further reducing their diversity. Such cycles might keep the species' diversity continuously low at species margins stopping the spread of a species into new environments.

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