The Molecular Ecologist: Reviewing the causes of population genetic structure

distance Photo by eliassss.

At The Molecular Ecologist today, I highlight a couple of recent literature reviews that seek to understand how natural populations are structured by the limitations of distance, and by local adaptation:

Taken together, these two papers are a nice compilation of a very large literature. If nothing else, they demonstrate that we’re well past the point of asking whether environmental isolation happens at all—in fact, it looks to be quite common—and we’re ready to start digging into the details of when and why it develops.

To see what broad patterns two different groups of authors were able to extract from their surveys of many population genetic studies, go read the whole thing.◼

The Molecular Ecologist: Genes … in … space!

(A) Geography, and (B) genetics. Figure 2 from Wang et al. (2012).

I’ve got a new post up over at The Molecular Ecologist, discussing a new paper that tries to take a quantitative approach to a phenomenon that keeps turning up in human population genomic datasets, in which genetic data mirrors the geography of the places it was collected.

It’s something of a classic result in human population genomics: Go out and genotype thousands of people at thousands of genetic markers. (This is getting easier to do every day.) Then summarize the genetic variation at your thousands of markers using Principal Components Analysis, which is a method for transforming that genetic data set into values on several statistically dependent “PC axes.” Plot the transformed summary values for each of your hundreds of samples on the first two such PC axes, and you’ll probably see that the scatterplot looks strikingly like the map of the places where you collected the samples.

Of course “looks strikingly like” is not a very quantitative statement. To see how the new study deals with that problem, go read the whole thing. And yes, I manage to shoehorn in a reference to the Muppets.◼

The Molecular Ecologist: Isolating isolation by distance

Linanthus parryae population Linanthus parryae. Photo by naomi_bot.

And now I present my first “real” post as a contributor at the Molecular Ecologist, a discussion of a new review article pointing out that population geneticists aren’t doing a great job dealing with one of the best-known patterns in population genetics, isolation by distance, or IBD. You may recall that I discussed IBD in a more historical context way back in the day on this very website. It’s simply a pattern in which populations located close to each other are more genetically similar than populations farther away from each other, which arises because most critters (or their seeds, or larvae, or pollen) are less likely to move longer distances. But IBD can be conflated with a number of other patterns population geneticists often try to detect:

So let’s say you’ve collected genetic data from sites on either side of a line you think might be biologically significant—a pretty standard-issue population genetics study. You run your data through Structure, and find two clusters of collection sites that line up pretty well with that Line of Hypothesized Biological Significance. As a followup, you conduct an AMOVA with the collection sites grouped according to their placement by Structure, and you find that the clusters explain a significant fraction of the total genetic variation in your data set. Therefore, you conclude that the LHBS is, in fact, a significant barrier to dispersal.

Except that as we’ve just discussed, everything you’ve just found could be a consequence of simple IBD plus the fact that you’ve structured your sampling so that your LHBS happens to bisect the landscape you’re studying. And just to add to the frustration, even if you’d started out by testing for IBD before you started with all of the tests for population structure, a significant result in a Mantel test for IBD wouldn’t necessarily mean that population structure wasn’t there.

To find out how the author of the new review article suggests we deal with the complications outlined above, go read the whole thing.◼