For many species, both local abundance and regional occupancy are
highest near the center of their geographic distributions. One
hypothesis for this pattern is that niche suitability declines with
increasing distance from a species geographic center, such that
populations near range margins are characterized by reduced density and
increased patchiness. In these smaller edge populations, genetic drift
is more powerful, leading to the loss of genetic diversity. This simple
verbal model has been formalized as the central-marginal hypothesis,
which predicts that core populations should have greater genetic
diversity than edge populations. However, demographic shifts over time
can generate a similar pattern. For example, in species with expanding
ranges, populations at the range edge experience serial founder effects,
creating a gradient of declining genetic diversity from the range core
to edge. Testing the central-marginal hypothesis properly thus requires
us to consider the confounding role of historical demography. Here, we
account for the role of history in testing the central-marginal
hypothesis using a genomic dataset of 25 species-level taxa of
Australian skink lizards (genus: Ctenotus and Lerista). We found support
for the central-marginal hypothesis in 16 of our 25 taxa, of which eight
taxa recovered significant support. Unexpectedly, species with the
strongest evidence for range expansion were the least likely to follow
predictions of the central-marginal hypothesis. The majority of these
species had range expansions that originated at the range edge, which
led to lower genetic diversity at the range edge compared to the core,
contrary to the central-marginal hypothesis.
Demographic Inferences and Loci Under Selection in a Recently Expanded Coral Population
