Partial support for the central-marginal hypothesis within a population: reduced genetic diversity but not increased differentiation at the range edge of an island endemic bird.
Large-scale population comparisons have contributed to our understanding of the evolution of geographic range limits and species boundaries, as well as the conservation value of populations at range margins. The central-marginal hypothesis (CMH) predicts a decline in genetic diversity and an increase in genetic differentiation toward the periphery of species' ranges due to spatial variation in genetic drift and gene flow. Empirical studies on a diverse array of taxa have demonstrated support for the CMH. However, nearly all such studies come from widely distributed species, and have not considered if the same processes can be scaled down to single populations. Here, we test the CMH on a species composed of a single population: the Island Scrub-Jay (Aphelocoma insularis), endemic to a 250 km2island. We examined microsatellite data from a quarter of the total population and found that homozygosity increased toward the island's periphery. However, peripheral portions of the island did not exhibit higher genetic differentiation. Simulations revealed that highly localized dispersal and small total population size, but not spatial variation in population density, were critical for generating fine-scale variation in homozygosity. Collectively, these results demonstrate that microevolutionary processes driving spatial variation in genetic diversity among populations can also be important for generating spatial variation in genetic diversity within populations.
Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, USA.
K M
KM
Langin
Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC, USA.
T S
TS
Sillett
Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, USA.
W C
WC
Funk
The Nature Conservancy, San Francisco, CA, USA.
S A
SA
Morrison
Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, USA.
C K
CK
Ghalambor
D063147:Animal Distribution; D000818:Animals; D002140:California; D019143:Evolution, Molecular; D051456:Gene Flow; D040961:Genetic Drift; D014644:Genetic Variation; D005828:Genetics, Population; D005838:Genotype; D062312:Islands; D018895:Microsatellite Repeats; D046109:Passeriformes / Q000235:genetics*; D011156:Population Density; D062206:Spatial Analysis