Boundary effects cause false signals of range expansions in population genomic data

Abstract

Studying range expansions (REs) is central for understanding genetic variation through space and time as well as for identifying refugia and biological invasions. Range expansions are characterized by serial founder events causing clines of decreasing diversity away from the center of origin and asymmetries in the two-dimensional allele frequency spectra. These asymmetries, summarized by the directionality index ({psi}), are sensitive to REs and persist for longer than clines in genetic diversity. In continuous and finite meta-populations, genetic drift tends to be stronger at the edges of the species distribution. Such boundary effects (BEs) are expected to affect geographic patterns in {psi} as well as genetic diversity. With simulations we show that BEs consistently cause high false positive rates in equilibrium meta-populations when testing for REs. In the simulations, the absolute value of {psi} (|{psi}|) in equilibrium data sets was proportional to the fixation index (FST). By fitting signatures of REs as a function of {varepsilon}=|{psi}|/FST and geographic clines in {psi}, strong evidence for REs could be detected in data from a recent rapid invasion of the cane toad, Rhinella marina, in Australia, but not in 28 previously published empirical data sets from Australian scincid lizards or the Indo-Australasian blacktip shark that were significant for the standard RE tests. Thus, while clinal variation in {psi} is still the most sensitive statistic to REs, in order to detect true signatures of REs in natural populations, its magnitude needs to be considered in relation to the overall levels of genetic structuring in the data.