Profound and rapid allopatric differentiation of Arctic charr Salvelinus alpinus on a microgeographic scale

It is considered that allopatric speciation proceeds slower than sympatric speciation and rarely results in rapid differentiation of populations. In particular, high intraspecific divergence in fishes from recently glaciated freshwater systems is typically observed within flocks of sympatric forms, not between geographically isolated populations. We present an alternative case of fast and profound eco-morphological and genetic divergence between two allopatric Transbaikalian populations of Arctic charr. In one, charr are predominantly piscivorous with sparse short gill rakers; in the other, predominantly planktivorous with dense long gill rakers. Gill raker number differs with no overlap and there is considerable differentiation in other meristic and morphometric characters. Both populations manifest exceptionally low variability at microsatellite loci coupled with substantial interpopulational differences in allele composition obviously resulting from genetic drift. Still, they form a monophyletic clade in microsatellite phylogenetic tree of Transbaikalian charr and share the same mtDNA control region haplotype, which evidences their common origin. According to genetic dating, the divergence of the populations occurred about 14 thousand years ago. Our study shows that high differentiation between local charr populations can emerge in allopatry over a short time if speciation is driven by trophic specialization and is facilitated by small population sizes.