A tree of tree frogs around the Black Sea

Abstract

Speciation, the process by which one species evolves into two or more, is a major focus of ongoing debate, particularly regarding the geographic context in which it occurs. Geographic models of speciation tend to fall into discrete categories, typically referred to as allopatric, parapatric and sympatric speciation, according to whether two groups evolve reproductive isolation while geographically isolated, differentiated but connected by gene flow, or completely co‐occurring. Yet molecular studies indicate that full development of reproductive isolation can take very long compared with the timescale at which climatic oscillations occur, such that the geographic context of differentiating forms might change often during the long process to full species. Studies of genetic relationships across the ranges of organisms with low‐dispersal distances have the potential to reveal these complex histories. In a particularly elegant example in this issue, Dufresnes et al. () use genetic variation and ecological niche modelling to show that a ring of populations of the eastern tree frog (Hyla orientalis) surrounding the Black Sea had a complex history of geographic differentiation. Alternating phases of geographic fragmentation and phases of gene flow between neighbouring populations have produced a pattern of gradual genetic change connecting the western, southern and eastern sides of the ring, with the northwestern and northeastern forms being most differentiated. In the north, a population in Crimea appears to have been produced through mixture of the two extreme forms. The overall genetic relationships are reminiscent of those found in ring species, which have been used as prime demonstrations of the process of speciation. The difference, however, is that the terminal forms appear to have mixed rather than be reproductively isolated, although more research is needed to infer whether there might be some reproductive isolation on the northern side of the ring.

How Ancient Sex Drove Mammalian Lineage Evolution

“…The connection between sex chromosome rearrangements and speciation is being intensively investigated. It was shown that non‐recombining regions of any chromosome might present hotspots for incompatibility that may be a key to speciation . In this article, Irwin argues that in ZW and XY sex‐determination systems, the Z and X chromosomes can play a dominant role in speciation.…”

Ring Patterns - distribution, diversification or speciation? A case study of two small mammals in the mountains of Southwest China

“…5). Intriguingly, both groups W2 of the two species belonged to the western lineage based on the nuclear DNA but had mitochondrial DNA from the eastern clades D4 and C2, suggesting a history of sex-asymmetric hybridization and selection for a particular mitochondrial type in the mixed population (Irwin, 2016). The asymmetric gene flow observed between group W2 and E2 were in support of gene exchange between the terminals of the western and eastern lineages, and the reduced level gene flow at the terminal of the rings support the existence of partial reproductive isolation (Table S5).…”