Evolution and stability of ring species

Abstract

Neutral models, in which genetic change arises through random variation without fitness differences, have proven remarkably successful in describing observed patterns of biodiversity, despite the manifest role of selection in evolution. Here we investigate the effect of barriers on biodiversity by simulating the expansion of a population around a barrier to form a ring species, in which the two ends of the population are reproductively isolated despite ongoing gene flow around the ring. We compare the spatial and genetic properties of a neutral agent-based population model to the greenish warblers' complex, a well-documented example of an actual ring species in nature. Our results match the distribution of subspecies, the principal components of genetic diversity, and the linear spatial-genetic correlation of the observed data, even though selection is expected to be important for traits of this species. We find that ring species are often unstable to speciation or mixing but can persist for extended times depending on species and landscape features. For the greenish warblers, our analysis implies that the expanded area near the point of secondary contact is important for extending the duration of the ring, and thus, for the opportunity to observe this ring species. Nevertheless it also suggests the ring will break up into multiple species in 10,000 to 50,000 y. These results imply that simulations can be used to accurately describe empirical data for complex spatial-genetic traits of an individual species.geography | topopatric | allopatric | birds | DNA

Diploid versus haploid models of neutral speciation

“…Ring species are interesting examples of such, where geography plays a crucial role in physically shaping the ring but does not block dispersal or gene flow along the ring [22][23][24][25]. A recent study of cluster formation with similar interactions was performed by Scott et al (2013) using a sympatric model [26].…”

Wherefore and Whither the Ring Species?

“…In Siberia the two distributional axes, represented by the subspecies viridanus and plumbeitarsus, came into secondary contact, but failed to interbreed freely. A distributional gap on the eastern side of the ring was interpreted as due to recent extinction, probably human mediated, and genetic studies using amplified fragment length polymorphisms (AFLPs) were consistent with isolation by distance and gradual genetic divergence throughout the ring (Irwin et al, 2005;Martins et al, 2013). Finally, studies of song structure showed that songs in the south are short and simple, whereas songs in the north, where the terminal forms viridanus and plumbeitarsus meet, are relatively complex (Irwin et al, 2001b(Irwin et al, , 2008Kovylov et al, 2012).…”

Ring species as demonstrations of the continuum of species formation

“…Importantly, both barriers are topographically identical to the Californian Central Valley (Monahan et al 2012), which was necessary for ring diversification in the Ensatina salamander complex. By modelling neutral evolution of a species around a barrier, Martins et al (2013) estimate that ring species can form, particularly when the termini have expanded over a large geographic area, as observed in Alophoixus. Yet, they also estimate that ring species are likely to break up into independent species or collapse into a single species in~130,000 generations, which is a relatively short period of time in the history of species complexes for ring species to be observed.…”

“…What conditions favour their formation? Modelling studies have attempted to address these knowledge gaps by estimating the biological parameters that result in stable ring species (Martins et al 2013), and determining the necessary topographic parameters of the barriers encircled (Monahan et al 2012). However, any generalization is undermined by a major limitation: only a handful of ring species are known to exist in nature.…”