Evolution and instability in ring species complexes: an in silico approach to the study of speciation

Ring species are a biological complex that theoretically forms when an ancestral population extends its range around a geographic barrier and, despite low-level gene flow, differentiates until reproductive isolation exists when terminal populations come into secondary contact. Due to their rarity in nature, little is known about the biological factors that promote the formation of ring species. We use evolutionary algorithms operating on two simple computational problems (SAW and K-max) to study the process of speciation under the conditions which may yield ring species. We vary evolutionary parameters to measure their influence on ring species' development and stability over evolutionary time. Using the SAW problem, ring species consistently form, i.e. fertility is negatively correlated with distance (R-values between -0.097 and -0.821, p<0.001), and terminal populations show substantial infertility. However, all SAW simulations demonstrate instability in the complex after sympatric zones are established between terminal populations. Higher mutation rates and larger dispersal/breeding radii promote ring species' formation and stability. Using a problem with a simple fitness landscape, the K-max problem, ring species do not form. Instead, speciation around the ring occurs before ring closure as good genotypes become locally dominant.