Diversity patterns and speciation processes in a two-island system with continuous migration

TL;DR

This study models how continuous migration influences speciation and diversity patterns in a two-island system, revealing complex relationships between migration, genome size, and species richness.

Contribution

It introduces a neutral model of speciation with continuous migration, analyzing how migration levels affect diversity and speciation modes across different genome sizes.

Findings

Low migration induces speciation in small genomes.

High migration reduces diversity, leading to a single species.

Large genomes enable sympatric speciation even with strict isolation.

Abstract

Geographic isolation is a central mechanism of speciation, but perfect isolation of populations is rare. Although speciation can be hindered if gene flow is large, intermediate levels of migration can enhance speciation by introducing genetic novelty in the semi-isolated populations or founding small communities of migrants. Here we consider a two island neutral model of speciation with continuous migration and study diversity patterns as a function of the migration probability, population size, and number of genes involved in reproductive isolation (dubbed as genome size). For small genomes, low levels of migration induce speciation on the islands that otherwise would not occur. Diversity, however, drops sharply to a single species inhabiting both islands as the migration probability increases. For large genomes, sympatric speciation occurs even when the islands are strictly isolated. Then species richness per island increases with the probability of migration, but the total number of species decreases as they become cosmopolitan. For each genome size, there is an optimal migration intensity for each population size that maximizes the number of species. We discuss the observed modes of speciation induced by migration and how they increase species richness in the insular system while promoting asymmetry between the islands and hindering endemism.