Isolating Mechanisms

Abstract Isolating mechanisms are characteristics of species that reduce or prevent successful reproduction with members of other species. Viewed genetically, they are characters that act as barriers to the exchange of genes between populations. Most of these barriers are incidental consequences of evolutionary processes within populations, but they can be elaborated directly by natural selection. They can take many forms, from mismatches between mating signals and preferences to genetic incompatibilities causing sterility of hybrids. Barriers may not be stable through time and can erode, for example due to change in the environment. A major outstanding challenge is to document the contributions of different forms of isolation to the overall barrier to gene exchange between species and to understand the order in which these barriers evolve, and how they interact with each other. Key Concepts Isolating mechanisms are intrinsic characteristics of species that reduce or prevent successful reproduction with members of other species. Many, perhaps most, isolating mechanisms are incidental consequences of evolutionary processes within a population, not fashioned by selection for the purpose of preventing gene flow. Individual barriers to gene exchange act sequentially which means that barriers acting early in the life cycle tend to have the greatest effect even though they may not have been the first to evolve. Ecological components of isolation evolve through adaptation to local conditions and may reduce the probability of mating or the fitness of hybrids. Behavioural components of isolation prevent mating even when reproductively active individuals meet and may evolve due to sexual selection on mating signals and responses. Barriers can occur after mating but before zygote formation. These barriers may be a side effect of evolutionary conflicts of interest between males and females. Divergent populations accumulate genetic differences. Inevitably, some of the new traits or genes are incompatible, resulting in isolation through reduced viability or fertility of hybrids, especially in the heterogametic sex. Reproductive isolation builds up by the accumulation of barriers to gene exchange. This process is neither linear nor deterministic. Instead, barriers accumulate at different rates during speciation and may act in combination in ways that are not predicted by the effects of single barriers. There is still a need to know more about the range of barriers operating in individual species pairs, the reasons for their evolution, the order in which they evolved and their genetic architecture.