The molecular evolutionary basis of species formation revisited.

How do new species arise? This is among the most fundamental questions in evolutionary biology. The first genetic model for how reproductive barriers lead to the origin of new species was proposed nearly 90 years ago. However, empirical evidence for the genetic mechanisms that cause reproductive barriers took many decades to accumulate. In 2010, Presgraves presented a comprehensive review of the literature on known 'speciation genes' and the possible evolutionary mechanisms through which they arose. Fifteen years later, with an explosion of studies that include both non-model and model organisms, the number of known hybrid incompatibility genes has increased approximately sevenfold. Here, we synthesize previous and new empirical examples to investigate the genetic mechanisms through which intrinsic incompatibilities in hybrids arise and highlight current gaps in our understanding.