Vicky Just in Phys.org:
Promiscuity mixes up the gene pool and dilutes genetic differences between populations, slowing down the evolution of new species, says new research by an international team led by the University of Bath's Milner Centre for Evolution.
Darwin's theory of evolution showed that new species evolve when natural selection favours individuals with particular characteristics, allowing them to survive, breed and pass on their genes more successfully than their peers. Over time, a group of individuals can evolve to adapt to their local environment and form a new species.
Previously it was thought that sexual selection, when one sex prefers to mate with individuals with specific characteristics, was a strong driver of the formation of new species. One of these processes is the Fisherian runaway selection whereby arbitrary traits such as conspicuous feathers or fancy songs attract female's attention and hence improve the mating success of the bearer. Due to local variations in female preferences, nearby populations can rapidly differentiate and over time evolve into new species.
However new research in birds, published in the leading academic journal Evolution, overturns the conventional wisdom and suggests that promiscuity actually slows down the evolution of new species.