Parallel Speciation
Kerstin Johannesson · 2024
Parallel speciation is a young concept, derived as a specific case from the more general concept of parallel evolution. It was coined by Dolph Schluter and Laura Nagel in a seminal paper from 1995. In this paper they illustrated parallel speciation using the three-spined stickleback that repeatedly form freshwater ecotypes from marine ancestral populations. However, already in 1993 Schluter and McPhail provided a review on “replicate adaptive radiation” discussing parallel evolution of fish ecotypes in postglacial lakes and of Anolis lizards on Caribbean islands. Parallel speciation is essentially equivalent to parallel evolution of reproductive isolation, and both can be partial rather than complete. And similar to the more general discussion on speciation, it is mostly impossible to draw the line between partial and complete. The core attribute of parallel speciation is that traits leading to reproductive isolation evolve repeatedly and independently when a species is distributed over contrasting environments. The driver of parallel speciation is divergent natural selection. Mostly the reproductive barriers are not the direct target of selection but evolve as a by-product of repeated adaptation to different environments. For example, a species evolves into a small ecotype in one type of habitat and a large in another, and due to the size differences individuals of the different ecotypes avoid mating with each other while individuals from different locations of the same ecotype remain compatible. The expectation under parallel speciation is that barriers to reproduction between different ecotypes establish in contact zones where the environment shifts, despite these populations being phylogenetically the most closely related. However, there are no barriers between individuals of similar ecotype coming from different areas (when these are brought together in experimental tests).