Experimental hybridization of two African Streptocephalus species (Crustacea, Branchiopoda: Anostraca)

Fairy shrimp are ancient crustaceans that live in islands of ephemeral waters all over the world. Although an ancient group, there is evidence that they evolve slowly and thus, genetic differences between species can be relatively small. Here, we hybridize two African species of Streptocephalus with overlapping geographical ranges, and find that hybrids have reduced viability. We postulate a model of allopatric speciation, and discuss what may happen if sympatry is secondarily regained. We argue that strong pre- and post-mating barriers should be favoured if speciation has proceeded in conditions where the risk of sympatry is non-negligible. In species that evolved in absolute allopatry (like in Africa and North America), no selection for such barriers exists, however. Man nowadays increasingly interferes with incipient speciation by artificially connecting ephemeral pools, in which case full introgression may occur.