Speciation by interspecific hybridization in animals

Speciation by hybridization occurs when new species arise from crosses between parental ones. Hybrid speciation has traditionally been considered prominent in plants, while rare in animals. This view has been changing in the last few decades, mainly due to the increased availability of various genetic markers. The number of animal hybrid species is steadily increasing and now includes lizards, geckos, frogs, salamanders, fishes, beetles, moths, grasshoppers, stick-insects, copepods, snails, turbellarians, nematodes, corals, etc. Several well documented cases will be examined, and a number of points will be considered, such as: (i) the modes of reproduction of animal hybrid species, such as thelytokous parthenogenesis, gynogenesis, hybridogenesis, selfing, fragmentation, bisexuality; (ii) the role of polyploidy; (iii) the short- and long-term evolutionary success of animal hybrid species. These species generally combine a heterotic advantage (from the coexistence of two or more parental genomes), with a demographic advantage (from the switch from sexuality to clonal or emiclonal reproduction). This explains the frequent successful spread and competitive dominance of hybrid species over their parental ones. Recently proposed cases of homoploid sexual hybrid species (e.g. in fishes, butterflies and flies) will be discussed, with particular regard to their recombinant genomes, ecological characteristics, and mating behaviour.