Speciation by hybridization in animals

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Abstract
An increasing number of animal species of hybrid origin has been detected in recent years in different groups. In the present paper many of such cases are reviewed in detail and the mechanisms involved in their evolution are examined. The origin of an animal hybridogen requires: a) an interspecific hybridization, and b) changes in the maturation divisions allowing the transmission of the hybrid genome to the next generation. The latter are not caused by hybridization itself, and are certainly rare events; on the contrary, the role of high rates of interspecific hybridization should be taken into account, and the conditions for their occurrence are discussed. Animal hybrid species reproduce in different ways: thelytokous parthenogenesis, gynogenesis, hybridogenesis, self-fertilization, or fission. The consequences of these reproductive strategies are examined. Two phenomena are considered particularly relevant for the evolutionary success of animal hybridogens: 1. hybrid species being generally unisexual, their reproductive potential is doubled with respect to that of their bisexual ancestors (“demographic advantage”); 2. interspecific hybridization dramatically enhances heterozygosity (“heterotic advantage”). The short- and long-term effects of these phenomena on animal hybrid species are discussed.