CL Hubbs Systematic Zoology (1955) 4 1-20

In an old, but very comprehensive paper Hubbs documents over 130 instances of interspecies hybridisations in fish species in nature.

Interestingly he notes that hybridization events seem to be much more common in fresh water fish than in sea water species and that the factors which seem to favour hybridization is when there is a large population of one species living in a close ecological niche to a much smaller population.

Clearly animals that utilise the same kind of external fertilization as fish do are much more prone to susceptible to errors in fertilization than, for example, mammalian species with elaborate mate recognition and courtship displays. However, Hubbs did document a hybridization he witnessed himself between two sunfish (Lepomis) species where the male did undergo a courtship 'dance' with the female before she laid her eggs and he fertilized them.

The point that should be noted here, however, is that if it were not for mate recognition it is likely that many more mammalian hybridisations would probably have been recorded than has hitherto been the case. Barriers of mate recognition, although clearly making hybridizations less likely and hence rarer in evolution, when viewed in evolutionary time are unlikely to have prevented hybridisation events to have taken place in the past. It is my view that this is particularly likely in primate social systems where more often than not sexually mature males are excluded from access to females by dominant males. It is such situations, where perhaps two closely related species inhabit adjacent habitats that hybridisation events are perhaps most likely in the animal world.

Algis Kuliukas
June 2003