EXPERIMENTAL STUDIES ON HYBRIDIZATION AND SEXUAL ISOLATION BETWEEN SOME APHYTIS SPECIES (HYMENOPTERA: APHELINIDAE). III. THE SIGNIFICANCE OF REPRODUCTIVE ISOLATION BETWEEN INTERSPECIFIC HYBRIDS AND PARENTAL SPECIES.

Abstract

Interspecific hybridization is of rare occurrence in animals, especially in comparison to plants, obviously because of their very different modes of reproduction, ecology, and perhaps especially, of behavioral isolation (Mayr, 1963). Within the animal kingdom, hybridization frequency differs with the class, being more characteristic of animals which reproduce by means of external fertilization, e.g., fishes and amphibia. To illustrate this rarity of animal hybrids in nature, Mayr (op. cit.) states that perhaps one out of 60,000 wild birds is a hybrid. In animals with internal fertilization, the rarity of hybrids may frequently be attributed to the presence of ethological isolating mechanisms. Where ethological isolating mechanisms are overcome, either under laboratory or natural conditions, and successful interspecific copulation occurs, hybrids may occasionally be produced. The steps in hybrid production subsequent to mating, such as fertilization of the egg by the foreign sperm, development of the embryo, etc., should proceed normally. These may be prevented by death of the sperm in the female genital tract, inability of the sperm to fertilize the egg, or by subsequent death of the zygote due to genetic imbalances. Even when all these isolational barriers are circumvented and hybrids are produced, poor genetic fit (brought about as a result