Coevolution at Two Trophic Levels
Most of the classical populations genetics theories have till today dealt with models regarding genetic changes in a single species as a result of its interactions with its physical environment. Models dealing with competition between species have received some attention (Roughgarden 1979, Matessi and Jayakar 1980, Christiansen and Loeschcke 1980). It is clear, however, that any serious theoretical consideration of evolutionary mechanisms must take into account the coevolution, besides its competitors, also of its prey, its predators, and its parasites. Several models of gene frequency changes of such interactions have been proposed, but they suffer from the drawback that they deal only with gene frequencies and not with population sizes or densities (Mode 1958, Leonard 1977, Lewis 1981, Jayakar 1970). Roughgarden (1979) proposed a general ecological model of species interactions and considered several particular examples. Population sizes must be an essential feature of models involving two trophic levels since the possible extinction of food sources (or hosts) must govern the choice of organisms on which to feed or parasitize.
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