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In spite of abundant evidence that intra- and inter-specific competition occurs in natural communities, there is surprisingly little to suggest it is a major force promoting genetic change. This report assesses the genetic effects of competition in two species of seaweed fly,Coelopa frigidaandC. pilipes. In laboratory cultures ofC. frigidathe relative survival of heterozygotes at theAdhlocus, which was being used as a marker for the large αβ chromosomal inversion, was greater than that of homozygotes. In monocultures ofC. frigidathis competitive superiority was dependent on larval density. At low densities facilitation was seen, whereas at high larval densities there was competition. In mixed cultures of the two species, interspecific competition contributed to the differential mortality ofC. frigida, and observations of natural populations suggested that competition may have similar effects to those described in laboratory culture. A possible mechanism involving the supply of nutritive microorganisms is proposed to underly both intra- and inter-specific competition. In seaweed flies, competition and the consequent differential mortality appear to be forces maintaining rather than reducing genetic variation. |
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