THE GENETIC-EFFECTS OF COMPETITION IN SEAWEED FLIES

In spite of abundant evidence that intra- and inter-specific competiti on occurs in natural communities, there is surprisingly little to sugg est it is a major force promoting genetic change. This report assesses the genetic effects of competition in true species of seaweed ny, Coe lopa frigida and C. pilipes. In laboratory cultures of C. frigida the relative survival of heterozygotes at the Adh locus, which was being u sed as a marker for the large alpha beta chromosomal inversion, was gr eater than that of homozygotes. In monocultures of C. fripida this com petitive superiority was dependent on larval density. At low densities facilitation was seen, whereas at high larval densities there was com petition. In mixed cultures of the two species, interspecific competit ion contributed to the differential mortality of C. frigida, and obser vations of natural populations suggested that competition may have sim ilar effects to those described in laboratory culture. A possible mech anism 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 for ces maintaining rather than reducing genetic variation.