Rollin C. Richmond

A SEARCH FOR EMERGENT COMPETITIVE PHENOMENA: THE DYNAMICS OF MULTISPECIES DROSOPHILA SYSTEMS'

Theoretical analyses of multispecies competition suggest that the stable co- existence of three competing species may occur even when all the pairwise combinations of the same species do not result in stable equilibria. In an attempt to discover such an emergent property of multispecies competition, we constructed laboratory populations consisting of one, two, and three species of Drosophila in all possible combinations. Two series of experiments at 19? and 220C were conducted. We found no evidence of emergent behavior; the outcome of competition in three-species populations could be adequately predicted from a knowledge of the pairwise interactions among the species.

GENETIC DIFFERENTIATION WITHIN THE ANDEAN SEMISPECIES OF DROSOPHILA PAULISTORUM

Species in statu nascendi (Dobzhansky and Spaasky, 1959) are relatively rare, perhaps because the critical stages of the speciation process are passed rather rapidly. The superspecies Drosophila paulistorum is an example of a form caught in these critical stages at our time level. It is in the process of evolving into six semispecies or incipient species. Although the semispecies show no taxonomically useful morphological differentiation, they do exhibit fairly strong, though incomplete, ethological isolation, as well as complete sterility of male F1 hybrids (Dobzhansky and Pavlovsky, 1969; Dobzhansky et al., 1969; Perez-Salas et al., 1970). D. pavlovskiana is a sibling species very closely related to D. paulistorum and was once thought to be a semispecies of paulistorum, but the failure of chromosome pairing in the salivary glands of hybrids is indicative of a more substantial genetic differentiation than that found between semispecies (Kastritsis and Dobzhansky, 1966). Ethological isolation among the semispecies of D. paulistorum is strong enough that two and sometimes three semispecies coexist sympatrically in many localities. No evidence of hybridization between semispecies in nature has been found although a careful attempt to detect such hybrids has not been made. However, some strains, particularly those of the Transitional semispecies, are capable of producing fertile hybrids with both the Centroamerican and Andean-Brazilian semispecies under laboratory conditions (Dobzhansky et al., 1969). Other strains of D. paulistorum which are intermediate between some semispecies have also been found. Dobzhansky and Pavlovsky (1966 and 1971) have described a strain of D. paulistorum which when originally established from a single wild female in 1958 was a member of the Orinocan semispecies. When this strain was reexamined in 1963, it no longer produced fertile hybrids and was subsequently found to produce fertile hybrids with the then undiscovered semispecies, the Interior. Dobzhansky and Pavlovsky (1975) have described more recently collected strains which give similar results. Ten strains of D. paulistorum, collected in Venezuela in 1972, produced fertile hybrids with both the Orinocan and Interior semispecies. When these strains were tested again a year later, six strains gave fertile hybrids with only the Orinocan semispecies. Clearly D. paulistorum presents an unparalleled and fascinating opportunity to study those events which lead to both genetic and ethological isolation between once similar populations. We describe here the results of genetic and ethological tests of strains of D. paulistorum and other members of the D. willistoni group collected in southern Venezuela in 1973. Strains from one of the localities sampled, Caicara, have the extraordinary property of producing fertile hybrids with five of the semispecies of D. paulistorum as well as with D. pavlovskiana.