No abstract

No abstract

Again we emphasize that the interpretation yields only the fact that the lower bound for the mean yield strength is lower slightly above 300 km than below 300 km; similar comparisons apply to the depths slightly above and below 550 km. Strengths are not compared at widely separated depths. No statistical analysis of these results has as yet been made. The statistical properties of the curve may modify the interpretation.

Certain local populations, or races, of the three sibling species, Drosophila tropicalis, D. paulistorum, and D. willistoni possess an interesting genetic structure.1' 2, 3 More than half the individuals in these populations are heterozygotes for the same inverted section in one of their chromosomes, and fewer than half are homozygous.In a panmictic population at equilibrium, such a state of affairs is possible only if greater proportions of the homozygotes than of the heterozygotes are removed by a differential mortality. Natural selection then establishes a situation known as "balanced polymorphism"; the Mendelian population in which it occurs may possess a high fitnees, since the hybrid vigor (heterosis) in the heterozygotes compensates for the low adaptive value of the homozygotes.4In D. tropicalis, a population in which 70 per cent of the individuals were heterozygous for a certain inversion, was encountered at Lancetilla, Honduras; elsewhere in Central America and in the West Indies the same chromosomal inversion had frequencies below 50 per cent, while in South American populations it was rare or altogether absent.1' I In D. paulistorum, the sample from Urubamba, Peru, contained significantly more than 50 per cent of heterozygotes, while in two other localities on the eastern slope of the Andes in Peru the heterozygotes amounted to less than 50 per cent.3 We have no evidence to show whether in these species the excesses of the heterozygotes are widespread or occur only in some small populations of isolated localities. In D. willistoni the situation is a little clearer. In at least three localities in northeastern Brazil the population samples contained more than 50 per cent of heterozygotes for a certain inversion (J, in the third chromosome), and in one of these localities samples taken on two successive years both showed this condition.3 Elsewhere in South and Central America this chromosomal inversion is heterozygous in 50 or less per cent of the individuals, but another inversion (E, in the right limb of the second chromosome) reaches frequencies higher than 50 per cent of the heterozygotes in the Brazilian state of Ceara.3The experiments reported below were designed to elucidate the nature of the genetic difference between populations in which the incidence of the heterozygotes is above and below 50 per cent. Our working hypothesis has been that this difference is quantitative rather than qualitative. Under balanced polymorphism, the incidence of the homo-and heterozygotes in a population at equilibrium is deter- PROC. N. A. S. 622 Downloaded by guest on August 3, 2020

No abstract