HYBRIDIZATION AND SEASONAL SEGREGATION IN TWO RACES OF A BUTTERFLY OCCURRING TOGETHER IN TWO LOCALITIES

Abstract

1. Two localities where the two races of Colias chrysotheme occur together are described (Mono Lake and Round Valley, Calif.). 2. In these places, the yellow-race has definite broods during the season. The orange race apparently does not. 3. The yellow-race has more seasonal generations when a population is at a lower elevation (Round Valley) than at a higher elevation (Mono Lake). This compares with latitudinal differences of the same type. 4. The two localities are 50 miles apart, but show no correlation in seasonal generations. 5. The yellow-race generations at the higher elevation are separated by interbrood periods with no adults. At the lower elevation, the generations merge one into the other. 6. Hybrid intermediates are present at one locality rather constantly at a frequency of about 10 per cent. 7. The range of color intermediates is not trimodal, but a U-shaped curve. This is probably due to a low viability of the F₁. A trimodal curve is expected under conditions of very recent hybridization and all intermediates with longtime hybridization. 8. Several reasons are given to account for the lack of complete blending between the races after years of hybridization.