Dynamics of natural selection on a lethal fourth chromosome of Drosophila. Twelve-generation study of experimental populations of D. melanogaster.

The dynamics of natural selection on a lethal fourth chromosome of Drosophila melanogaster was studied in replicated half-pint bottle and cage populations over 12 generations. Population numbers fluctuated widely in all populations, but there was no association between fluctuation in numbers and change in lethal frequency. In the bottle populations the lethal heterozygote frequency decreased to a low of 0.04 to 0.05 and then increased to 0.18 to 0.30, suggesting that the selection coefficients were not constant. In the cage populations heterozygote frequency decreased to about 0.35 to 0.40. In the cage populations the data suggest that the lethal chromosome is overdominant for both viability and fertility selection.