AN EXPERIMENTAL DEMONSTRATION OF FISHERS PRINCIPLE - EVOLUTION OF SEXUAL PROPORTION BY NATURAL-SELECTION

Most sexually reproducing species have sexual proportions around 1:1. This major biological phenomenon remained unexplained until 1930, when FISHER Proposed that it results from a mechanism of natural selection . Here we report the first experimental test of his model that obeys a ll its assumptions. We used a naturally occurring X-Y meiotic drive sy stem-the sex-ratio trait of Drosophila mediopunctata-to generate femal e-biased experimental populations. As predicted by FISHER, these popul ations evolved toward equal sex proportions due to natural selection, by accumulation of autosomal alleles that direct the parental reproduc tive effort toward the rare sex. Classical Fisherian evolution is a ra ther slow mechanism: despite a very large amount of genetic variabilit y, the experimental populations evolved from 16% of males to 32% of ma les in 49 generations and would take 330 generations (29 years) to rea ch 49%. This slowness has important implications for species potential ly endangered by skewed sexual proportions, such as reptiles with temp erature sex determination.