Comparative studies suggest that sex chromosomes begin as ordinary aut
osomes that happen to carry a major sex determining locus. Over evolut
ionary time the Y chromosome is selected to stop recombining with the
X chromosome, perhaps in response to accumulation of alleles beneficia
l to the heterogametic but harmful to the homogametic sex. Population
genetic theory predicts that a nonrecombining Y chromosome should dege
nerate. Here this prediction is tested by application of specific sele
ction pressures to Drosophila melanogaster populations. Results demons
trate the decay of a nonrecombining, nascent Y chromosome and the capa
city for recombination to ameliorate such decay.