EACH sex is part of the environment of the other sex. This may lead to
perpetual coevolution between the sexes, when adaptation by one sex r
educes fitness of the other. Indirect evidence comes from experiments
with Drosophila melanogaster indicating that seminal fluid reduces the
competitive ability of sperm from other males, thereby increasing mal
e fitness(1,2). It also reduces a female's propensity to remate and in
creases her egg-laying rate(3). In contrast to these benefits to males
, seminal fluid has substantial toxic side effects in females, with in
creasing quantity leading to decreasing female survival(4,5). Here I s
how that when female D. melanogaster are experimentally prevented from
coevolving with males, males rapidly adapt to the static female pheno
type. This male adaptation leads to a reduction in female survivorship
, which is mediated by an increased rate of remating and increased tox
icity of seminal fluid.