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(b)
(a)
0.6
0.5
0.4
0.3
sib+sib
non+non sib+non non+sib
Female mating treatment
Fig. 7.22 (a) Field cricket, Gryllus bimaculatus . Photo © Tom Tregenza. (b) Results of a
double mating experiment. Females mated to two of the female's own siblings suffer
reduced hatching success due to inbreeding depression. Females mated to a sibling plus
a non-sibling (in either order) do just as well as those mated to two non-siblings,
suggesting the female can bias fertilization success in favour of the unrelated male.
From Tregenza and Wedell (2002).
accessory gland proteins in Drosophila have high levels of amino acid polymorphism
and differ markedly between species; they are estimated to evolve at twice the rate of
non-reproductive tract proteins, which is strongly suggestive of a continuing sexual
arms race.
The power of sexual conflict to produce antagonistic coevolution between the sexes
has been demonstrated by some wonderfully elegant artificial selection experiments
with Drosophila melanogaster . Bill Rice and Brett Holland asked the question: are evolving
male adaptations kept in check by evolving female counter-adaptations? In one
experiment (Holland & Rice, 1999), they had two selection lines, each run for 47
generations in the laboratory, during which they selected for the most successful males
and females. In one line, there was intense sexual selection: each vial had three males
and one female. In this environment there was strong selection for males who were
successful at sperm competition and for females able to cope with male-male
competition. In the second line, sexual selection was eliminated altogether by the neat
trick of enforced random monogamy. In these vials, one male and one female spent their
whole lives together. Here, in the absence of male-male competition, a male was
guaranteed paternity of all his female's ova and it obviously paid him to behave in a way
which maximized her lifetime success.
The results showed that in the monogamous line males did indeed evolve to be less
harmful to females. For example, they had a decreased courtship and mating rate. As a
result, female survival and fecundity was greater than in the sexual selection line. Did
females evolve too? If sexual conflict leads females to evolve costly defences against
Selection
experiments with
Drosophila reveal
antagonistic
co-evolution
between the
sexes
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