Sexual conflict

Sexual conflict is a term in evolutionary biology. It occurs in a species when what benefits the females is different from what benefits the males.

The word 'benefit' here means 'what benefits the reproduction of the female's (or male's) genes. It is expressed as a difference in biological fitness.

The conflict can lead to an evolutionary arms race between males and females. It has primarily been studied in animals, though it can in principle apply to any sexually reproducing organism, such as plants and fungi.

Fundamental difference of interest

In sexual reproduction there is, from the point of view of evolution, a fundamental difference of interest between males and females. This difference of interest plays out differently in different species.

Males

Their interest is to mate with a large number of completely faithful females, thus spreading their genes widely in the population.

Females

Their interest is to mate with a large number of fit males, thus producing a large number of fit and varied offspring.

Examples

A well-documented example is the seminal fluid of Drosophila melanogaster, which up-regulates females' egg-laying rate and reduces her desire to re-mate with another male (serving the male's interests), but also shortens the female's lifespan, so reducing her fitness.
"A hormone called 'sex peptide'... causes the female to become uninterested in mating... also males have components in their sperm which kill old sperm from previous males".^[1]⁹²
The same set of alleles in males and females may have different optima: they are expressed differently in the sexes. A classic example is the human pelvis, where females need larger hips for childbirth. A narrower hip size is better for locomotion.
The genes that affect hip size must reach a compromise that is at neither the male optimum nor the female optimum. In some cases, the loci involved are expressed differently in males and females. Evidence indicates that intralocus conflict is important in the evolution of many traits.^[2]
Sexual conflict may lead to antagonistic co-evolution, in which one sex (usually males) evolves a favorable trait which is countered by a trait in the other sex.
For example, male bean weevils (Callosobruchus maculatus) have spiny genitalia. These allow them to copulate for a longer time without getting dislodged, and so transfer more sperm.
However, this damages the female and reduces her fitness. Females have evolved the counteradaptation of kicking at males during mating, which reduces the time spent in copulation.^[3]

Some regard sexual conflict as a subset of sexual selection (which was traditionally regarded as mutualistic), while others suggest it is a separate evolutionary phenomenon.^[4]

Related pages

Courtship in animals

References

↑ Schilthuizen, Menno 2001. Frogs, flies and dandelions: the making of species. Oxford University Press. ISBN 019850392X
↑ Arnqvist G. and Rowe L. 2005. Sexual conflict. Princeton University Press, Princeton New Jersey
↑ Crudgington H. & Siva-Jothy M.T. 2000. Genital damage, kicking and early death. Nature. 407: 855-856.
↑ Lodé, Thierry 2006. La guerre des sexes chez les animaux. Jacob, Paris. ISBN 2738119018

[Menno-1] Schilthuizen, Menno 2001. Frogs, flies and dandelions: the making of species. Oxford University Press. ISBN 019850392X

[2] Arnqvist G. and Rowe L. 2005. Sexual conflict. Princeton University Press, Princeton New Jersey

[3] Crudgington H. & Siva-Jothy M.T. 2000. Genital damage, kicking and early death. Nature. 407: 855-856.

[4] Lodé, Thierry 2006. La guerre des sexes chez les animaux. Jacob, Paris. ISBN 2738119018

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