Day 059 - Evolutionary arms races

Submitted by Sam on 18 July, 2011 - 23:56

Co-evolution can engender an arms race between competing genes, as predators adapt to better catch their prey and prey correspondingly counter-adapt to better evade their predators. Each genetic lineage 'races' against the other, progressively improving and counter-improving over many generations, always trying (in so far as a gene that produces a particular protein which has some effect on behaviour or morphology can be said to 'try') to out-compete the rival. A common-sense example is the asymmetric genetic arms races between cheetahs and gazelles, where one evolves adaptations which tend to make it better at chasing whilst the other consequently co-evolves adaptations which favour a greater evasive ability. A symmetric arms race occurs in the height of trees in a forest, where the selection pressure for access to light tends to cause trees to grow taller, amongst other adaptations.

These intuitive examples are inter-species arms races, but there are subtler, more insidious intra-species conflicts, even between the sexes. One interesting example is provided by the powerful effect a male canary's birdsong has on a female canary's reproductive behaviour; she initiates nest-building behaviour and increases the size of her ovary when she hears it. This has been shown to happen even when the stimulus is only a recording of a male canary's song, suggesting that the song acts like a drug, manipulating her to be primed for the male's advances. One reading of this phenomenon is that the male has 'won' an arms race, manipulating the nervous system of the female to his own ends, using his song to create an electrochemical pattern in her brain, which in turn stimulates her pituitary to synthesize the required hormones to bring her into an appropriate reproductive condition. Alternatively, the female may exhibit this behaviour as a counter-adaptation which requires the male to give an exhaustive performance of his song before she is willing to mate, signalling that he is a robust partner and one that is willing to commit time and energy to the relationship (having already spent time and energy wooing with a song).

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