Sex Please, We're Birdish

Though Darwin takes trouble to stress that the Struggle for Existence (and reproduction) is more than simply kill or be killed, he chooses to delineate from Natural Selection the phenomenon of Sexual Selection. "This depends," he writes, "not on a struggle for existence, but on a struggle between the males for possession of the females: the result is not death to the unsuccessful competitor, but few or no offspring." The partitioning off of sexual from natural selection is in part due to the desire to emphasise reproduction as well as survival, but partly because Darwin saw how sexual selection could be largely driven by social selection. Returning to the topic with 1871's The Descent of Man, and Selection in Relation to Sex, Darwin can be accused of confusing biological imperatives with social constructs. In man he believes that the female choice evident in nature has passed to the male, and that the differences in ability (particularly intellectual ability) between the sexes was biological and inevitably set to advance. Darwin was in fact derided by his scientist contemporaries for his views that female mate-choice predominates in the natural world, though today there is considerable evidence for widespread and evolutionarily significant female choice. Naturally though, the whole biological picture of mate- choice and the role of the sexes is far more complex in both the animal and the human spheres than Darwin or his contemporaries comprehended.

Darwin adds to his first words on the subject that "Sexual selection is… less rigorous than natural selection," a claim that would be hard to justify in the light of discoveries since Darwin. Sexual selection has been proved to be a crucial battle in the evolutionary struggle - a battle that can prove as fatal as it can natal. And it is still an area of research that still attracts a great deal of study as well as its share of controversy. A favourite topic with regard to sexual selection has been why the males of many species, particularly birds, display such prominent - and apparently hugely inconvenient for the task of purely survival - decorations, displays and behaviours. Darwin comments that, "I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect." Exactly as to why they would select these most peculiar specimens has attracted a range of theories - like all evolutionary theory not illuminated fully until the application of later develops in the field. (See Further Reading)

However, an important observation that Darwin does makes (though he is unaware of its full significance) is that sex produces greater levels of variation than parthenogenesis (such as vegetative reproduction in plants, or the 'virgin birth' of aphids.) Such 'scrambling' of genes has been since shown to be all-important in the evolutionary arms race - generally in terms of producing variation, and specifically with regard to the immune system. Very basically, sexual reproduction 'jumbles up' the genes and thus the immune systems of each generation, giving the new offspring an in-built advantage over the micro-organisms that have been busy evolving to attack the parental generation. Animals that reproduce without sex are therefore often susceptible to these micro-organisms: they have no natural variation of immune systems between individuals or generations, and if one is susceptible, in theory they all are. We have sex to confound our germs. (see Further Reading)

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