Hypotheses on Sex
There have been many hypotheses about the origin and maintenance of sex. While sex remains a contentious area of theoretical evolutionary biology, competing views pretty much answer the same question the same way.
What compensates for the sex tax? The answer invariably involves adaptability.
Broadening genetic diversity renders a population more robust: a concept with several facets, including the ability to evade and overcome disease. Sex may well have evolved by virtue of presenting a moving target to pathogens. Sex and a microbiome-based immune system were the evolutionary insurance policies taken out to raise the odds of a population surviving nefarious microbes.
Paradoxically, though theorists agree that adaptability through diversity accounts for sex, their theories do not account for that very thing. Instead, many reproductive-advantage theories, beginning with Darwin, foolishly focused on competition. Darwin believed that sexual selection was a powerful force: a special case of natural selection that emphasized mating success for males. Propagation of a male’s genes were secured by maximizing progeny, whereas a female’s concern was optimizing quality: raising the odds of success to maturity and reproduction, not merely the number of offspring.
Darwin’s sexual selection theory was treated skeptically for decades. English evolutionary biologist Ronald Fisher invigorated the theory in the 1930s with the observation that supernormal ornaments could arise simply from female choice.
Nature sometimes equips animal males with mate-attracting ornaments and display behaviors that go so far as to be life-threatening: hence the idea of supernormality leading to survival of the fittest. If a male can flash self-endangering bling and survive, he must be well endowed where it counts; or so it was supposed. The implication was that, in evolutionary time, traits desired by females would become further exaggerated. Preference for long tails would result in longer and longer tails. This became known as the runaway hypothesis.
The Darwin/Fisher runaway hypothesis assumed that supernormality correlates with fitness. But a female-desired trait is not necessarily an evaluation of fitness. It may simply be that females favor a certain trait for its aesthetic value.
Seemingly capricious choices may become quite widespread in a population simply by being desirous. The senses are often positively responsive to supernormal features, such as men liking lips enhanced by red lipstick. In this regard, sensory biases toward supernormal features are biologically favorable, at least in stimulating the desire for copulation.
Females that mate with males because of their popular ornaments may benefit because they produce sons with similarly charming features that can attract mates, and so perpetuate the lineage.
Sexual selection theories evolved after Fisher. Many centered on the iffy proposition that the desirous trait was indicative of a male’s quality. While appearances are often an honest signal of a male’s overall health, there are a multitude of exceptions where appearance does not equate with reproductive fitness or success, including humans. From an evolutionary perspective sex is not straightforward.