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Can Sex Explain Evolution?


Last time out, we looked at Darwin's theory of natural selection, alleged by some to be the single best idea anyone ever invented. The mere process of eliminating unfit examples of a type in a given environment builds up information over time, resulting in huge new layers of complexity.

Talk to the Fossils.jpgBut if no one can say what is fit or unfit according to natural selection, because nature has no direction, why must we pay attention to claims about natural selection? Why is there supposed to be anything to know?

Then there is Darwin's theory of sexual selection, with its famous exemplar: the peacock's tail. An illustration may help us see why reasonable persons continue to doubt.

Picture a triplex: Tom, a world class cribbage addict in Apartment A, does no work and has no money (apart from social assistance and charity). Dick, in Apartment B, works eight shifts a week in trucking, so has no trouble paying his bills. Harry, formerly in Apartment C, went off and became a multimillionaire (legally) in packaging and shipping for the software industry.

Does work alone explain Harry's success? Did he work a thousand times harder and more often than Dick? Is that even possible? Or is it all an accident of fate, such that Tom or Dick might have stumbled down the same way and done the same thing?

Most human beings tend to doubt that it is so simple. Also, there are not a billion generations between Tom, Dick, and Harry. Not even one, actually.

And if each of these guys somehow ends up with fertile heirs, is any of them "unfit"?

Very well, so let us now look at Darwin's other theory, sexual selection:

Reproduction is expensive and can exert an additional evolutionary pressure. Darwin defined this pressure as sexual selection. Sexual selection operates through some members of a species having an advantage over others in terms of mating. It is the selection for traits that are solely concerned with increasing the mating success of an individual.

So fitter animals pass on their genes more frequently, so we don't know anything except that they do pass them on.

Traditionally, that has often been interpreted as, a more combative male wins more mates and sires more offspring, furthering evolution via his genes.

From a common sense perspective, there is a problem with the underlying assumption: The combative male may get himself seriously injured and be unable to protect his offspring. He may also be susceptible to a serious Y-linked disease to which the vanquished males are immune. So it is unclear whether fitter offspring reliably result from success in sexual competition.

Incidentally, human breeding is not a good analogy because breeders are "designers" who avoid breeding aggressive animals or those with serious genetic disease in their pedigree.

After a fruit fly experiment, some researchers offered an additional reason why sexual dominance does not necessarily speed evolution:

Females are frequently harassed and harmed by males attempting to obtain matings. When these males are also "choosy" with their courtship, there may be negative consequences to the species' ability to adaptively evolve.

Similarly, another study revealed many secrets of the convoluted sex lives of crickets, including the fact that "dominant males had fewer mates than subordinate males, but they had similar numbers of offspring."

Recently, a key sexual selection theory, Bateman's theory that promiscuity benefits males but not females, was subjected to replication studies (repeating the experiment to see if it works out) and it didn't replicate.

Sexual selection, despite its immense cultural popularity as an idea, does not seem to work as a mechanism for major evolutionary changes.

We are still stuck for a mechanism that replaces intelligence.

See the rest of the series to date at "Talk to the Fossils: Let's See What They Say Back."

Image credit: Jebulon (Own work) [GFDL or CC BY-SA 3.0], via Wikimedia Commons.