2015-08-02 11:36:46
Sexual reproduction is a mechanism for cleaning up harmful mutations.
Although it may seem incredible, one of the mysteries of Biology that has not yet been completely elucidated is what sex is for, that activity for which many, and even many, would prefer to lose a hand rather than not be able to enjoy it. Believe it or not, biologists still debate the reason for its existence.
And sex, as a reproductive mechanism, is quite ineffective, despite how fun it is, in general. Sex allows only half of the population to generate offspring. In the human case (also in almost all animal species), only women can give birth, which leaves men as mere repositories of genetic material for their use in the very important task of maintaining the species. It would be more effective if all of us, males or females, men or women, could generate offspring independently, without having to mix our cells and genes with another individual. However, what biologists do not doubt today is that if sexual reproduction has been selected during billions of years of evolution, it is because this way of reproducing is what allows adaptation to the environment and the survival of the species. Very good, but why?
Several hypotheses have been considered to try to explain this incontrovertible fact. The most plausible of them is that sexual reproduction is a mechanism for cleaning up harmful mutations that, if not carried out, would generate an accumulation of these that would lead to the extinction of the species. It is true that if we reproduced by budding – for example, generating new human beings from the fingers of our hands, which would fall off, once converted into tiny babies, and would then be replaced by new fingers that would become more babies – , the mutations that occurred in our genome as the cells reproduced would accumulate over generations. Finally, there would be no individual in the population without a set of mutations that would prevent it from reproducing, or simply surviving, which would ensure extinction.
Sexual reproduction would hinder this accumulation of harmful mutations in the population through several mechanisms, one of which is sexual selection. In general, one sex (usually the male) competes for access to the other sex, and the second sex (usually the female) selects the winners of the previous competition. This sexual selection, together with the mixing of the genomes of two individuals that sexual reproduction entails, and that generates new genetic diversity, favors the reproduction of only the genetically healthiest, and is believed to eliminate the most pernicious mutations from the population. whose accumulation could lead to the extinction of the species.
Inbreeding and extinction
Until today, this remained a hypothesis that was only supported by theoretical considerations and computer simulations. For this reason, a group of researchers set out to carry out an evolutionary experiment with living beings to try to confirm or refute this hypothesis in the complex reality of life. In 2005, they began to breed the small Tibolium castaneum in the laboratory, better known as the red flour beetle, which we can sometimes enjoy baked in a piece of bread.
The researchers maintained four different populations of these beetles for seven years. The first population was subjected to strong sexual selection by the simple method of placing males and females in a ratio of nine to one, respectively. Under these conditions, access to females was limited to only the “best” males. In another population, the bias went the other way, with one male for every nine females, a much funnier ratio as far as I’m concerned. Still another population of beetles was maintained at the ratio of one female to five males. Finally, the fourth population remained in a state of obligate monogamy, with one male for each female. These populations reproduced for around fifty generations, which for us humans would have meant around a thousand years.
After this period of different sexual selective pressure, the researchers determined the number of harmful mutations that had accumulated in the different populations of beetles through the procedure of allowing brothers and sisters to reproduce, that is, generating a situation of inbreeding, the type sexual reproduction more similar to reproduction by budding. As is well known, inbreeding generates individuals that have inherited practically the same gene variants from both parents, which in the case of these beetles will reveal the proportion of harmful mutations accumulated in the different reproductive regimes. If sexual selection serves to clean the population’s genes of these types of mutations, it is expected that beetles reproduced under strong sexual selection will have a lower proportion of them in their genome.
And this was precisely what was observed. The beetles reproduced in a proportion of nine males to each female survived up to twenty generations in inbreeding before becoming extinct. On the contrary, the population maintained under the regime of least sexual selection (nine females for every male) became extinct only after ten generations of inbreeding. These results, published in the journal Nature, seem to indicate that we poor and vilified males finally do serve a purpose: nothing less than to prevent the extinction of species.
Referencia: Sexual selection protects against extinction. Alyson J. Lumley et al. Nature (2015). www.nature.com/doifinder/10.1038/nature14419
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