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The World's Oldest "Profession"
It’s highly controversial, and too often the butt of many jokes; a taboo subject in several cultures, and yet considered the "world’s oldest profession;" sex seems to be the hottest thing to have graced this planet. But has it always been, and will it always be? Has anyone ever really wondered, jokes aside, why we have sex? Despite its central role in biology, sex is a bit of an evolutionary mystery. It may sound like fun, but seeking evidence of sex is perhaps one of the most difficult endeavours in palaeontology (http://news.nationalgeographic.com/news/2009/02/090225-ancient-fish-sex.html), partly because it appears to make little scientific sense. It is time-consuming, exhausting, and not to mention dangerous. In reality (or rather, technically speaking), sex appears to have more disadvantages than advantages. Think, for example, of the logistical problem of finding a mating partner, which is a big problem in many species where mobility or population density is low. Then there is always the risk in some species, such as the praying mantis and black widow spiders, of being eaten alive right after the mating ritual. It's not even essential to reproduction, at least not in asexual organisms such as microbes, certain fungi, some plants, and even a few reptiles, which can multiply without sex. Furthermore, in sexual species, two individuals have to combine in order to reproduce one offspring, giving each generation of asexual organisms twice the reproductive capacity of sexual ones.Why, then, did sex become the dominant strategy when the do-it-yourself approach is so much more efficient?
First and foremost, while the puzzling question of how sex began in the first place still cannot be fully addressed, there are theories that high mutation rates, under the conditions of a stressful environment, can force an asexual organism to become sexual (http://news.nationalgeographic.com/news/2001/07/0709_sexorigin.html). Chris Adami, who is co-publishing a report in the Scientific Journal of the Royal Society says that, "the reason the origin of sexual reproduction has been such a big mystery is that we look at the world as it is now. But the early world was a much more stressful place, sometimes changing very rapidly," which may shed light to the speed at which asexual organisms were “forced” to become sexual, due to the rapid environmental changes.
So, then, moving on to the question of why sex became the better, or rather, the more dominant choice for reproduction, despite the seemingly overwhelming cons in comparison to asexual reproduction. Going back to the 19th century, scientists have proposed that sexual reproduction makes natural selection more effective because it increases genetic variation. (http://news.nationalgeographic.com/news/2005/03/0330_050330_sexevolution.html). One hypothesis is that parasites keep asexual organisms from getting too plentiful. When an asexual creature reproduces, it makes clones—exact genetic copies of itself. Since each clone has the same genes, each has the same genetic vulnerabilities to parasites. Thus, if a parasite emerges that can exploit those vulnerabilities, it can wipe out the whole population. That, in theory, should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites. Moreover, new studies also show that a sexual population evolves faster than an asexual population when challenged by a new environment. In a harsher environment, the sexual strain reached an increase in growth rate of 94 percent, but the asexual strain only 80 percent. Furthermore, there are four main possible conditions as to why sex is preferable than asexual reproduction:
- Selection Varies over Time: When the environment changes rapidly over time, genetic associations built up by past selection can become detrimental, and thus recombination can break apart these associations and improve the fitness of offspring.
- Selection Varies over Space: When selection varies over space, genetic associations created by migration can be locally detrimental. Somewhat similar to the idea of selection varying over time, what was once beneficial in a certain environment may not be suitable in another, and thus, recombination can also break apart these genetic combinations and improve the fitness of the offspring.
- Rates of Sex Vary among Individuals: Most models of the evolution of sex assume that individuals are equally likely to engage in sex, regardless of their condition in the current environment. Many organisms, however, are more likely to engage in sex when they are in poor condition. If individuals in good condition reproduce asexually while less fit individuals reproduce sexually, then sex as a means of reproduction readily evolves. It is even possible that sex evolved for the simple reason that genetic elements that have the ability to cause their carriers to engage in sex when condition is poor will spread—not because they enhance variation, not because they break apart unfavourable gene combinations, but because they are selfish and can escape bad genetic backgrounds via sex.
- Selection in Finite Populations: Scientists have argued that the genetic recombination brought about by sex means that favourable combinations of genes that have accumulated by selection are forever at risk of being broken up. However, when you put into consideration that we are talking about finite population, the genetic shuffling from sex actually allow a greater chance that a useful suite of genes will come together, especially when environmental change is another factor. In an infinite population, all possible combinations of mutations would arise instantaneously, so recombination would not be necessary to bring together advantageous mutations. Similarly, the best genotype could be lost by drift in finite populations, and, once lost, this genotype could be rapidly regenerated by recombining the remaining chromosomes in sexual populations. However, an asexual population would have to await the long-term right back mutations, leading to the eventual decay of fitness in asexual. With population drift and natural selection, species rapidly use up accessible variation, where beneficial alleles are found together on the same chromosomes, but hidden variation, where beneficial alleles are found on chromosomes with deleterious alleles, persists over time. Sex and recombination are then favoured to bring together fit alleles that tend to be found in different individuals.
So, in general, we have sex because when we spread the love, the best genes from a pool of genes (not just the same ones) prevail, and when the best genes prevail, our species continues to be able to adapt to the environment and exist. We can also add one more thing to be thankful for regarding sex: it adds meaning and excitement (again, jokes aside) to life. As we talked about in class, once every 100,000 times, there is a “kink” in the DNA and RNA replication that leads to variation, which is fundamental to the slow but eventual process of evolution. This “kink” is a kind of mutation itself, and I doubt that it occurs to us how frequent we mutate, or rather, evolve, and this evolution is sped up by sex. It’s amazing, really, that this happens; that sex became the dominant reproductive way, all because of the idea that variation is needed in order for a species to survive. Moreover, variation is a vital element of life—and not only the literal meaning as talked about, but in the metaphoric and poetic interpretation as well. If we reproduced asexually, it would mean that we would all eventually look alike, feel alike, and think (thus even act?) alike. I, for one, cannot imagine living in such a world, if you could even call that “living.” What would be the point?
A couple of lab classes ago, Will expressed something that I myself realized growing up (and a realization that has become more pronounced after this paper): every individual has a perfectly unbroken chain of genes that connects them to their ancestral lineage all the way back to the beginning of life. Every organism presently living contain all the genetic material it needs in order to have been born, created, or simply brought out into this world into the improbable assembly of life that they are now. Tall or short, fat or thin, more or less all the combinations of genes that survive today are a testament to Darwin’s theory of the “survival of the fittest.” They say nobody is perfect, which is of course true (as everything is constantly evolving and mutating for the better, as this paper has explored), but as of this moment, of this generation, and in this line of reasoning, we all pretty much are.
Reference:
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| This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated. |
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Comments
sex and life
Its an interesting idea indeed that sex is a favored mechanism for trying out new things, particularly important in a varying/unpredictable world. And that it in turn contributes to the variation/unpredictability, and hence to "meaning and excitement." For humans? For all organisms that reproduce sexually?