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Why Have Sex?

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Biological evolution is not wasteful. Because all organisms evolved under resource scarcity, energy was almost always hard to come by. As a result, all available energy was conserved and directed towards immediately useful adaptations. One of the most widespread and powerfully useful adaptations over evolutionary time was sexual reproduction. For about 2 billion years the large majority of species on Earth have utilized this energy-intensive and overly complicated form of reproduction.

Why?

Asexual reproduction is much less energy-intensive and simple. Asexual organisms don’t have to spend time searching for a mate; they are their own mate! And asexual organisms don’t have to worry about the parental cost-benefit analysis when caring for and raising offspring. Their offspring is 100% theirs and 100% identical (minus random mutations). All the genes win! And yet, sexual reproduction rules pretty much all eukaryotic kingdoms (i.e., animals, plants, fungi, protozoans).

If you google “why have sex?” the internets provides us with a lot of reasons why (via WebMD):

  • pleasure
  • stress relief
  • exercise
  • sexual curiosity
  • attraction
  • love
  • commitment
  • procreation
  • social status
  • revenge
  • boost self-esteem

And I’m sure there are many more. But all of these reasons are proximate, and therefore, do not tell us anything about why organisms procreate using sexual reproduction, rather than asexual reproduction. In fact, if you really think about it, all of the above “reasons for sex” could in theory be achieved by asexual reproduction (except maybe revenge?).

My point is that we need to understand the ultimate cause of sex, not the proximate causes. Charles Darwin died never knowing the answer to why sexual reproduction was so widespread. In the early 1860s he wrote:

We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements. The whole subject is as yet hidden in darkness.

The reason why Darwin was unable to uncover the profound mystery of sexual reproduction was because he was unaware of genetics. And at a fundamental level, the answer to why organisms reproduce sexually is all about genes.

1. Genetic Variation

Asexual reproduction is easy, but it is also problematic because adaptations are slow to take hold in a changing environment. Asexual organisms are basically clonal populations. Their only variation comes from random mutation. In contrast, sexual reproducing organisms can utilize recombination (50% of each sexes genome) to create novel genomes. This novelty is the variation natural selection needs to produce new successful adaptations to changing environments. In fact, many asexual species start reproducing sexually when under extreme environmental stress for this reason.

2. Bad Mutations

Another reason sexual reproduction became so successful is because of “bad” mutations. Mutations happens whenever an organism reproduces. Mutation is simply a copying error during transcription. These copying errors provide evolution with new information. The downside is that mutations are almost always bad or neutral (because they are copying errors). In asexual organisms all bad mutations are retained. This gives an enormous competitive advantage to sexual reproducers. Organisms that reproduce sexually can “purge” bad mutations through recombination. Recombination either turns harmful genes into silent alleles or enables selection to weed them out over a few generations.

3. Parasites

Finally, asexual organisms struggle to cope with parasites (i.e., viruses and bacteria). All organisms must cope and co-evolve with parasites in order to predominate. However, asexual reproducers have an essentially static genome. This allows viruses to quickly breakdown the organisms defence system and decimate the population. In contrast, sexual reproducers have very complex and varying genomes that can adapt quickly to intruders. New combination of genes make it very difficult for parasites to win the evolutionary arms race.


I have always felt like there is a strange force behind sexual bonds. No other bond I’ve experienced is quite as powerful or intensely emotional. There is nothing quite like sharing your body with another human. And for many of us, sex becomes such a dominant aspect of our adult life. So much of our time and energy is dedicated towards finding and keeping a partner(s). So much of adult identity is constructed because of (and as well as a result of) sexual relationships. And all human culture can in some sense be viewed as directly or indirectly produced by the desire to have sex.

But at its core sex is all about swapping genomes. Sex is a gene’s trick to ensure that they exist within bodies devoid of bad mutations, and with abundant coping mechanisms for parasites and changing environments. Very romantic.

I’m not sure that this view of sex is welcomed by many. Even to me, I find thinking about the function of sex to be sobering. But I do enjoy contemplating what this knowledge means for the future of sex. Technological evolution is “waking evolution up” in a sense. We are going to be able to intelligently direct our evolution in the future. Will we be able to fulfill the genetic function of sex better and more quickly than sex can? If so, how will sex change? No one has the answer to that question. But it is still worth giving some thought.

Discuss this on Hubski or let me know what you think on Twitter!

 
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