Sexual reproduction, evolutionary phenomenon

Modern human society is sex obsessed. It’s everywhere, and it triggers all sorts of curious behaviours. Yet, social pressure and humiliation aside, sex is crucially important. Well, without it, none of us would be here, and reproduction would seem incredibly problematic. Without sexual reproduction as we know it, life may not have evolved
from simple organisms and life forms.

The word ‘sex’ has various meanings to different people. In general terms, sex is an act that combines the genetic material from two individuals, to produce a new, single individual. By this definition however, ‘parasexual’ events (genetic exchange) observed in bacteria and viruses would be included, alongside the more familiar, and extravagant cycle demonstrated by eukaryotes. Sexual cycles do differ between organisms; however, the core components of the sexual cycle are highly conserved between eukaryotes, from microorganisms such as algae to larger mammals such as elephants, suggesting the selecting forces responsible must be both strong and ubiquitous.

Sexual reproduction is the process by which gametes fuse to recombine the parent’s genomes to create a new genoty-

Image: Flickr

pe via complex mechanisms such as crossing over and gene fusion. Sexual reproduction is likely to have evolved around 1.2 billion years ago in primitive eukaryotes and needless to say, allowed evolution to switch gears from a slow walking pace to supersonic speeds, which lead to the creation of more complex and diverse life forms. Before sex, genetic diversity could only arise through random mutations, which has since been altered profoundly.

Around 1.5 billion years ago, bacterial forms (prokaryotes) demonstrated the most basic form of sex to minimise mutation load: bacterial conjugation. Bacterial cells can transfer their DNA through an intercellular bridge (the pilus) which allows bacteria to replace their faulty DNA by replicating intact sequences from donor cells. However, the basis of the sex cycle has been present in the common unicellular ancestor of all extant eukaryotic lineages. But evolutionary biologists have been asking themselves for years: why is it advantageous for offspring to differ genetically to their parents, despite a suspected twofold cost of sex?

Scientists have spent years trying to discover why sexual reproduction is so abundant in the natural world. In brief, an EMBO article suggested it could have evolved as an incidental consequence of the success of eukaryotes; after all, the first appearance of sex does pre-date the diversification of eukaryotes. Another theory is that genetic complementation compensates for the effects of deleterious genes in one genome by providing a functioning copy of the gene from the other genome: homologous recombination. More specifically, meiosis allows cells to repair DNA damage, and produces large scale genetic diversity, such that all offspring are genetically different to their parents. “Sex produces higher quality genotypes and the genes for sex hitch hike to high frequency on the back of the high fitness genotypes that they create.” A mutation in a haploid individual is more likely to become resident than in a diploid organism, as the DNA repair machinery has no way of knowing what the original undamaged sequence was.

The combination of two sets of parental alleles can create greater diversity than any random mutation. This means that mutations that create advantageous characteristics are less likely to disappear. This allows humans, and countless other species to constantly adapt and create new phenotypes better suited for survival in a changing world.

Scientists have concluded alternative theories, such as that sex evolved as a form of cannibalism. This suggests that sexual reproduction originated from selfish parasitic
genetic elements that exchange genetic material for their transmission and propagation. Sex may also have been present even earlier, in the RNA world that is considered
to have preceded DNA cellular life forms, for example an interaction that occurs in the influenza virus.

Image: Wikimedia Commons

Given the ubiquity of sex, it is by no means essential for reproduction,
and many species can reproduce without bothering with it at all, or alternatively have lost the ability to have sex. Asexual reproduction is abundant in the natural world, and many species can reproduce by both means. This is highly advantageous when members of the
opposite sex are scarce, and has the potential to recover decreasing populations. Some species of sea stars, the female Komodo dragon, parasitic wasps and hammerhead sharks can all reproduce asexually, and hydra, yeast and jellyfish may reproduce by both
methods. It seems an advantage of sexually reproducing organisms to hold a greater genetic diversity, evolving at a much faster rate than those who do not. This is further
supported by asexually reproducing organism’s placement at the tips of the tree of life- suggesting they have a relatively short evolutionary lifespan.

Sex does not only flag importance in the biological sense; history, art, culture are all profoundly influenced by sex. Our modern day society has caused a shift from the original purpose of sex, reproduction, such that sex and sexual identity have become forms of expression. This obsession with sex is by no means limited to humans; any natural world documentary will demonstrate the strange adaptations countless numbers of animal species have acquired in order to attract a mate. Modern scientists increasingly separate sexual pleasure from its biological function. The phenomenon of contraception has allowed couples to enjoy sex without the reproductive consequences, and has largely liberated women from unwanted pregnancies.

It would take a lot to write about everything related, from courtship behaviour in animals, evolution, and the biochemistry of sexual desires, but how and why sex really evolved is unknown. “Ultimately, we might be limited to plausible stories and might never
have a conclusive answer to why sex evolved in the first place.”

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