The first person who wondered about the evolution of sex did so while looking at his penis. Illustration by Allan Turton
The first person who wondered about the evolution of sex did so while looking at his penis. Illustration by Allan Turton

To this day, evolutionary biologists are trying to understand why evolution would favour the seemingly inconvenient process of sex, when asexual reproduction allows for organisms to reproduce without the trouble of finding a mate, while avoiding the inefficiency of having to produce offspring of two different sexes.

 From a biological perspective, sexes differ only in that one sex (females) produces large gametes while the other (males) produces small ones.  In many cases this means that  males compete to give their small gametes access to the large gametes of females, and thus a greater proportion of females get a chance to pass on their genes than do males. Therefore, from a certain evolutionary perspective, potentially giving birth to males seems like a bad genetic strategy.

So why then has sex evolved, and for that matter, why has it come to be the dominant form of reproduction? There are several theories that attempt to answer this question, but many centre around the same broad assumption: the efficiency of asexual reproduction allows for the proliferation of all mutations, even those that are disadvantageous.

Sex, however, allows for the recombination of genes to increase fitness in subsequent generations. One model, known as the Red Queen hypothesis, suggests that organisms must continuously adapt, so that their potential parasites and predator do not adapt disproportionately advantageous traits. Sex allows organisms to keep up in this evolutionary race.

 Another model, Muller’s Ratchet focuses on the fact that once a mutation develops in an asexual individual, it is almost impossible for their offspring not to inherit it, leading to the proliferation of potentially harmful mutations in subsequent generations. Sexual individuals can overcome Muller’s Ratchet, however, by recombining their genes in their offspring, decreasing their chances of passing on harmful mutations.

 UofT’s own Agrawal lab is amongst those trying to provide empirical models to match pre-existing theories about the evolutionary advantages of sex. One of the lab’s model organisms, Brachionus calyciflorus, a species of rotifer, is used to provide insight into questions about sexual advantages. Though perhaps not one of the world’s most known animals, the rotifer is an abundant and fascinating water dwelling invertebrate. The vast majority of rotifers are asexual females, however, when exposed to a triggering level of pheromones released from other rotifers they can lay summer eggs containing sexual males and females. Male B. calyciflorus are much smaller than females and only live until they reproduce, which they do by piercing female body cavities to inject their sperm.

 Because the genetic compositions of rotifers can allow them to be provoked to become sexual at different pheromone thresholds, they are an ideal organism for testing whether there are evolutionary advantages for sexual as opposed to asexual reproduction.

 A prime advantage of sex is that it allows organisms to adapt to a steadily changing earth.  Theoretically therefore, if the world’s environments become stable, asexual reproduction may become the biological norm. Since that will almost certainly never happen,  however, sex is here to stay; and whatever its social and medical shortcomings, it will continue to serve the interests of humans and rotifers alike.  

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