The Birds and the Bees

Why does nature favour sex?

The natural world revolves around sexual reproduction. Through it, the majority of species guarantee their survival. If we are to believe Freud, it also drives our own behaviour. It may come as a shock then, to realise that some of the best minds in the field have struggled to explain why sexual reproduction exists.

There are a number of families of species that breed asexually. One group of these are the Bdelloid rotifers. These water-dwelling, microscopic animals have been abstinent for the last eighty million years. To asexually reproduce, the female produces an embryo identical to itself without fertilization. As there are no males in the population, every individual can bear young. This potentially allows the population to grow at twice the rate, because no resources are wasted on males who cannot produce offspring.

There are even cases of asexual reproduction amongst reptiles and birds. Turkeys and Komodo dragons are just two of the species that can produce offspring without fertilization. This form of reproduction carries a number of benefits. Asexual individuals need not worry about finding and attracting a mate, competing with rivals, or the risk of sexually transmitted diseases (STDs)—costs which we as humans are prone to struggle with.

Given these disadvantages, why does nature bother with sexual reproduction? A possible answer is that sexual reproduction produces a faster means of evolution, due to the recombination (mixing) of parental genomes, which is lacking in asexual reproduction.

In animals, sexual reproduction involves the meeting of sperm and ova (collectively known as gametes), which are produced in males and females respectively. Gametes are produced when a single cell containing two copies of each chromosome (one from each parent of the individual) divides to form two new cells that carry single copies of each chromosome. Each chromosome in a new gamete is a unique combination of maternal and paternal genomes. This is the reason why we are similar, but not identical to our siblings. In contrast, asexual species pass down genes from a single individual and as a result produce identical offspring.

In the case of sexual reproduction, natural selection will result in the spread of mutations of beneficial effect throughout the population. In an asexual population, each mutation occurs in one individual and its progeny. Different lineages have different beneficial mutations. Because there is no exchange of genes between these lineages, it is impossible for the population to accumulate all the positive mutations at once. Instead, the lineages compete with each other. The lineage with the most beneficial mutations outcompetes the others, causing their extinction. While this provides a reason for sexual reproduction, it is unlikely that the benefits are sufficiently large to overcome the problems that sex entails.

Several other theories have evolved in an attempt to understand nature’s preference for sex. One of the most prominent is the “Red Queen hypothesis”, named after the scene in Lewis Carrol’s classic where the Red Queen tells Alice, “It takes all the running you can do, to keep in the same place”. The evolution between parasites and their hosts is an example of this: parasites evolve ways to exist undetected, while their hosts evolve immune mechanisms to avoid them. Parasites target the most common genotype in a host population, hence efficient exchange of genetic material in the host species is favourable as it results in a rapidly changing common genotype, offering protection against parasites. This could only occur via recombination, which, in turn, occurs through sexual reproduction.

Given the benefits of sexual reproduction, the existence of asexual species seems odd. In fact, asexual reproduction allows the survival of species in extreme circumstances—for example, the female Komodo dragon will reproduce asexually when there are no potential mates.

However, in general, sexual reproduction is the rule and asexual reproduction is the exception. While there is not yet a generally accepted theory to explain this, it seems likely that the ability of sexually reproducing species to acquire multiple genetic advantages through recombination plays a key role.

Art by Anna Pouncey.

About Jeremy Brown

Jeremy Brown '12 is an undergraduate studying Biological Sciences at Queen’s College.