Sex and evolution

What role does sexual reproduction play in evolution? In this article, we will consider how recombination and genetic variation drive evolution.

Almost all species of animals and plants reproduce sexually, a fact that is a fundamentally important part of evolution. There are males that produce huge numbers of tiny male sex cells or gametes (called spermatozoa in animals and pollen in flowering plants), and there are females that produce many fewer, but vastly larger female gametes called egg cells. Fusion of one male with one female gamete re-combines genes from the two parents, to create a fertilised cell, called a zygote, which will eventually develop into a new offspring. Note that a female provides all the energy, nutrients and protection that the zygote needs for its growth and development. In contrast, the male invests extremely little, because the spermatozoa contributes nothing other than a set of his genes carried on chromosomes. This very unequal investment between female and male in the offspring has fascinating consequences.

Why males exist – the meaning of recombination

The first question that arises is why sex exists at all! As we know, evolution is driven by the success with which an individual passes on its genes to future generations. Why then does a female go to the trouble of finding a mate, and then spending a large amount of her time and resources on the eggs and often the young, when the result is to pass on as many of his genes as her own? Instead she could pass on twice as many of her own genes for the same effort.  The female pays what is called the “twofold cost of sex”, as the diagram below illustrates.

Asexual vs sexual reproduction: twofold cost
Asexual vs sexual reproduction: the twofold cost

The left side shows asexual reproduction without males. If a female produces two offspring, they are both females, and these in turn produce four granddaughters, all carrying only her genes. The right side shows that in sexual reproduction, the female produces on average one daughter and one son, each carrying only half her genes. The daughter in turn produces only one granddaughter and one grandson each carrying half her genes.

There must be a huge selective advantage to sexual reproduction in order to balance the disadvantage of the twofold cost. The benefit lies in the greatly increased genetic variation amongst the offspring that comes from the random recombination of half the genes from each parents, although there is a good deal of argument about why this is so important. In part it may simply ensure that at least some of the offspring are well adapted even if the environment is unpredictably variable. Recombination is also a way of getting rid of a disadvantageous mutation. Each offspring unlucky enough to have inherited one from a parent is less likely to survive, and therefore the deleterious mutation will soon disappear from the population. Variation of successive generations also helps to prevent disease microorganisms from overwhelming a population.

Asexual reproduction, called parthenogenesis, is actually found in quite a lot of species, especially amongst plants but also in several fishes and reptiles. They consist only of females but, as we would expect from their lack of variation, they eventually become extinct.

Why there is the same number of each sex – the cost of sons and daughters

Even accepting the advantages of sex, you might still wonder why there are usually an equal number of males as females in a population, when only a small number of males are needed to fertilise many females. Indeed this is often obvious in practice such as in the elephant seal, where a single male guards a harem of a dozen or more females. But think what would happen if there were in fact fewer males. Each one would have a higher chance of finding a mate. Therefore a mother would improve her chances of passing on her genes if she produced more sons than did other females. But if all the females did that, there would soon be more males than females, at which point the reverse would happen and producing daughters would become more advantageous. Overall therefore, selection for gender balance maintains the two sexes roughly equally.

Giant seal fights off rival male (BBC Earth)

Why males are often larger and showier – sexual selection

Given that more males exist than are necessary, they have to compete for the opportunity to attract and mate with a female. The males of most mammals for example are larger than the females, reflecting the use of direct aggression and physical combat towards rivals. But in most animals, the competitiveness is more indirect.  The male peacock opens his absurdly showy tail to the dull brown female. Male birds of paradise have the most fantastic colours, feather forms, and patterns of courtship behaviour, as this video shows.

Bird of paradise courtship spectacle (BBC Earth)

Male frogs croak their loud nocturnal mating choruses, as you can hear in the coqui frogs of Puerto Rico in the video below.

Coqui frogs

These and the enormous number of other examples of what are called secondary sexual characters evolve thanks to a process called sexual selection. If a difference in appearance or behaviour of one male compared to the others is positively attractive to females, he has a higher chance of mating and so passing on his genes. Furthermore, the greater the expression of a secondary sexual character, the more successful it often is. The character evolves more and more prominently for as long as its attractiveness to females continues to increase. Theoretically a point is reached where the advantage gained by the signal is balanced by a reduction in fitness caused, for example, by increased vulnerability to predators because of conspicuous plumage, or by the surprisingly high amount of energy it takes a frog to croak all night. As to why such expensive signals should be attractive to females, one theory is called the handicap principle. A male that can survive despite the cost of extravagantly showing off must be a pretty strong, healthy individual, and consequently a desirable father for her offspring.

Think about this further…

Humans, Homo sapiens, is also a biological species. Think about how much of our appearance and behaviour today might be a result of evolution by sexual selection millions of years ago. Now consider how much our behaviour has changed due to the development of society and civilisation over the last few thousand years, too short a length of time for much genetic change to have occurred.

Dr Tom Kemp
Dr Tom Kemp

Dr Tom Kemp, Emeritus Research Fellow in Biology

I spent many years at Oxford University’s famous Natural History Museum, researching fossils of vertebrate animals, and thinking and writing about evolution. At the same time, I was lucky enough to be Biology Tutor at St John’s College, where I grew to love teaching these subjects. Even though I am supposed to be retired now, I still write and teach, and it is a privilege to be involved in helping you come to understand how important evolution is to understanding much about the world we live in.