When posed with this question, most people will answer some combination of “to make babies” or “it feels good.” These answers are both true, but they are only part of the story — they describe what biologists call “proximate causes.” A proximate cause is one that explains the presence of a trait during the lifetime of the individual, compared to “ultimate causes” which are the reason why the trait evolved in the first place.
Take, for example, the opposable thumb. The proximate causes for this trait include the genes that code for it and the developmental processes that create the right environment for it to occur. The ultimate cause of the trait is that it allows us to manipulate objects in our environment with greater dexterity, therefore allowing us to get more food and survive better. All traits have an ultimate cause and a proximate cause, and both are integral to understanding the trait.
To make things a little bit more complicated, “to make babies” can be both a proximate and an ultimate cause. One thing that motivates people to have sex is the intention of making a baby. This motivation is a proximate cause. Making babies is also the ultimate cause of sexual reproduction. But even this is not the whole story. There are two basic methods of reproduction: sexual and asexual. Across all life, sexual reproduction is not the default position. Of the three lineages of life — bacteria, archaea and eukarya — only one (eukarya; the lineage to which we belong) typically reproduces sexually, and there are many species in this lineage that are asexual. Why do some types of organisms reproduce sexually and others asexually?
To put this question into perspective, let’s talk about the down-sides of sexual reproduction:
First, there is the problem of males. What exactly do males do? In a population of asexually-reproducing organisms, every individual is reproducing. Because males cannot have babies, a population of sexually-reproducing organisms can only reproduce half as quickly (because only half of the individuals are capable of reproducing). Males consume resources that would otherwise be available for females and young. Males provide parental care in relatively few species.
Second, there is the efficiency of passing on genes. Everyone knows that reproduction is about passing on your genes. When reproducing asexually, each offspring is a genetic clone of you, meaning that you are related to each offspring by 100%. With sexual reproduction, each offspring is only related to you by 50%, meaning that you need to have twice the number of offspring when reproducing sexually to match what you would have when reproducing asexually. For accounting purposes, each offspring only counts as a half.
Third, there is the problem of finding mates. In an asexual species, an individual may spend all of their energy on survival and reproduction. In a sexual species, individuals must expend energy on finding mates at the expense of survival and actually producing offspring.
I could make this list longer, but I hope I’ve convinced you that sexual reproduction has it’s shortcomings. So why on earth would the most complex organisms on earth reproduce this way?
The answer is genetic diversity. Sexually reproducing organisms are much less vulnerable to changes in the environment than asexual ones, especially when it comes to infectious diseases. Normally, individuals of a species differ in their susceptibility to a particular pathogen, and a large part of this variation is due to genetics. Some people don’t usually get the flu, whereas other people get it often and badly. This is because people are physiologically different, and therefore the flu virus cannot infect all people equally well. When two individuals reproduce sexually, their genes get mixed together in new ways. This creates a constantly moving target for pathogens. Asexual organisms do not need another individual to mate with, but all of their offspring are genetic clones of the parent. When there is lower genetic diversity, a pathogen can evolve to be better at infecting the host species to devastating effect. For example, the “regular” bananas that we buy in the store are a single strain called “cavendish” bananas. Not only are they a single strain, but every tree that produces them are genetic clones of each other. There is essentially zero genetic diversity among them, and as a result, a fungal disease is currently decimating the world’s crops.
Infectious disease is one of the biggest problems for life on earth — even many organisms that are themselves diseases can become infected by diseases. The costs of sexual reproduction may be high, but not as high as the costs of reproducing asexually when there are infectious diseases around.