Why have sex?
Why is sex a better reproductive strategy than cloning yourself?
Humans, like all mammals, reproduce sexually. Many organisms, though, don’t bother. In fact, asexual reproduction seems to have many advantages – it’s quick and easy because you don’t need to find a suitable partner.
Pond-dwelling algae called Volvox, for example, simply clone themselves. Bacteria divide in two. Even some animals, like Komodo dragons, can make babies without mating.
Every new organism produced by asexual reproduction is genetically identical or very similar to its parent. Over the long term, however, this might not be the best way to ensure your survival (although bacteria and viruses have done quite well…).
More than a century ago, August Weismann recognised that sexual reproduction – the marriage of genes from two parents – increases genetic variation. Too little variation within a population can make it susceptible to disease or changing conditions.
Bringing together new combinations of genes helps species evolve and adapt to their environment. One of the reasons humans are successful as a species is that sex mixes up our genes in a way that asexual reproduction cannot. This makes each of us different from our parents. It also means that although we each have our own genetic weaknesses, we won’t all be wiped out by a flu epidemic.
Genetic shuffling (random assortment) in sexual reproduction occurs due to meiosis and the fusion of male and female gametes. During meiosis – a special type of cell division that produces the gametes – chromosomes can ‘cross over’ at points called chiasmata. The shuffling process leads to new, potentially advantageous combinations of genes coming together and allows for harmful mutations or variations to be eliminated or masked.
On the other hand, sex has its drawbacks. It takes two organisms to have offspring, while each member of an asexual species can produce its own young. So an asexual population can grow more rapidly.
Many animals are capable of sexual and asexual reproduction. Starfish-like brittle stars can split themselves in two, but they can also make eggs and sperm. Strawberry plants can produce plants identical to themselves, but they can also be pollinated by bees carrying pollen from other plants.
However they do it, sex is a big part of many organisms’ lives. And it has led to astonishing diversity in the animal world.Lead image:
Illustration © Glen McBeth