Why Do We Have Sex? Part 2

Although there are countless exceptions and variations when it comes to reproduction, there is one fundamental characteristic that typically distinguishes between sexual and asexual organisms - the structure of their genes.

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Most asexually reproducing plants and animals carry a single complete set of genes in their cells. In biological terms their genes are haploid, which is just a Latin word for simple.

Sexually reproducing organisms, on the other hand, are generally die-ploid, meaning they carry two complete sets of genes. Every one of your cells effectively has twice the genetic information it would take to make a single person. Your mother and father each contributed genes that encode the color of your hair and eyes, the size of your nose, the proportions of your limbs, and so on. Your mother's genes determine some of your traits, your father's genes determine others, and some of your traits are determined by both your mother's and father's genes.

Asexual reproduction has some distinct benefits. For one thing, it's fast. If a typical bacterium takes twenty minutes to split in two, it will have eight descendants in an hour, and sixty-four in two hours.

If there's sufficient food around to support a population explosion, a single germ dividing at this rate could boast more than a million offspring in seven hours, and over a billion in ten hours. It's what physicists call exponential growth.

Sexual creatures can grow their numbers exponentially as well, although not as fast as asexuals. If two populations of organisms, identical in every way except for their mode of reproduction, were to squeeze into the same ecological niche, the asexual population wins, at least in the short term.

Rabbits are the iconic example of animals that breed like, well, rabbits. Imagine a fertile valley that's settled by two colonies of ten rabbits each. One colony consists of five male and five female rabbits that reproduce once a year in the usual way. The other is a colony of ten asexual females that also reproduce once a year, but have no need of males (fortunately this is a purely hypothetical type of rabbit).

If every female rabbit can bear a litter of ten babies each season, then in one year the sexual rabbits increase their numbers by five litters - one litter from each female - resulting in fifty babies plus the ten original colony members, for a total of sixty sexual rabbits. The asexual colony has ten litters, one hundred babies, plus the original ten for a total of a hundred and ten rabbits.

Presumably, the amazons give birth only to females, while the sexual rabbits produce half male and half female babies. After a single breeding season, the females in the colony of asexual rabbits outnumber the sexual females by nearly four to one.

The all-female asexuals will swamp the sexual rabbits in a few generations. Even if there are limitations of food and water in the valley that keep the total number of rabbits in check, the asexuals efficient breeding scheme allows them to overrun the sexual rabbits in short order.

As you can see, males are the true liability in breeding populations - they eat food that could go to the girls, produce waste, and take up precious space in the colony. But they are of little help in increasing population numbers other than donating sperm, which asexuals can live without.

Why don't we see asexually reproducing rabbits, squirrels, rats, elephants, or humans in the real world? The answer lies in adaptation to stress. And we can thank males for that.

Asexual populations consist essentially of clones, with each child carrying exactly the same genetic material as its parent. If a new disease, parasite, or predator were to come along with a particular talent for attacking our asexual rabbits, the whole population could be rapidly decimated.

Sexual rabbits have a better chance of surviving in the face of stress thanks to the presence of the boys. In mating, the male and female of a species each contribute a portion of the offspring's genetic material, which means babies are always at least slightly different from their parents. Sex stirs the genetic pot, leading to combinations that may occasionally handle stresses better.

When a fox finds a valley full of bunnies, you might imagine that it eats the slowest ones first. All the asexual rabbits are equally swift because they're identical. If the fox can catch one it can catch them all.

Some of the sexual bunnies however, will be faster than others as a result of the variability that comes from male-female breeding. Pretty soon, the pressure of having a fox hanging around might lead to natural selection of fleet-footed bunnies. Of course, rabbits could deal with foxes in other ways - developing better camouflage, enhancing their wariness, or growing wickedly sharp claws. But in any case it's the sexual ones that have the potential of finding solutions, while the asexuals are doomed.

Once sexual rabbits have developed an adaptation to deal with a specific stress, you might wonder what is to prevent them from spontaneously changing reproductive tactics to become a new asexual super rabbit that can fend off a given type of threat.

Based on some physicists' models, the primary reason is that foxes, germs and parasites evolve as well. A rabbit that adapts to the stress of a certain fox causes stress for the fox by denying him food, which in turn leads to the evolution of better hunters, forcing rabbits to evolve further, and so forth. Populations of rabbits and foxes ebb and flow as each adapt to changes in the other, leading to long-term stability of predators and prey that is maintained by sexual mixing of each species' genetics.

As organisms evolve, they face lots of shifting stresses, which firmly establishes sexual reproduction as the procreation method among just about everything larger than an amoeba.

Even if there were no threat of predators, parasites or diseases, all life faces the risk of random genetic mutations. Mutations are changes that arise from errors that occur when DNA replicates, or from exposure to things like radiation and chemicals. Asexual organisms that have a single precious copy of their DNA are in deep trouble as errors accumulate. Sexual organisms gain protection through their genetic redundancy - if an error develops in a gene contributed from one of your parents, repair mechanisms in your DNA can use the genes from your other parent as a map for repairing the problem. Or the problem may be moot if a healthy gene is dominant over the flawed copy.

In short, sex provides multiple levels of genetic protection. It offers a route to adaptation through gene shuffling, ensures backup copies of genes are available, and keeps genes in good shape with DNA repair mechanisms.

Tune in next time for part three of, “Why Do We Have Sex.”

I'm Kate. Thanks for subscribing to The Physics of Sex podcast.