Evolution of Separate Sexes and the Sex Ratio

Why do humans — and a lot of other species — have different sexes? Why are some individuals males and others females? Evolutionary biologists have offered a couple of suggestions to answer these questions.

Viral sex

Sex in viruses is quite a bit different from what we commonly think of as sex. In fact, viruses typically reproduce asexually. In phi-6, sex works like this:

1. The virus injects its genome, consisting of three segments, into an unlucky bacterium.

2. The virus hijacks the cell's biochemical machinery to make more copies of itself.

3. Out pop progeny viruses, each of which has copies of the three parental genome segments.

Nothing about this process requires a second virus. Viruses don't need mates and are perfectly able to reproduce asexually. Sometimes, though, two viruses simultaneously infect the same host cell, and all the various replicating bits get mixed together. Now when the new viruses pop out, they can have genome segments from both of the original viruses (that is, one segment from one virus, and the other two segments from the other virus).

These new viruses have two "parents" instead of just one. This process is what scientists are referring to when they talk about viral sex.

Given that two sexes do exist, the next question for evolutionary theory is how many offspring of each sex an individual should make. And how is it that species with separate sexes end up with a 50-50 sex ratio: 50 boys for every 50 girls? It turns out that 50-50 sex ratio is something that is perfectly and easily explained by natural selection.

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