Nick Lane

And this goes even for single-cells things that don't have any obvious differences between gametes, which is to say they don't have oocytes and sperm or anything like that.

They produce little motile gametes that look more like sperm than anything else.

Both sexes would do that.

But...

By definition, the female sex passes on the mitochondria and the male does not.

And that's an approximation.

It's not always true.

There's exceptions to that rule, but it's a kind of a rule of thumb in biology that the females pass on the mitochondrial DNA.

So why would that happen?

With sex, what you're doing is you're increasing the variance in the nuclear genome.

and you're subjecting that to selection and the winners are coming through that and everything which is worse than it would have been gets eliminated by selection.

So you're effectively, you're increasing variance on nuclear genes, the genomes, and then selecting for what works.

With the mitochondria, they're passing on asexually down the generations.

It's a very small genome, but there's multiple copies of it.

And so the question is, well, how do you keep that clean?

How do you prevent that from degrading and degenerating over time?

Because if you've got, let's say, if you've got 100 copies of mitochondrial DNA and two of them acquire mutations, but you've still got 98 which are doing their job fine, what's the penalty for those two mutations?

It's not very much.

You'll hardly notice them.

So now you require another couple of mutations and you can degenerate over time.