Nick Lane

One of them is the population size.

So in bacteria, if you've got a small population and they're not sexual, then you accumulate mutations in that population.

But if you've got a much larger population, the closer you kind of get towards an infinitely large population, they're not all going to accumulate the same mutations.

And so the population as a whole is going to be fine.

And this kind of goes back decades in population genetics.

But the other thing which is less explored in population genetics is the size of the genome.

So if you, with bacteria, if you increase their genome size up to eukaryotic-sized genomes, you can't maintain a larger genome.

You'll accumulate mutations in that genome, and it'll shrink again.

And with the Y chromosome, yes, it shrunk.

It's a tiny chromosome in comparison with all of the rest.

So it's really how many genes can you maintain in a good state?

And with the Y chromosome, basically, you only need a couple of genes in there.

Basically, it's the SRY gene is saying grow faster.

And you only need that to remain functional.

And then selection at the level of fertile or infertile men will kind of weed out the ones that have got a non-functional SRY gene.

So it's not as if you've got a patchwork.

You can afford to degenerate your Y chromosome down to almost nothing, and you'll still be functional.

It's gone down from, say, 3,000 or 4,000 genes to, in our own case, 37 genes.

So you cannot sustain a large genome if you're inside.

I said population size matters.