Munch Lab

Waiting for good mutations

We know intuitively that individuals from small isolated populations are often more related with each than individuals in large populations are. Whereas closely related individuals have a shared relative in the recent past, distantly related individuals will have to look back many many generations to find such a common ancestor. This leaves more time for genomic mutations to produce differences between the two individuals, and this is why individuals from large populations show more differences between their genomes than individuals from a small one do.

So large population size go hand in hand with large genetic diversity. Species are isolated populations because they do not interbreed with the populations of other species. Among the great apes, bonobos and chimpanzees exhibit the least genetic diversity. Gorillas have larger populations and higher diversity only superseded by orangutans.

In each species, the genetic diversity is not even across the genome. Some regions of the genome have lower diversity because natural selection removes harmful mutations in regions, such as genes, of particular importance to the organism. This produces a landscape of higher and lower diversity that is shared between the great apes because the genes and other important regions are located in roughly the same places.

Temporary differences in this landscape of diversity may arise from events of adaptation. Once in while a beneficial mutation comes along, which gives the individuals that carry it an advantage in the natural selection. When such a mutation quickly spreads to the entire species the mutation and the region around it, becomes shared by all individuals – which obviously depletes genetic diversity in that part of the genome. Such depletions in diversity are called “selective sweeps” and supply evidence of how often new beneficial mutations are introduced. In a paper that is published in PNAS today, we use this principle to show that larger populations seem to experience such sweeps more often than small ones: orangutangs more so than gorillas more so than chimpanzees and bonobos. We think this reflects that adaptation is limited by how often good mutations arise in the population. If twice as many good mutations happen in a population with twice as many individuals, then twice as many sweeps should occur.

It turns out that humans are the great apes with the slowest rate of sweeps. You may think this is odd considering that we are six billion people. The reason is this only happened in the last ten thousand years or so, a mere blink of an eye to evolution. In the rest of our history, we were only a small crowd. – But maybe, from now on, we will not have to wait so long for the good mutations.

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