Good point, I wasn’t thinking about that mechanism.
However, I don’t think this creates an information bottleneck in the sense needed for the original claim in the post, because the marginal cost of storing more information in the genome does not increase via this mechanism as the amount-of-information-passed increases. Each gene just needs to offer a large enough fitness advantage to counter the noise on that gene; the requisite fitness advantage does not change depending on whether the organism currently has a hundred information-passing genes or a hundred thousand. It’s not really a “bottleneck” so much as a fixed price: the organism can pass any amount of information via the genome, so long as each base-pair contributes marginal fitness above some fixed level.
It does mean that individual genes can’t be too big, but it doesn’t say much about the number of information-passing genes (so long as separate genes have mostly-decoupled functions, which is indeed the case for the vast majority of gene pairs in practice).
Here’s the argument I’d give for this kind of bottleneck. I haven’t studied evolutionary genetics; maybe I’m thinking about it all wrong.
In the steady state, an average individual has n children in their life, and just one of those n makes it to the next generation. (Crediting a child 1⁄2 to each parent.) This gives log2(n) bits of error-correcting signal to prune deleterious mutations. If the genome length times the functional bits per base pair times the mutation rate is greater than that log2(n), then you’re losing functionality with every generation.
One way for a beneficial new mutation to get out of this bind is by reducing the mutation rate. Another is refactoring the same functionality into fewer bits, freeing up bits for something new. But generically a fitness advantage doesn’t seem to affect the argument that the signal from purifying selection gets shared by the whole genome.
Good point, I wasn’t thinking about that mechanism.
However, I don’t think this creates an information bottleneck in the sense needed for the original claim in the post, because the marginal cost of storing more information in the genome does not increase via this mechanism as the amount-of-information-passed increases. Each gene just needs to offer a large enough fitness advantage to counter the noise on that gene; the requisite fitness advantage does not change depending on whether the organism currently has a hundred information-passing genes or a hundred thousand. It’s not really a “bottleneck” so much as a fixed price: the organism can pass any amount of information via the genome, so long as each base-pair contributes marginal fitness above some fixed level.
It does mean that individual genes can’t be too big, but it doesn’t say much about the number of information-passing genes (so long as separate genes have mostly-decoupled functions, which is indeed the case for the vast majority of gene pairs in practice).
Here’s the argument I’d give for this kind of bottleneck. I haven’t studied evolutionary genetics; maybe I’m thinking about it all wrong.
In the steady state, an average individual has n children in their life, and just one of those n makes it to the next generation. (Crediting a child 1⁄2 to each parent.) This gives log2(n) bits of error-correcting signal to prune deleterious mutations. If the genome length times the functional bits per base pair times the mutation rate is greater than that log2(n), then you’re losing functionality with every generation.
One way for a beneficial new mutation to get out of this bind is by reducing the mutation rate. Another is refactoring the same functionality into fewer bits, freeing up bits for something new. But generically a fitness advantage doesn’t seem to affect the argument that the signal from purifying selection gets shared by the whole genome.