“Given sexuality and chromosome assortment but no recombination, a species with 100 chromosomes can evolve much faster than an asexual bacterial population!”
No, it can’t. Suppose that you want to maintain the genome against a mutation pressure of one hundred bits per generation (one base flip/chromosome, to make it simple). Each member of the population, on average, will still have fifty good chromosomes. But you have to select on the individual level, and you can’t let only those organisms with no bad chromosomes reproduce: the chances of such an organism existing are astronomical.
A sexual population can’t handle more lethal mutations than an asexual one. Since the mutation is lethal, before a cell carrying it can have sex and reproduce, it dies.
And just as an asexual population can’t handle a mutation rate that swamps it (say, 1 very deleterious mutation per cell per generation) a sexual population with 100 chromosomes can’t handle 100 per cell per generation.
But still the sexual population can do better. Here’s why:
Since back-mutation is rare, an asexual population gets onto a slippery slope when it loses all its nonmutants. If the time comes when there are no individuals without an unfavorable mutation, then the exact same mechanism which got them there will get them to a time when there are no individuals without two unfavorable mutations and so on. (See “Muller’s Ratchet”.)
But a sexual population can maintain an equilibrium just fine where the average individual has 8 or 10 unfavorable mutations, provided those individuals are still viable.
Consider the simple example—one individual has one mutation, a second individual has another unlinked one. They mate and produce 4 offspring, and the average result would be 1 wild-type, 2 with one mutation and 1 with 2 mutations. More room for selection than just 2 cells with a mutation each.
Sexual populations depend on the average cells, and the distribution of mutations is dependably binomial. Asexual populations depend on the wild-type to outcompete everything else, and when the superior wild-type gets to be too small a fraction they become undependable.
“Given sexuality and chromosome assortment but no recombination, a species with 100 chromosomes can evolve much faster than an asexual bacterial population!”
No, it can’t. Suppose that you want to maintain the genome against a mutation pressure of one hundred bits per generation (one base flip/chromosome, to make it simple). Each member of the population, on average, will still have fifty good chromosomes. But you have to select on the individual level, and you can’t let only those organisms with no bad chromosomes reproduce: the chances of such an organism existing are astronomical.
A sexual population can’t handle more lethal mutations than an asexual one. Since the mutation is lethal, before a cell carrying it can have sex and reproduce, it dies.
And just as an asexual population can’t handle a mutation rate that swamps it (say, 1 very deleterious mutation per cell per generation) a sexual population with 100 chromosomes can’t handle 100 per cell per generation.
But still the sexual population can do better. Here’s why:
Since back-mutation is rare, an asexual population gets onto a slippery slope when it loses all its nonmutants. If the time comes when there are no individuals without an unfavorable mutation, then the exact same mechanism which got them there will get them to a time when there are no individuals without two unfavorable mutations and so on. (See “Muller’s Ratchet”.)
But a sexual population can maintain an equilibrium just fine where the average individual has 8 or 10 unfavorable mutations, provided those individuals are still viable.
Consider the simple example—one individual has one mutation, a second individual has another unlinked one. They mate and produce 4 offspring, and the average result would be 1 wild-type, 2 with one mutation and 1 with 2 mutations. More room for selection than just 2 cells with a mutation each.
Sexual populations depend on the average cells, and the distribution of mutations is dependably binomial. Asexual populations depend on the wild-type to outcompete everything else, and when the superior wild-type gets to be too small a fraction they become undependable.