“Let’s say that you have a complex adaptation with six interdependent parts, and that each of the six genes is independently at ten percent frequency in the population. The chance of assembling a whole working adaptation is literally a million to one; and the average fitness of the genes is tiny, and they will not increase in frequency. ”
Right—but look at the premise. Genes have linkage to other genes on the same chromosome—and so their frequencies may be far from independent. The existence of this possibility actually creates a selection pressure for interdependent genes that contribute to an adaptation to migrate towards each other on chromosomes—so they have more chance of being inherited together.
Two other factors:
1) Population sub structure matters. Suppose a population of one million is divided into mating bands with 30 individuals. Small bands tend to lose diversity so some bands would have some of the minor alleles at higher frequency. Now suppose band X has minor alleles A1, A2, and A3 at high frequency while band Y has minor alleles A4, A5, and A6 at high frequency. The two bands meet and party. The result is kids with all 6 minor alleles. Those kids have big fitness advantage and those minor allele frequencies are significantly boosted in those bands. The high local concentration of those alleles means even more kids with all 6 alleles are born, further increasing their frequency. (If individuals were equally likely to mate with anyone in the population then local concentrations would be diluted in one generation and there would be no effective selection. But individuals are far more likely to mate with related nearby bands, so high local concentrations of the minor alleles are maintained while the minor alleles slowly become the major alleles.)
2) Gene variants tend to have additive affects. Also most genes affect multiple traits simultaneously. So the all or nothing scenario given above would be rare. More likely you would have a diversity of environmental niches. In some of those niches the minor alleles would provide benefit due to one of their affected traits becoming more important. The frequencies of that minor allele would locally rise (while its frequency in the total population would remain low). E.g., a minor allele might provide protection against a specific pathogen. So there might be local environments where the probability of 6 minor alleles combining could be much higher than would occur in one large population in a uniform environment mating randomly.
I checked the Wiki here: http://wiki.lesswrong.com/wiki/Human_universal
“Let’s say that you have a complex adaptation with six interdependent parts, and that each of the six genes is independently at ten percent frequency in the population. The chance of assembling a whole working adaptation is literally a million to one; and the average fitness of the genes is tiny, and they will not increase in frequency. ”
Right—but look at the premise. Genes have linkage to other genes on the same chromosome—and so their frequencies may be far from independent. The existence of this possibility actually creates a selection pressure for interdependent genes that contribute to an adaptation to migrate towards each other on chromosomes—so they have more chance of being inherited together.
Two other factors: 1) Population sub structure matters. Suppose a population of one million is divided into mating bands with 30 individuals. Small bands tend to lose diversity so some bands would have some of the minor alleles at higher frequency. Now suppose band X has minor alleles A1, A2, and A3 at high frequency while band Y has minor alleles A4, A5, and A6 at high frequency. The two bands meet and party. The result is kids with all 6 minor alleles. Those kids have big fitness advantage and those minor allele frequencies are significantly boosted in those bands. The high local concentration of those alleles means even more kids with all 6 alleles are born, further increasing their frequency. (If individuals were equally likely to mate with anyone in the population then local concentrations would be diluted in one generation and there would be no effective selection. But individuals are far more likely to mate with related nearby bands, so high local concentrations of the minor alleles are maintained while the minor alleles slowly become the major alleles.)
2) Gene variants tend to have additive affects. Also most genes affect multiple traits simultaneously. So the all or nothing scenario given above would be rare. More likely you would have a diversity of environmental niches. In some of those niches the minor alleles would provide benefit due to one of their affected traits becoming more important. The frequencies of that minor allele would locally rise (while its frequency in the total population would remain low). E.g., a minor allele might provide protection against a specific pathogen. So there might be local environments where the probability of 6 minor alleles combining could be much higher than would occur in one large population in a uniform environment mating randomly.