In my post below there is a reference to fast evolution in a relatively slow-breeding species, H. sapiens. For this to be plausible you need high selection coefficients, that is, the difference between mean population fitness and the fitness of those who are carrying the favorable allele. How plausible is this? R.A. Fisher argued against selection coefficients of large effect because he believed that mutations of large effect would usually "overshoot" the idealized fitness peak, and it was mutations of small gradual effect which were the real drivers of evolution. But recent work has reemphsized the possible importance of fast sweeping positively selected mutations across populations. And with that, I point you to How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles. Relevant sections:
Crude estimates on the strength of selection on phenotypic traits and effect sizes of quantitative trait loci (QTL) suggest that selection coefficients for leading QTL underlying phenotypic traits may be high enough to permit their rapid spread across populations. Thus, species may evolve collectively at major loci through the spread of favourable alleles, while simultaneously differentiating at other loci due to drift and local selection.
The full text is available for those without academic access at the link provided.
Comments
I think that the historical record implies numerous genes of large effect. Plagues that wipe out 30%-90% of the population are not uncommon, not to mention the continuing plague throughout history that killed off 80% of children before they could reproduce. When Europeans came in contact with agriculture, there were likely new plagues that killed of 90% of the population, similar to when the Americas came into contact with Europeans. Similarly, human aggression (and subsequent assimilation of remnant populations) could have a similar effect.
Also, there's a tipping-point effect here, that you don't need genes of large effect, in a measurable sense, to get a large selection gradient.
Posted by: David Boxenhorn | March 15, 2006 11:11 PM
Those numbers are mostly wrong, David. The idea that 80% of kids didn't make it to reproductive age is especially wrong.
Posted by: gcochran | March 16, 2006 9:06 AM
The idea that 80% of kids didn't make it to reproductive age is especially wrong.
That a relief. But I was under the impression that 10 births per woman was the norm (for those who lived a full life span). In general, populations increased only very slowly until recently, so that means that only about 2 managed to reproduce. (BTW I said "reproduce", not "reproductive age".)
In any case, we see everywhere that the transition from hunter-gatherer society results in a population crash, so it's not hard to imagine genes with more than 5% fitness boost in that context.
Posted by: David Boxenhorn | March 16, 2006 11:27 PM