[sgt101]

I think GCA originated in 1954; I don't know how many generations of people breeding it would take to get a selected trait - humans are genetically very similar compared to other species; there is not much variation... so I don't have any idea of what the breeding program length would be, but I assume that it would be several generations... so I don't think that an idea that has been around for 60 years would really qualify as fixed and agreed in that time scale.

Also I don't think it's agreed.

Also it's not clear that your statement that population level changes can be created by selective breeding is true - if a characteristic is dictated by alleles that damage fertility or other fitness indicators then it may well not be continuous. In the case of human intelligence the increased child birth mortality associated with large heads and the increase dependence of infants on parents are examples. There may be others which are more relevant to modern life, like propensity to depression, disinterest in sex etc (note : I am using these as examples, not saying that this is so).

[danieltillett]

If there are some individual in the population with the particular phenotype you are selecting who are not infertile then selective breeding can increase the frequency of that phenotype in the population. In the case of GCA we know that there are fertile individuals with very high GCA so we know it is possible to selectively breed for high GCA.

Of course selecting only for GCA may well increase the frequency of other phenotypes, but unless GCA is directly linked to that phenotype then there is no reason you can’t select against any undesired phenotype at the same time. This is done all the time with the selective breeding of animals and plants. I know of no phenotype shared by all individuals with a high GCA so there should be no phenotype that would inherently be a consequence of selecting for high GCA, certainly not infertility.

[kanzure]

> I don't know how many generations of people breeding it would take to get a selected trait - humans are genetically very similar compared to other species; there is not much variation... so I don't have any idea of what the breeding program length would be, but I assume that it would be several generations

Use whole-genome mutagenesis and increase the size of your population. This will reduce the total number of generations that you require to find the novel mutations that you seek.