[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.