Imagine a gene that caused 9/10 of the humans who have it to be twice as fertility and attractiveness as the population that did not have it, while 1/10 of the humans who have it can't reproduce at all.
btw. you don't have to imagine. sickle cell is like this. a proportion of the population gets increased benefit from having the gene, and a proportion gets decreased benefit, in the ratio of heterozygotes (those who carry one sickle cell allele and one normal) and homozogytes (those who carry two alleles), i.e., 2pq:q^2. that's not species selection, it's standard balancing selection upon one gene.
Imagine a gene that caused 9/10 of the humans who have it to be twice as fertility and attractiveness as the population that did not have it, while 1/10 of the humans who have it can't reproduce at all.
btw. you don't have to imagine. sickle cell is like this. a proportion of the population gets increased benefit from having the gene, and a proportion gets decreased benefit, in the ratio of heterozygotes (those who carry one sickle cell allele and one normal) and homozogytes (those who carry two alleles), i.e., 2pq:q^2. that's not species selection, it's standard balancing selection upon one gene.