My assumption was that some of the more obvious racial traits, like the Garlean third eye, would be a recessive gene and so it takes two of that race to produce it. Elezen ears are obviously the exception since Hilda got at least a little point to them.

Another thing we can infer from lore is that the races themselves predate the Sundering, since some of the races exist on the First albeit in slightly different proportions to the Source at this point. For example, Lalafell are some of the most common on the Source, but they're relegated to a handful of tribes on the First, and even that number probably wasn't much higher before the Flood of Light. Garleans do not exist on the First, interestingly enough, which makes me curious if they were a post-Sundering evolution.. (Like we know that Ixali do not exist on the First because they are a novel Allagan creation.)