I could totally see us evolving into a non-Fisherian species in a few million years.
How? What evolutionary incentive is there?
If there’s a genetic factor that leads to any given baby having only a 1/6 chance of being male, then the more widespread that gene is, the greater the advantage held by the competing “1/2 male” gene. Because the more skewed the population ratio, the more advantageous it becomes to have sons.
For a simplified model, imagine a binary-gender population strictly divided into generations of fixed size, with children of one generation born from random pairings within the previous. On average, women can be expected to have 1.2 children each, with five women having one son and five daughters. Men, however, have on average
six children each. That means if you introduce a “1/2 male” gene, it has an evolutionary advantage, because the average number of
grandchildren is going to be more.
(I’ve run this simulation. The skewed-gender gene sticks around for a surprisingly long time, because as the sex ratio approaches 1:1 its disadvantage shrinks. Never seems to actually stop declining in prevalence from one generation to the next, though.)