Basically the Title. How are our body’s so good at regulating how much males and females are born, if they aren’t aware of the Balance at all?
In: Biology
So a lot of people have answered this question citing the mechanism as to how this occurs but have not actually answered the question as to why this mechanism exists in the first place, or as to why there is no bias in fertilization rates, or miscarriage rates. The answer is because a 50/50 split is the most evolutionary stable ratio.
Imagine a mutation occurs that slightly favours male births over females such that the ratio is 51/49. At first this mutation is completely benign, so it spreads through the population as it is not selected for or against via evolution. Fast forward a few generations and the population starts to have more males than females. Under these conditions, females are more likely to be reproductively successful due to the abundance of males, and males find reproduction harder. Thus the male bias mutation will be selected against, and an opposite female bias mutation will be selected for, thus the birth rates move towards to the 50/50 ratio again. Ergo, the population will inevitably end up at 50/50 regardless of any deviation from this balance.
A phenomenon like this is evolutionary stable. A mutation can’t thrive under these circumstances because it’s own success creates the seeds of it’s failure.
HOWEVER what if a species has a higher infant/juvenile mortality rate for one sex? We would expect the birth rates to be skewed towards that sex such that the species would be balanced 50/50 upon reaching sexual maturity. And lo’ and behold, this is exactly what we see in humans. There are on average 105 males for every 100 females born. So our ratio is NOT actually 50/50.
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