Would it not be more beneficial for a species (population wise) to have more individuals able of giving birth rather than individuals that only increase the population during reproduction?
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Let’s assume 75% of all humans were female and 25% of all humans were male. In such a world, each man would have on average 3 female partners to reproduce. Let’s say he has 4 children with each female partners. All 12 children would have 50% of the man’s genes, but only 4 children would have 50% of each mother’s genes.
That means that it’s evolutionarily beneficial to have sons. If I have a son, he will pass on my and my wife’s genes to 12 children. If I have a daughter, she will only pass our genes on to 4 children.
Now, imagine I have a mutation that increases the likelyhood of producing sons. That would be a huge evolutionary benefit for my genes. If my genes cause 50% (instead of 25%) of my children to be male, that means that those 6 sons will pass on my genes to 72 grandchildren and my 6 daughters will pass on their genes to 24 grandchildren – a total of 96. For everyone without this mutation it would be 3 sons with 36 grandchildren and 9 daughters with 36 grandchildren – a total of 72.
Thanks to my mutation, I get an extra 24 grandchildren. On top of that, I also pass on the 50%-male-children gene to half of my offspring. That means half of my offspring also has this evolutionary benefit.
Over generations, this mutation spreads throughout the population until we at some point have about 50% males and 50% females.
The same would happen with an imbalance in the other direction.
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