Or, we can simply say that if a gene makes a male mate with 2x as many females but also makes 20% of the females die with the child dying too, it still propagates.
Propagates for a very short while. If you initial population was stable (which means that each female had, on the average, two children which survive until they breed), introducing a mutation which kills off 20% of the females during birth is likely to lead to this population dying out pretty quickly. Yes, you'll have lots of males around, but they can't give birth.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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