The trait may or may not be advantageous. Diversity is. In this case the selective pressure is poaching, but selective pressures are constant. Diversity is a bank of genetics that the population can draw from to adapt and survive.

Traits that are rare, are often rare because of shortcomings. But if environmental changes make that tradeoff worthwhile, it can be made. Meanwhile, evolution doesn't stop. Once the trait/gene is common there is positive selective pressure on complementary genes. Over time, these may mitigate or compensate for the negative traits that have acquired.

Of course, on the timescales that humans tend to selectively pressure species, there isn't time for all this elegance to emerge, usually.

This is all true. You make very good points.

My parent said:

> And this is why genetic diversity is a good thing.

I guess what I meant was: genetic diversity is a good thing. But this particular case is not a great example for illustrating why.

Why do they die? Because they can't protect themselves? Or does the trait come with other less desirable side effects for males?

The trait appears to be lethal to males and they do not make it to birth.

It may be the case that at least one typical X chromosome is necessary for survival, and males only get one X chromosome.