[vidarh]

A gene survives and spreads if the gene helps increase the average fitness of the part of the population having the gene.

There is nothing about evolution that requires a gene to benefit any given individual. There are plenty of species where individuals are metaphorically "thrown under the bus" for the survival of genetically similar enough individuals to be worth it.

And for any given individual, a gene that confers high fitness on the population it exists in as a whole can still be disastrous in certain configurations.

An example of that would be the sickle-cell trait, which causes lots of deaths amongst those unlucky enough to get it from both parents, as it leaves them prone to sickle cell anaemia and related problems.

It survives in many regions such as West Africa because while receiving it from both parents cuts an average 20-40 years of your life expectancy in developed countries, "just" having the trait from one parent is a net positive because it makes it a lot less likely you will get malaria, which is a far bigger killer in countries with poor healthcare.

This is the convoluted way of saying that you can't look at the effect on individual survival - there can be convoluted mechanisms that makes a gene that makes your copies less likely to be propagated more fit on average.