A little more than 40 years ago, Richard Dawkins wrote his landmark book, The Selfish Gene, and changed our view of evolution forever. Dawkins, drawing on work by several contemporary scientists, made a simple argument, very persuasively, that evolution by natural selection operates on individual genes rather than on the organisms that host them.
It follows from this selfish gene hypothesis that a genetic mutation that’s harmful to its host (and hence limits its fitness, or ability to reproduce) is unlikely to make it into the next generation and will gradually disappear from the general population. This is because if a gene limits its host’s reproductive ability, it’s effectively limiting its own heritability.
Or so we’ve thought for the past four decades.