According to the rules of natural selection, genes in the next generation are all that matter in terms of evolutionary success. It works like this. Imagine two animals, one with a gene that helps it have lots of healthy offspring and one with a gene that allows it to have fewer healthy offspring. The animal with the high-reproduction gene will leave many descendents who also carry that gene — and those descendents will reproduce a lot, passing the gene on to their descendents…and so on. The animal with the low-reproduction gene will leave some offspring who carry the low-reproduction gene in the next generation — but not very many — and those descendents will, in turn, leave behind few offspring. In each generation, more and more of the genes in the population will be of the high-reproduction type. That is the basic idea of how natural selection works.