The rules of natural selection: It's all about the genes
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.