Here are the basics: If a gene pro-duces a trait that makes an organism less likely to survive and reproduce, that gene (and thus,that trait) won’t get passed on, at least not for very long, because the individuals who carry it are less likely to survive. On the other hand, when a gene produces a trait that makes an organism better suited for the environment and more likely to reproduce, that gene (and again, that trait) is more likely to get passed on to its o? spring. he more advantageous a trait is, the faster the gene that produces it will spread through the gene pool.
From Survival of the Sickest