 as the ability of a living thing to survive / to reproduce increases , the likelihood of passing on its traits to its decendents will increase & people with sediment that positively african their microorganisms have a greater inability to survive / reproduce -> the decendants of an animal with traits that positively impacted their survival will also have traits that positivley impact their survival the decendants of an animal with traits that positively impacted their survival will also have traits that positivley impact their survival