According to neo-Darwinian evolutionary theory, the genetic traits of a given population of plants or animals track, over generations of time, one or more optimal character states that are specific to the organism and its environment. Changes in phenotype are thought to result most commonly from individual-level natural selection (together with some forms of kin and group selection), which acts to preserve those genotypes that direct the formation of phenotypes best suited to the prevailing conditions. This theory has now proven to be very successful in explaining the genetically coded traits of most organisms.