=Natural Selection= Natural selection is the gradual process by which heritable traits become either more or less common in a population as a result of their impact on reproductive success. It is a key mechanism of evolution. '''Biological Fitness''' Populations of sexually reproducing organisms consist of varied individuals. Some of these will be better adapted to their environmental conditions than others. The fittest individuals are those that are best adapted to their environment. Their adaptations increase their chance of survival and therefore reproduction. We can say that the fittest individuals are ''selected for''. '''Natural Selection''' The fittest individuals (the best adapted) have greater reproductive success; they pass on their genes to a greater number of offspring. As a result the frequency of the alleles (versions of a gene) that code for their adaptations will increase over subsequent generations. This process, sometimes called survival of the fittest, is better known as natural selection. [image:] '''Selection Pressures''' The factors that determine which characteristics are favourable (those that increase fitness / reproductive success) are known as ''selection pressures.'' These can include environmental conditions, predators, prey, etc. Any factor that will select for or against individuals with certain heritable characteristics. In the example (left) selection pressures are selecting against individuals with the recessive characteristic (aa). As a result, the frequency of the recessive allele (a) will decrease and the numbers of those individuals with the dominant characteristic will increase. However, because carriers (heterozygous individuals) are not selected against, it may take a very long time before the recessive allele is eliminated.