Natural selection is the process by which organisms that are better adapted to their environment survive, reproduce and pass on their alleles to the next generation.Different environmental conditions favour different individuals and those individuals that are not well adapted to their environment tend not to survive.A few characteristics are determined by a single gene. Most characteristics are determined by more than one gene (polygenes) and may also be influenced by the environment. Characteristics influenced by more than one genes are affected by the genes and the environment and produce a variety of individuals that vary about a mean. When you plot the variation on a graph you get a normal distribution curve.Selection may favour one extreme (directional selection). This changes the characteristics of the population. Selection may favour 'average' individuals (stabilising selection) and preserve the average characteristics whilst selecting against characteristics that are very different from the average.