Interdependence between species illustrates coevolution. Evolution is best explained by natural selection. Natural selection is the process where some individuals in a population have genetic traits that give them features promoting their survival and the survival of their offspring in a particular environment. Eventually individuals, within a population, will compete for resources. Most genes occur as alleles, which give rise to differences in phenotype.
Some phenotypes are better than others at helping the individual compete, survive, and reproduce.
These alleles, therefore, increase in the population while others decrease. A population is a group of individuals of the same species that occupy a given area. In theory, the entire population shares a gene pool, which is the genetic resources that is passed to the next generation. Alleles are two or more slightly different molecular forms of the same gene. Movement of alleles between populations is called gene flow.