The aim of fish breeding is not to change individual fish, but rather the fish population. Thus, our knowledge about Mendelian inheritance must be extended from the level of the individual to the level of the population. Population genetics may be defined as the study of Mendelian genetics in a group of interbreeding individuals who share a common gene pool. The number of genes in this pool depends on the species, ranging from about 4000 in the bacterium E. coli to about 30.000 in the vertebrate genome. Due to the accumulation of mutations over very long periods of time, the DNA sequence of these genes can vary slightly among individuals in the population. These different forms of the genes are called alleles. If the different alleles result in differences of the gene products, the result can be genetic variation for that trait within the population. The proportions of the different alleles for each gene in the pool, the allelic frequencies, determines the genetic characteristics of the population. The product of expression of these alleles interacts with the environment in which the population exists. Some of these interactions give the individuals carrying a particular allelic conformation an advantage over other combinations. Natural selection results in an accumulation of these favorable alleles in the population and leads to a change in allele frequency.