Natural selection is a natural process by which the fittest species or beings adapt to survive, while the weakest go extinct. Charles Darwin was one of the concept's founders, believing that the indvidual genotype (the genes of the individual) combine with the environment to form the individual phenotype (the physical features of a being). Populations are made up of individual phenotypes and genetic mutations, which are the biological basis for the variation of characteristics seen in populations of individuals. External environment acts on individuals in the population and selects which individuals survive, and therefore which individuals reproduce and contribute their genetic makeup to the next generation. The selection in nature is natural selection, but humans have developed methods of artificial selection. Selection pressures can be positive (certain genotypes reproduce more), negative (prevent certain genotypes from reproducing), or both. Beings who adapt well can reproduce more and therefore ensure that their offspring are also adapted to the environment.