Natural selection of a population for dark coloration.
Natural selection is the differential reproduction of genotypes due to factors in the environment. In the words of Charles Darwin, who first expounded the idea: "Natural Selection is the principle by which each slight variation, if useful, is preserved."
In the process of natural selection, one genotype (the hereditary constitution of an individual) leaves more offspring than another genotype because of superior life attributes, known as fitness. Natural selection acts on genetic variation by conferring a survival advantage to those individuals harboring a particular mutation that tends to favor a changing environmental condition. These individuals then reproduce and pass on this "new" gene, altering their gene pool. Natural selection, therefore, decreases the frequencies of alleles that reduce the fitness of an organism and increases the frequency of alleles that improve fitness.
It is important to understand that natural selection does not always represent progress, only adaptation to a changing surrounding, that is, evolution attributable to natural selection is devoid of intent – something does not evolve to better itself, only to adapt. Because environments are always changing, what was once an advantageous mutation can often become a liability further down the evolutionary line.