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n Charles Darwin's 1859 seminal book, On the Origin of Species, he tried to answer the question of how species originate. He saw a paradox. On the one hand, all living organisms attempt to perpetuate their kind by producing many more offspring than are necessary to maintain their numbers. Yet, the actual size of natural populations usually remains relatively constant over time. How could this be? Darwin's answer was that many of the offspring do not survive to reproduce. This phenomenon can be illustrated by considering the common housefly (Musca domestica). Females lay up to 500 eggs at a time. The eggs hatch into larvae which go through several molting stages and then transform into pupae. Thirty-six hours after emerging from pupae, females are receptive for mating. Adult flies live 15-30 days, during which time, females lay eggs repeatedly though they mate only once. Over a 4-5 month period, the descendents of a single mating pair of house flies potentially could number 1920. If that actually occurred, we very quickly would be up to our armpits in fly bodies all over the planet and the piles would grow at a rapidly increasing rate. Fortunately, most fly eggs, larvae, and pupae are killed by other insects and microscopic parasites. This keeps the total fly population more or less constant over time. Darwin surmised that the environment operated in a selective way, reducing the number of poorer-adapted variants of a species while increasing the proportion of better-adapted ones. This process became known as natural selection.
Darwin correctly understood that natural selection is usually the most powerful mechanism of evolution. However, he did not fully comprehend how it operates. This was due to the fact that he was largely ignorant of the mechanisms of genetics. That knowledge mostly came after his time. We now know that natural selection's effect on individuals depends on their phenotypes which in turn are determined mostly by their genotypes. The environment ultimately selects individuals with the best suited genotypes to survive to reproduce. Those individuals who have more surviving offspring pass on more of their genes to the next generation. As a consequence, the gene pool frequencies shift in the direction of their more adaptive alleles. However, the alleles that provide an advantage now may not in the future as new environmental stresses appear. Natural selection acts as a constantly changing template in its selection of winners and losers. This introduces chance into the equation. It is largely a matter of luck in having the right combination of genes at the right time to survive as the environment changes. Extinction occurs if those genes are not present.
For natural selection to cause evolution, it must select for or against one or more of the genotypes for a trait. In the simple case of a trait that is determined by a single gene with two alleles, there are five combinations of genotypes that nature can select:
Explanation:
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