Struggle for existence. Natural selection

Natural selection is the survival and greater reproduction of individuals with useful hereditary traits in the struggle for existence, and lesser reproduction of the others; it is the main directing factor of evolution. Its material is hereditary variation; the unit that is selected is the individual, while the unit that changes is the gene pool of the population. Stabilising selection preserves individuals with the average trait and removes the extremes; it works when the environment stays unchanged for a long time. Example: during a severe storm sparrows with average wing length survived more often (H. Bumpus’s observation); “living fossils” such as the coelacanth and tuatara. Directional selection shifts the average of a trait in one direction when the environment changes: with the growth of industry in England, dark forms of the peppered moth (Biston betularia) increased, because tree bark darkened with soot and birds found pale moths more easily; after pollution fell, the pale form increased again. Other examples: the spread of resistance to antibiotics in bacteria and to pesticides in insects – which is why antibiotics should be taken only as prescribed by a doctor and the full course completed; the abundance of wingless and reduced-wing insects that Darwin saw on very windy ocean islands is another example. Disruptive selection lowers the intermediate form and keeps two or more extreme forms: for example, the reddish form of the two-spot ladybird survives winter better while the dark form breeds more in summer, so both forms persist in the population. Sexual selection preserves traits that raise success in mating (peacock tail, appearance, song).