Darwin’s theory of evolution depends on reproduction (the capacity of individuals to survive and produce offspring), on heredity (the tendency of offspring to resemble their parents), and on variation (individuals differing from one another). Darwin recognized that natural selection occurs because the differences among individuals affect their survival and the numbers of offspring they produce. If the variations that affect reproduction are heritable, they increase in frequency and the outcome is evolutionary change. Many generations of natural selection in a particular direction, for example for moving efficiently through the air, increase the frequency of variants that eventually can give rise to complicated structures like wings and coordinated processes like flying.