Are there any specific examples with this question??
Darwin's main contribution to evolution was the METHOD that species evolve, which is via Natural Selection. Random gene mutations over considerably long periods of time lead to differences in traits of individuals within a species. Those with traits for adaptation that confers an advantage to procreate or survive longer to procreate more, are selected for by nature, hence Darwin's term. Over time those genes for traits giving advantages will increase in a population. Then the population of this species is said to have evolved (changed).
