Darwin's idea of natural selection

according to Darwin's theory of evolution, species go through a natural selection over long periods of time. meaning that only the fittest of a species survive as it is the organisms of a species who are the most fit, that have the greater capability to hunt or gather food versus weaker organisms of that species who die out as the fitter organisms out-compete them for food or resources. for example, giraffes who have longer necks are considered to be more fit as they can more easily reach tall trees and gain access to food, as opposed to giraffes with shorter necks who can't reach enough food and therefore die out. it is considered natural selection as certain animals are "fitter" than others because of a mutation they have (like longer necks) that put them at an advantage.