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Introduction

Organisms can differ from each other in ways that affect their biological functionality and thus their, or their groups' (see Kin selection), probability of surviving and reproducing. Natural selection is the phrase used to refer to the totality of the biological processes that participate in determining such non-random differences in survival and reproduction, differences which in technical biological language are called differences in "fitness". Natural selection is therefore the proxy term for the causation of differences in fitness. Variant traits are at times heritable so that the preferential reproduction of individuals that display heritable variant traits which boost reproduction should let such traits become more common over the generations, i.e., natural selection can result in evolution and adaptation. Becuase of adaptive evolution, populations of a single species that live in different environments can become progressively different from each other and even different species. Biological research had provided ample support for natural selection being the force driving the evolution of the astounding ways in which organisms cope with their environments and the evolution of the millions of living and extinct species as well as for adaptive divergence being the reason why there is a myriad of species rather than a single one that monopolizes the whole biosphere.

The phrase "natural selection" conjures the image of nature letting fitter organisms reproduce more than others, which is a direct analogy to a human breeder choosing as breeding stock individuals that display desirable variant traits in the hope of improving an animal breed or a crop (see Artificial selection). Modern evolutionary biology envisions, and studies, the process of natural selection as a complex causal chain that goes from the genetics and developmental biology of the generation of trait differences between individuals, to the biomechanics and ecology of how such trait differences result in differences in the performance of individual organisms or groups thereof, and ultimately, when reproductive output rather than only survival is affected, to the reproductive biology of how such differences in organismic performance translate into non-random differences in reproduction among individuals. Therefore, natural selection cannot be summarized by the circular phrase "preferential reproduction of those who reproduce most", as it is often charged by those who forget, or have never been exposed to, the just mentioned chain of causation that underlies fitness differences.

The reality and pervasive agency of natural selection throughout the biological world makes the fact that individuals can differ from each other into a crucial property of life. Before Darwin the prevalent view among naturalists was that differences among individual organisms within a species are uninteresting departures from each species' Platonic ideal or typus. But by the 19th century the Platonic metaphysical straightjacket was already being challenged by emerging evidence that gradual but colossal geological change had occurred throughout the history of the earth; and some evolutionists were embracing the view that adaptive evolution can occur when organisms transmit to their progeny the modifications they acquire while dealing with the environment (which proved to be false; see Lamarckism). In contrast, Darwin argued that adaptive evolution results from the fact that heritable differences between individuals which arise without directionality can allow some individuals to perform better in the struggle for existence and thus to reproduce more, so that over the generations the functionally superior traits will increase in frequency.