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The fundamental statement of '''natural selection''' is that heritable traits which are beneficial will become more numerous in successive generations, while heritable traits which are harmful will become more scarce.  This principle is general, and applies to any system of individuals which reproduce; it is most commonly applied to systems of living organisms.   

The fundamental statement of '''natural selection''' is that heritable traits which are beneficial will become more numerous in successive generations, while heritable traits which are harmful will become more scarce.  This principle is general, and applies to any system of individuals which reproduce; it is most commonly applied to systems of living organisms.   

Together with [[mutation]] (an altogether distinct phenomenon that should not be confused with natural selection), natural selection forms the basis of evolutionary theory.  As stated above, in any system where selection combines with random variation, successive generations will become better adapted to reproduce.  [[Evolution]] is the theory that this combination of genetic variation (which may be either mutation of the parent or, in most cases, sexual reproduction) and natural selection leads to [[speciation]].

Together with [[mutation]] (an altogether distinct phenomenon that should not be confused with natural selection), natural selection forms the basis of evolutionary theory.  As stated above, in any system where selection combines with random variation, successive generations will become better adapted to reproduce.  [[Evolution]] is the theory that this combination of genetic variation (which may be either mutation of the parent or, in most cases, sexual reproduction) and natural selection leads to [[speciation]].

Natural selection has been used to explain many organism traits. For example, deer run fast because slower deer have been eaten by predators, and the faster deer are more likely to pass their traits to the next generation. Long-tailed peacocks are less able to escape from predators, but their existence implies that they have been able to reproduce (perhaps because the females prefer them), so natural selection explains them also.

Natural selection has been used to explain many organism traits. For example, deer run fast because slower deer have been eaten by predators, and the faster deer are more likely to pass their traits to the next generation. [[Selection pressure]] is seldom so one-sided; for example, the long tail of the [[peacock]] leaves it vulnerable to predators.  However, since peahens are far less likely to mate with short-tailed peacocks, a long tail is an overall advantage and thus is selected for.

== An Example ==

== An Example ==

== Local and Non-Local Improvement ==  

== Local and Non-Local Improvement ==  

The above section illustrates how natural selection and random variation can combine to create improved individuals.  However, it is important to note that there is no long-term planning involved in natural selection.  This means that all changes must be locally beneficial in order to survive.  Essentially, no improvement can take place if a deterioration must take place first.  In order for a change to propagate to future "generations" of individuals, it must not be significantly harmful to ''any'' generation.  In the language of the above example, let us say that one of the dams would be much improved if the curvature were increased by 10%, but would be worsened considerably if the curvature was 3 to 7% higher.  Since curvature can only change by 1% at a time, and a regime of "bad" curvature lies between the current regime and the regime of "good" curvature, the dam will never reach the "good" regime.  Thus, in order for a large change to take place over several generations, it must be beneficial (or at least not significantly harmful) at ''each'' generation.

The above section illustrates how natural selection and random variation can combine to create improved individuals.  However, it is important to note that there is no long-term planning involved in natural selection.  This means that all changes must be locally beneficial in order to survive.  Essentially, no improvement can take place if a deterioration must take place first.  In order for a change to propagate to future "generations" of individuals, it must not be significantly harmful to ''any'' generation.  In the language of the above example, let us say that one of the dams would be much improved if the curvature were increased by 10%, but would be worsened considerably if the curvature was 3 to 7% higher.  Since curvature can only change by 1% at a time, and a regime of "bad" curvature lies between the current regime and the regime of "good" curvature, the dam will never reach the "good" regime.  Thus, in order for a large change to take place over several generations, it must be beneficial (or at least not significantly harmful) at ''each'' generation.  Again returning to the peacock, it might be an overall advantage to have a very short tail or no tail at all, since predators would be easy to avoid.  However, since a ''slightly'' shorter tail gives little help in avoiding predators and also is much less attractive to females, short tails are unlikely to come about.

* [[Theory of evolution through natural selection]]

* [[Theory of evolution through natural selection]]