Natural selection

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Natural selection 


Some members of a population may have genetic traits that enhance their ability to survive and produce offspring with these traits.

Natural selection occurs when some individuals of a population have genetically based traits that increase their chances of survival and their ability to produce offspring with the same traits.
Three conditions are necessary for evolution of a population by natural selection to occur. First, there must be genetic variability for a trait in a population. Second, the trait must be heritable-that is, it can be passed from one generation to another. Third, the trait must lead to differential reproduction-that is, it must enable individuals with the trait to leave more offspring than other members of the population. Note that natural selection acts on individuals, but evolution occurs in populations.

An adaptation, or adaptive trait, is any heritable trait that enables an organism to survive and repro- duce better under prevailing environmental conditions. Structural adaptations include coloration (allowing more individuals to hide from predators or to sneak up on prey), mimicry (looking like a poisonous or dangerous species), protective cover (shell, thick skin, bark, or thorns), and gripping mechanisms (hands with opposable thumbs.) Physiological adaptations include the ability to hibernate during cold weather and give off chemicals that poison or repel prey. Behavioral adaptations include the ability to fly to a warmer climate during winter and various interactions between species.

When faced with a change in environmental conditions, a population of a species has three possibilities: adapt to the new conditions through natural selection of an increasing number of individuals in the population, migrate (if possible) to an area with more favorable conditions, or become extinct.

The process of microevolution can be summarized as follows: Genes mutate, individuals are selected, and populations evolve.


Materials from   http://biology.brookscole.com/miller11

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