Evolution describes the changes in the gene pool of a species over time.  These changes are the results of mutations, natural selection and genetic drift.

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Often populations produce more offspring than the environment can support; in other words, they often exceed the carrying capacity.  Over-production of offspring leads to intraspecific competition and survival of the individuals best suited to that particular environment.  This process is known as natural selection.

Within a species, there is genetic variation.

Remember that sexual reproduction promotes variation in a species.  Creating gametes by meiosis provides variation through random assortment.  The fusing of two different gametes leads to even further variation in the new offspring.

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Variation in a species can also come about through mutations.  Variation is non-directional, in other words, random.  The selection process is dictated by the environment and leads to differential survival.

The result is that the individuals best adapted to a particular environment will survive.  They will be able to get the most food, find the best shelter, find a mate and most importantly reproduce and care for their offspring.  If the environments are different, the “best adapted” may be different too.  The environment can also change either gradually or suddenly due to natural disasters.  If this occurs, then the criteria for “best adapted” also changes.

READ:
Darwin's Theory: Homologous, Analogous, Vestigial Features

Natural selection can lead to changes in the species.  It can also lead to speciation. – the production of two or more groups of species that may have once been able to interbreed.  When two groups of a species are in different environments and cannot interbreed, selection pressure will be different and eventually they become different species (e.g. many of the species of finches in the Galapagos islands)

If a species cannot adapt to a changing environment, then the species will die out.

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10 years ago

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