Biology  5/e   Raven/Johnson
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Only identical twins and clones have identical genetic makeups. Most members of a population have unique genotypes and therefore also exhibit unique phenotypes. The suite of characteristics that one individual has may make it more or less likely to survive and reproduce than other members of its population. Individuals that are better adapted to their environment tend to live longer and produce more and better offspring than more poorly adapted individuals. Thus the next generation contains more genes contributed by the better-adapted individuals than by the more poorly adapted individuals. These changes in gene frequencies within a population are called microevolution, and over many generations can lead to such significant changes in the genetic makeup of the population that a whole new species is created. This second process of change is macroevolution. All the species that have ever lived on earth, including the ones alive today, are the result of such selection, adaptation, and evolution. Natural selection tends to make populations better adapted to their environments over time, and as a result, evolution occurs. If there were no mutation, migration into or out of populations, genetic drift, nonrandom mating, or selection, there would be no changes in allele frequencies over time and thus no evolution. But these factors do affect populations, and evolution does occur.

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