Biology  5/e   Raven/Johnson
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Natural selection: Evolutionary change occurs because of natural selection. Differential survival and reproduction of individuals within a population is the driving force behind microevolution, and over time microevolution leads to macroevolution.

Fitness: An individual's fitness is a measure of its genetic contribution to future generations relative to the contributions of other individuals in the population. The more fit an individual is, the greater its relative contribution to the future gene pool.

Microevolution: Microevolution is progressive change in allele frequencies in a population over time. These changes occur because of natural selection. Traits that make an individual more fit are selected for; traits that make an individual less fit are selected against. As a result of natural selection and microevolution, populations are adapted to their environment.

Change in populations: Over time, populations change on both a microevolutionary and a macroevolutionary scale. Via natural selection, populations adapt to their environments and undergo the changes in allele and gene frequencies that represent microevolution. Clear evidence of such changes can be seen in the sickle cell allele and industrial melanism. Eventually, enough microevolution occurs that populations are no longer considered to be members of the same species. At that point, macroevolution – the origin of a new species – has occurred. Again, there is much evidence to support the theory of macroevolution, just as there is for microevolution.

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