Biology  5/e   Raven/Johnson
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Adaptation: An adaptation is a feature that promotes the likelihood of survival and reproduction by an organism in a particular environment. It results from natural selection and microevolution. The better-adapted individuals are more fit (have greater fitness) and produce more offspring than more poorly adapted individuals in the population. When we look at a population today, we see the results of a long, long history of natural selection and evolution. Thus the population and its individual members seem well adapted to the surrounding environment, both the nonliving environment and other organisms.

How species originate: New species are formed when enough microevolution has occurred to cause populations to diverge into separate species. Such a process is called macroevolution. Hybridization barriers help keep species separate. Strong selection pressure and available habitats can result in rapid speciation.

Major features of evolution: Life first appeared on earth about 3.5 billion years ago, within a billion years of the formation of the earth. By about 1.5 billion years ago eukaryotic cells appeared, and multicellular organisms had appeared by 600 million years ago. The evolution of the organisms that inhabit the earth today continued, all via micro- and macroevolution. For example, terrestrial plants and insects evolved approximately 410 million years ago, amphibians about 360 million years ago, and mammals about 200 million years ago. Our genus, Homo, appeared only 2 million years ago and our species, Homo sapiens is very much a newcomer, appearing only about 200,000 years ago.

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