Chapter 21 Introduction

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CHAPTER 21

Evolution

How Genotypes and Phenotypes Change over Time

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Roc Canals Photography/Getty Images.

Core Concepts

  1. Genetic variation refers to differences in DNA sequences.
  2. Patterns of genetic variation can be described by allele frequencies.
  3. Evolution is a change in the frequency of alleles or genotypes over time.
  4. Natural selection leads to adaptations, which enhance the fit between an organism and its environment.
  5. Migration, mutation, genetic drift, and non-random mating are non-adaptive mechanisms of evolution.
  6. Molecular evolution is a change in DNA or amino acid sequences over time.

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Variation is a fact of nature. A walk down any street reveals how variable our species is: Skin color and hair color, for example, vary from person to person. Until the publication in 1859 of Charles Darwin’s On the Origin of Species, scientists tended to view all the variation we see in humans and other species as biologically unimportant. According to the traditional view, not only were species individually created in their modern forms by a divine Creator, but, because the Creator had a specific design in mind for each species, they were fixed and unchanging. Departures or variations from this divinely ordained type were therefore ignored.

Since Darwin, however, we have appreciated that a species does not conform to a type. Rather, a species consists of a range of variants. In our own species, people may be tall, short, dark-haired, fair-haired, and so on. Furthermore, variation is an essential ingredient of Darwin’s theory because the mechanism he proposed for evolution, natural selection, depends on the differential success—in terms of surviving and reproducing—of variants. Darwin changed how we view variation. Before Darwin, variation was irrelevant, something to be ignored; after Darwin, it was recognized as the key to the evolutionary process.