CHAPTER 21 INTRODUCTION

CORE CONCEPTS

21.1 Genetic variation is the result of differences in DNA sequences.

21.2 Information about allele frequencies is key to understanding patterns of genetic variation.

21.3 Evolution is a change in the frequency of alleles or genotypes over time.

21.4 Natural selection leads to adaptations, which enhance the fit between an organism and its environment.

21.5 Migration, mutation, and genetic drift are non-adaptive mechanisms of evolution.

21.6 Molecular evolution looks at changes in DNA or amino acid sequences.

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 at the time, not only were species individually created by God in their modern forms, but, because the Creator had a specific design in mind for each one, 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-skinned, fair-skinned, and so on. Furthermore, variation is an essential ingredient of Darwin’s theory because 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, but after him it was recognized as the key to the evolutionary process.