Chapter 6 Summary

CONCEPT 6.1 Evolution can be viewed as genetic change over time or as a process of descent with modification.

• Biologists often define evolution in a relatively narrow sense as change over time in the frequencies of alleles in a population.
• Evolution can also be viewed as descent with modification, a process in which populations accumulate differences over time and hence differ from their ancestors.
• Natural selection modifies populations by favoring individuals with some heritable traits over others.
• Although individuals are acted on by natural selection, an individual does not evolve—it either has a favored trait or it does not. Only populations evolve.

CONCEPT 6.2 Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change over time.

• Mutation and recombination are the sources of new alleles and new combinations of alleles, thereby providing the genetic variation on which evolution depends.
• Natural selection occurs when individuals with certain heritable phenotypic traits survive and reproduce more successfully than individuals with other traits.
• Genetic drift, which occurs when chance events determine which alleles are passed from one generation to the next, can have negative effects on small populations.
• Gene flow, the transfer of alleles between populations, makes populations more similar to one another genetically and can introduce new alleles into populations.

CONCEPT 6.3 Natural selection is the only evolutionary mechanism that consistently causes adaptive evolution.

• By consistently favoring individuals that have advantageous alleles over individuals that have other alleles, natural selection can cause adaptive evolution, in which the frequency of an advantageous trait in a population increases over time.
• Natural selection can increase the frequency of advantageous traits rapidly—in days to years, depending on the generation time of the organism under selection.
• Gene flow can limit the extent to which a population is adapted to its local environment.
• Constraints on adaptive evolution result from factors such as lack of genetic variation, evolutionary history, and ecological trade-offs.

CONCEPT 6.4 Long-term patterns of evolution are shaped by large-scale processes such as speciation, mass extinction, and adaptive radiation.

• The diversity of life has resulted from speciation, the process by which one species splits into two or more species.
• Biological communities are devastated by mass extinctions, global events in which large proportions of Earth’s species are driven to extinction in a relatively short time.
• An adaptive radiation occurs when a group of organisms gives rise to many new species that expand into new habitat or fill new ecological roles.
• Adaptive radiations can be stimulated by the removal of competitor groups by a mass extinction or by the evolution of a major new adaptation.

CONCEPT 6.5 Ecological interactions and evolution exert a profound influence on one another.

• Ecological interactions among organisms and between organisms and their environment can cause evolutionary change, ranging from allele frequency change in populations to the formation of new species.
• Similarly, evolutionary change can alter the outcome of ecological interactions, thus having a large influence on biological communities.