SUMMARY

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

6.1.1 Summarize the genetic basis for the evolution of traits in organisms.

Biologists often define evolution in a relatively narrow sense as change over time in the frequencies of alleles in a population.

6.1.2 Explain how evolution can be considered the accumulation of trait differences between populations.

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 other individuals.

Although natural selection acts on individuals, an individual does not evolve—either it 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.

6.2.1 Summarize how mutations contribute to the process of

evolution.

Mutation and recombination are the sources of new alleles and new combinations of alleles, thereby providing the genetic variation on which evolution depends.

6.2.2 Compare the effects of stabilizing selection and disruptive selection on the temporal changes in the genetic structure of a population.

Natural selection occurs when individuals with certain heritable phenotypic traits survive and reproduce more successfully than individuals with other traits.

Natural selection may favor the average traits (stabilizing selection) or traits near the extremes in a population (disruptive selection).

6.2.3 Evaluate how random events can affect populations through time via genetic drift.

Genetic drift, which occurs when random events determine which alleles are passed from one generation to the next, can have negative effects on small populations.

6.2.4 Describe the role of gene flow among populations in terms of homogenizing genetic structure as well as enhancing evolutionary change.

Gene flow, the transfer of alleles between populations, makes populations more genetically similar to one another and can introduce new alleles into populations, enhancing evolutionary change.

CONCEPT 6.3 Natural selection is the mechanism for adaptive evolution.

6.3.1 Explain how natural selection can lead to adaptations in populations.

By 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 organism and the selection pressure.

6.3.2 Evaluate the conditions in which gene flow can promote or deter adaptations.

Gene flow can promote the extent to which a population is adapted to its local environment by introducing new alleles into a population.

Gene flow can also limit the extent to which a population is adapted to its local environment by preventing fixation of the favored allele.

6.3.3 Describe factors that limit the development of adaptations in populations.

Constraints on adaptive evolution result from factors such as lack of genetic variation, evolutionary history, and ecological trade­offs.

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

6.4.1 Describe the process by which isolation of populations can lead to speciation.

The genetic divergence of populations over time can lead to speciation, the process by which one species splits into two or more species. Speciation requires the evolution of reproductive barriers between populations.

6.4.2 Evaluate the roles of speciation and extinction in determining the diversity of species.

The number of species in a group of organisms increases when more species are produced by speciation than are lost to extinction, and decreases when the reverse is true. The outcome of this process can be visualized with an evolutionary tree.

6.4.3 Explain how mass extinctions and rapid adaptations have influenced long-term patterns in diversity.

Biological communities can lose much of their diversity in 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 over a short period of time that expand into new habitat or fill new ecological roles.

Adaptive radiations can be promoted by factors such as the removal of competitor groups by a mass extinction or the evolution of a major new adaptation.

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

6.5.1 Evaluate how ecological processes can result in evolutionary changes in populations.

Ecological interactions among organisms and between organisms and their environment can cause evolutionary changes, ranging from allele frequency changes in populations to the formation of new species.

6.5.2 Describe how an evolutionary change in a population has the potential to impact ecological processes.

Evolutionary change can alter the outcomes of ecological interactions, thus having a large influence on biological communities.

REVIEW QUESTIONS

Natural selection acts on individuals, yet one of the points made in this chapter is that populations evolve, but individuals do not. Explain how natural selection works and why the italicized statement is true.

What causes adaptive evolution? Explain in your answer why each of the three primary mechanisms of allele frequency change in populations causes or does not cause adaptive evolution.

What large-scale processes determine patterns of evolution observed over long time scales? Explain how each process that you describe has this effect.

Explain why ecological interactions and evolutionary change have joint effects, each affecting the other.

More than 100 years ago, Rutter (1902) expressed concern about the effects of fishing on river salmon. He wrote (p. 134), “A large fish is worth more on the market than a small fish; but so are large cattle worth more on the market than small cattle, yet a stock raiser would never think of selling his fine cattle and keeping only the runts to breed.” From an evolutionary perspective, summarize the reasons for Rutter's concern, and describe how harvesting-induced evolution is thought to affect fish populations today.

HONE YOUR PROBLEM-SOLVING SKILLS

Stuart and colleagues (2014) studied how the invasion of islands in Florida by the Cuban brown anole lizard (Anolis sagrei) affected the native anole, Anolis carolinensis. After A. sagrei invaded, A. carolinensis moved to higher tree perches. Stuart and colleagues tested whether this change in habitat use caused evolution in the A. carolinensis toepad area; a larger toepad area improves the ability of lizards to grasp the slender branches found high in trees. They measured the toepad area of A. carolinensis caught in the wild on islands that were uninvaded and on islands invaded by A. sagrei. They also measured A. carolinensis toepad area in a “common garden experiment” in which A. carolinensis eggs collected on uninvaded and on invaded islands were reared to adulthood under identical conditions. Average toepad areas are shown in the table.

1.

2. When traits such as toepad area differ consistently between lizards living on uninvaded versus invaded islands, those differences may be due to evolution, phenotypic plasticity,[*] or both. Suppose that evolution had been the primary cause of differences in toepad area between lizards from uninvaded versus invaded islands. Under that assumption, predict whether wild-caught results would differ from common garden results. Explain.

3. Suppose that phenotypic plasticity had been the primary cause of differences in toepad area between lizards from uninvaded versus invaded islands. Under that assumption, predict whether wild-caught results would differ from common garden results. Explain.

4. Did the invasion of Florida islands by A. sagrei lead to eco- evolutionary effects? Explain.

LIST OF KEY TERMS

adaptive evolution adaptive radiation alleles clines

Directional selection disruptive selection evolution evolutionary tree fixation

Gene flow genetic drift genotype mass extinction mutation natural selection phenotype recombination speciation

stabilizing selection trade-offs