lesson overview 17.1 genes and variation. lesson overview lesson overview genes and variation...

Lesson OverviewLesson Overview17.1 Genes and Variation17.1 Genes and Variation

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Genetics Joins Evolutionary Theory

How is evolution defined in genetic terms? In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time.

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Genotype and Phenotype in Evolution

Natural selection acts directly on phenotype, not genotype. Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

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Populations and Gene Pools

A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population.

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Populations and Gene Pools

Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations. Populations, rather than individuals, evolve.

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Sources of Genetic Variation

What are the sources of genetic variation? Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

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Mutations

Mutations that produce changes in phenotype may or may not affect fitness. Some mutations may be lethal or may lower fitness; others may be beneficial.

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Lesson OverviewLesson Overview17.2 Evolution as Genetic 17.2 Evolution as Genetic

Change in PopulationsChange in Populations

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How Natural Selection Works

How does natural selection affect single-gene and polygenic traits? Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.

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Directional Selection

Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

For example, if only large seeds were available, birds with larger beaks would have an easier time feeding and would be more successful in surviving and passing on genes.

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Stabilizing Selection

Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end.

For example, very small and very large babies are less likely to survive than average-sized individuals. The fitness of these smaller or larger babies is therefore lower than that of more average-sized individuals.

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Disruptive Selection

Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

For example, in an area where medium-sized seeds are less common, birds with unusually small or large beaks would have higher fitness. Therefore, the population might split into two groups—one with smaller beaks and one with larger beaks.

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Lesson OverviewLesson Overview17.3 The Process 17.3 The Process

of Speciationof Speciation

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Isolating Mechanisms

Reproductive isolation occurs when a population splits into two groups and the two populations no longer interbreed.

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Behavioral Isolation

Behavioral isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors.

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Geographic Isolation

Geographic isolation occurs when two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. Separate gene pools formed, and genetic changes in one group were not passed on to the other.

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Temporal Isolation

Temporal isolation happens when two or more species reproduce at different times. For example, three species of orchid live in the same rain forest. Each species has flowers that last only one day and must be pollinated on that day to produce seeds. Because the species bloom on different days, they cannot pollinate each other.

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Founders Arrive

Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands, as shown in the figure. Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population.

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Geographic Isolation

Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands, as shown in the figure. Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population.

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Changes in Gene Pools

Over time, populations on each island adapted to local environments. Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.

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Behavioral Isolation

Over time, populations on each island adapted to local environments. Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.

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Competition and Continued Evolution

Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E).

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Lesson OverviewLesson Overview17.4 Molecular Evolution17.4 Molecular Evolution

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Gene Duplication

Where do new genes come from? One way in which new genes evolve is through the duplication, and then modification, of existing genes.

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Molecular Clocks

What are molecular clocks? A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.