Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall
16-1 Genes and Variation Genes and Variation 16-1 Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises Copyright Pearson Prentice Hall

How Common Is Genetic Variation?
Many genes have at least two forms, or alleles. All organisms have genetic variation An individual organism is heterozygous for many genes. Copyright Pearson Prentice Hall

Variation and Gene Pools
Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population. Copyright Pearson Prentice Hall

The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage. Copyright Pearson Prentice Hall

Gene Pool for Fur Color in Mice When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. Copyright Pearson Prentice Hall

In genetic terms, evolution is any change in the relative frequency of alleles in a population. Copyright Pearson Prentice Hall

Sources of Genetic Variation
The two main sources of genetic variation are mutations genetic shuffling that results from sexual reproduction. Copyright Pearson Prentice Hall

Mutations A mutation is any change in a sequence of DNA. Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism’s phenotype. Copyright Pearson Prentice Hall

Gene Shuffling Most heritable differences are due to gene shuffling. Crossing-over increases the number of genotypes seen in offspring. Sexual reproduction produces different phenotypes, but does not change the relative frequency of alleles in a population. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Traits
A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes. In humans, a single gene with two alleles controls whether a person has a widow’s peak (left) or does not have a widow’s peak (right). As a result, only two phenotypes are possible. The number of phenotypes a given trait has is determined by how many genes control the trait. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Trait
The allele for a widow’s peak is dominant over the allele for a hairline with no peak. However, the presence of a widow’s peak may be less common in a population. In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are dominant or recessive. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Trait
Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait. Normal Distribution Copyright Pearson Prentice Hall

Which of the following statements is TRUE? The relative frequency of an allele is not related to whether the allele is dominant or recessive. Mutations always affect an organism's phenotype. Crossing over decreases the number of different genotypes that appear in an offspring. Evolution does not affect the frequency of genes in a gene pool. Copyright Pearson Prentice Hall

Most inheritable differences are a result of gene shuffling. frequency of alleles. mutations. DNA replication. Copyright Pearson Prentice Hall

The main sources of inherited variation are gene shuffling and mutations. gene pools and frequencies. single-gene and polygenic traits. genotypes and phenotypes. Copyright Pearson Prentice Hall

A widow's peak in humans is an example of a(an) invariable trait. single-gene trait. polygenic trait. mutation. Copyright Pearson Prentice Hall

A graph of the length of the little finger on the left hand versus the number of people having fingers of a particular length is a bell-shaped curve. This indicates that finger length is a single-gene trait. polygenic trait. randomly inherited trait. strongly selected trait. Copyright Pearson Prentice Hall

16-2 Evolution as Genetic Change
16-2 Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises Copyright Pearson Prentice Hall

Evolution as Genetic Change
Natural selection affects which individuals survive and reproduce and which do not. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. Copyright Pearson Prentice Hall

Evolution as Genetic Change
Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, can evolve over time. Copyright Pearson Prentice Hall

Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Copyright Pearson Prentice Hall

Organisms of one color may produce fewer offspring than organisms of other colors. Those most visible to predators will be less likely to survive and reproduce. Therefore, that allele will become rare. Copyright Pearson Prentice Hall

Black lizards may warm up faster on cold days. This may give them energy to avoid predators. In turn, they may produce more offspring. The allele for black color will increase in relative frequency. Copyright Pearson Prentice Hall

Natural Selection on Polygenic Traits
Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection Copyright Pearson Prentice Hall

Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. The range of phenotypes shifts as some individuals survive and reproduce while others do not. Copyright Pearson Prentice Hall

Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. This keeps the center of the curve at its current position, but it narrows the overall graph. Copyright Pearson Prentice Hall

Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. In this example of stabilizing selection, human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. Copyright Pearson Prentice Hall

Disruptive Selection When individuals at the extreme ends of the curve have higher fitness than individuals near the middle If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes. Copyright Pearson Prentice Hall

If average-sized seeds become scarce, a bird population will split into two groups: one that eats small seeds and one that eats large seeds. In this example of disruptive selection, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds. Copyright Pearson Prentice Hall

Genetic Drift A random change in allele frequency is called genetic drift. Copyright Pearson Prentice Hall

Genetic Drift In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. Copyright Pearson Prentice Hall

Genetic Drift Genetic drift may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. The new population will be genetically different from the parent population. Copyright Pearson Prentice Hall

Genetic Drift When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect. Founding Populations Descendent Populations Original Population Copyright Pearson Prentice Hall

Evolution Versus Genetic Equilibrium
The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium. Copyright Pearson Prentice Hall

Five conditions are required to maintain genetic equilibrium from generation to generation: there must be random mating, the population must be very large, there can be no movement into or out of the population, there can be no mutations, and there can be no natural selection. Copyright Pearson Prentice Hall

Random Mating Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits. Copyright Pearson Prentice Hall

Large Population Genetic drift has less effect on large populations than on small ones. Allele frequencies of large populations are less likely to be changed through the process of genetic drift. Copyright Pearson Prentice Hall

No Movement Into or Out of the Population Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. The population's gene pool must be kept together and kept separate from the gene pools of other populations. Copyright Pearson Prentice Hall

No Mutations If genes mutate, new alleles may be introduced into the population, and allele frequencies will change. Copyright Pearson Prentice Hall

No Natural Selection All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. There can be no natural selection operating on the population. Copyright Pearson Prentice Hall

Which of the following patterns of natural selection on polygenic traits favors both extremes of a bell curve? stabilizing selection disruptive selection directional selection genetic drift Copyright Pearson Prentice Hall

Which of the following events could lead to genetic drift? A few new individuals move into a large, diverse population. A few individuals from a large, diverse population leave and establish a new population. Two large populations come back together after a few years of separation. The mutation rate in a large population increases due to pollution. Copyright Pearson Prentice Hall

The situation in which allele frequencies remain constant in a population is known as genetic drift. the founder effect. genetic equilibrium. natural selection. Copyright Pearson Prentice Hall

Which of the following conditions is required to maintain genetic equilibrium in a population? movement in or out of the population random mating natural selection small population Copyright Pearson Prentice Hall

According to the Hardy-Weinberg principle, no evolution will take place if all five of the Hardy-Weinberg conditions are met. any one of the Hardy-Weinberg conditions is met. at least three of the Hardy-Weinberg conditions are met. none of the Hardy-Weinberg conditions are met. Copyright Pearson Prentice Hall

END OF SECTION

16-3 The Process of Speciation
16-3 16-3 The Process of Speciation Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises Copyright Pearson Prentice Hall

The Process of Speciation
Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring. Copyright Pearson Prentice Hall

Isolating Mechanisms What factors are involved in the formation of new species? The gene pools of two populations must become separated for them to become new species. Copyright Pearson Prentice Hall

Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. Copyright Pearson Prentice Hall

Isolating Mechanisms Reproductive isolation can develop in a variety of ways, including: behavioral isolation geographic isolation temporal isolation Copyright Pearson Prentice Hall

Isolating Mechanisms Behavioral Isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. Figure 14.3B Copyright Pearson Prentice Hall

Isolating Mechanisms Geographic Isolation occurs when two populations are separated by geographic barriers such as rivers or mountains. A. harrisi A. leucurus Figure 14.4 Copyright Pearson Prentice Hall

Isolating Mechanisms Geographic barriers do not guarantee the formation of new species. If two formerly separated populations can still interbreed, they remain a single species. Potential geographic barriers may separate certain types of organisms but not others. Copyright Pearson Prentice Hall

Isolating Mechanisms Temporal Isolation
occurs when two or more species reproduce at different times.

Testing Natural Selection in Nature
Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galápagos Islands. The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor. Copyright Pearson Prentice Hall

Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions: For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection. Differences in beak size and shape must produce differences in fitness, causing natural selection to occur. Copyright Pearson Prentice Hall

The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galápagos Islands. During the rainy season, there is plenty of food. During droughts, food becomes scarce. Individual birds with different-sized beaks had different chances of survival during a drought. Copyright Pearson Prentice Hall

When food was scarce, individuals with large beaks were more likely to survive. This graph shows the survival rate of one species of ground-feeding finches, the medium ground finch. Copyright Pearson Prentice Hall

The Grants provided evidence of the process of evolution. Beak size can be changed by natural selection. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches
Speciation in the Galápagos finches occurred by: founding of a new population geographic isolation changes in new population's gene pool reproductive isolation ecological competition Copyright Pearson Prentice Hall

A few finches—species A—travel from South America to one of the Galápagos Islands. There, they survive and reproduce. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Changes in the Gene Pool Seed sizes on the second island favor birds with large beaks. The population on the second island evolves into population B, with larger beaks. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Reproductive Isolation If population B birds cross back to the first island, they will not mate with birds from population A. Populations A and B are separate species. Copyright Pearson Prentice Hall

Ecological Competition As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them. A new species—C—may evolve. Copyright Pearson Prentice Hall

Continued Evolution This process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galápagos island chain. Copyright Pearson Prentice Hall

Studying Evolution Since Darwin
Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past. Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved. Copyright Pearson Prentice Hall

When two species do not reproduce because of differences in mating rituals, the situation is referred to as temporal isolation. geographic isolation. behavioral isolation. reproductive isolation. Copyright Pearson Prentice Hall

The most important factor involved in the evolution of the ground squirrels of the Grand Canyon appears to be temporal isolation. geographic isolation. behavioral isolation. different food sources. Copyright Pearson Prentice Hall

One finding of the Grants' research on generations of Galápagos finches was that natural selection did not occur in the finches natural selection can take place often and very rapidly. beak size had no effect on survival rate of the finches. natural selection was slow and permanent. Copyright Pearson Prentice Hall

All of the following played a role in speciation of Galápagos finches EXCEPT no changes in the gene pool. separation of populations. reproductive isolation. natural selection. Copyright Pearson Prentice Hall

Beak size in the various groups of Galápagos finches changed primarily in response to climate. mating preference. food source. availability of water. Copyright Pearson Prentice Hall

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