Interest Grabber Yes, No, or Maybe Section 16-1 Yes, No, or Maybe Some traits, such as a widow’s peak, fall into neat categories: You either have a widow’s peak or you don’t. Other traits, such as height, aren’t so easy to categorize.

Interest Grabber continued Section 16-1 Make a list of physical traits that you think are influenced by genes. Then, write next to each trait whether you have the trait or not (e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color). 2. Are most of the traits you listed clear-cut or are they mostly traits that have many variations? Which traits in your list are difficult to categorize? 3. Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not?

Answers Students’ answers will include dimples and detached earlobes. 2. Most traits listed likely have many variations. 3. Some students may suggest that patterns of inheritance for traits with many variations (polygenic) are more complex than for clear-cut (single-gene) ones.

Section Outline 16–1 Genes and Variation A. Darwin’s Ideas Revisited B. Gene Pools C. Sources of Genetic Variation 1. Mutations 2. Gene Shuffling D. Single-Gene and Polygenic Traits

Darwin’s Ideas Darwin did not know how heredity worked: He did not know the source of the variation that was so central to his theory. 2. He could not explain how inheritable traits were passed from one generation to the next.

Words to Know Gene pool – combined genetic information of all the members of a particular population Relative frequency – the number of times an allele occurs in a gene pool compared with the number of times other alleles occur The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. Mutation – any change in the sequence of DNA

Mutations Can occur because of mistakes in the replication of DNA Can be a result of radiation or chemicals in the environment Can be limited to one or a few bases of DNA Can affect lengthy segments of a chromosome Do not always affect an organism’s phenotype – its physical, behavioral, and biochemical characteristics (Example: A DNA codon altered by a point mutation from GGA to GGU will still code for the same amino acid, glycine.) Many mutations do change the phenotype Some mutations affect fitness; others do not affect the organism’s ability to survive and reproduce

Words to Know The number of phenotypes produced for a given trait depends on how many genes control the trait. Single-gene trait – a trait controlled by a single gene that has two alleles (Widow’s peak) Polygenic traits – traits controlled by two or more genes; each gene of a polygenic trait often has two or more alleles (Height)

Generic Bell Curve for Polygenic Trait Section 16-1 Frequency of Phenotype Phenotype (height)

Figure 16–2 Relative Frequencies of Alleles Section 16-1 Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

Figure 16–3 Phenotypes for Single-gene Trait Section 16-1 100 80 60 40 20 Frequency of Phenotype (%) Widow’s peak No widow’s peak Phenotype

Interest Grabber . . . All the Help I Can Get Section 16-2 . . . All the Help I Can Get Natural selection operates on traits in different ways. You might be able to predict which traits natural selection would favor if you think about the demands of an organism’s environment. 1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other. 2. Under Trait, write in several of the animal’s traits. 3. Under Advantage, write in how you think the trait would be helpful to the animal.

1.Animal choices should be sufficiently familiar that students can describe several traits. 2. Students should list traits such as size, color, and specialized behavior. 3. Students should indicate that adaptive value is clearer for some traits than for others. For example, white-tailed deer raise their tails upon sensing a predator. This may be an alarm signal for other deer, or it may induce the predator to chase the now-conspicuous deer.

Section Outline 16–2 Evolution as Genetic Change A. Natural Selection on Single-Gene Traits B. Natural Selection on Polygenic Traits 1. Directional Selection 2. Stabilizing Selection 3. Disruptive Selection C. Genetic Drift D. Evolution Versus Genetic Equilibrium 1. Random Mating 2. Large Population 3. No Movement Into or Out of the Population 4. No Mutations 5. No Natural Selection

Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Example: If a population of lizards lives in dark soil, those with red skin coloring would be easier prey. Eventually, more lizards with dark coloring would survive and change the gene pool frequencies.

Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways: Directional selection Stabilizing selection Disruptive selection

Directional Selection Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end; entire curve shifts Example: An increase in the average size of the beaks in a particular species of Galapagos finches; better fitness as they competed for food

Figure 16–6 Graph of Directional Selection Section 16-2 Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce.

Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end of the curve; keeps the center of the curve at its current position, but it narrow the overall graph Example: Human infants at birth- low birth weight babies are less likely to survive and large birth weight babies are more likely to have difficulty being born

Figure 16–7 Graph of Stabilizing Selection Section 16-2 Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Disruptive Selection Individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle; the single curve splits into two curves Example: A population of birds lives in an area where medium-sized seeds become less common. Birds with unusually small or large beaks would have higher fitness.

Figure 16–8 Graph of Disruptive Selection Section 16-2 Disruptive Selection Largest and smallest seeds become more common. Key Low mortality, high fitness Population splits into two subgroups specializing in different seeds. Number of Birds in Population Number of Birds in Population High mortality, low fitness Beak Size Beak Size

Genetic Drift A random change in allele frequency In small populations, individuals that carry a particular allele may leave more descendents than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. Can occur when a small group of individuals colonizes a new habitat

Founder Effect A situation in which the allele frequencies change as a result of the migration of a small subgroup of a population Example: Evolution of several hundred species of fruit flies on the Hawaiian Islands (All descended from the same mainland)

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Interest Grabber Country Cousin/City Cousin Section 16-3 Country Cousin/City Cousin What happens when a population or group of living things is divided into two separate groups in two separate environments? To understand what goes on, think about someone who lives in another part of the United States or in another country. 1. Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours? 2. All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described?

Answers Country Cousin/City Cousin Students’ lists should include several social/environmental factors. 2. Students may understand that humans would evolve separately in response to different environmental pressures.

Section Outline 16–3 The Process of Speciation A. Isolating Mechanisms 1. Behavioral Isolation 2. Geographic Isolation 3. Temporal Isolation B. Testing Natural Selection in Nature 1. Variation 2. Natural Selection 3. Rapid Evolution C. Speciation of Darwin’s Finches 1. Founders Arrive 2. Separation of Populations 3. Changes in the Gene Pool 4. Reproductive Isolation 5. Ecological Competition 6. Continued Evolution

Reproductive Isolation Concept Map Section 16-3 Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation Temporal isolation Geographic isolation produced by produced by produced by Behavioral differences Different mating times Physical separation which result in Independently evolving populations which result in Formation of new species