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Interest Grabber 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,

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Presentation on theme: "Interest Grabber 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,"— Presentation transcript:

1 Interest Grabber 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. Section 16-1

2 Interest Grabber continued 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? Section 16-1

3 Answers 1.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.

4 Section Outline 16–1Genes 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 Section 16-1

5 Darwin’s Ideas Darwin did not know how heredity worked: 1.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.

6 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

7 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

8 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)

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

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

11 Frequency of Phenotype (%) Widow’s peakNo widow’s peak Phenotype Figure 16–3 Phenotypes for Single-gene Trait Section 16-1

12 Interest Grabber... 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. Section 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.

13 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.

14 Section Outline 16–2Evolution 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 Section 16-2

15 Natural Selection on Single- Gene Traits 1.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.

16 Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways: 1.Directional selection 2.Stabilizing selection 3.Disruptive selection

17 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

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

19 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

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

21 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.

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

23 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

24 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)

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

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

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

28 Interest Grabber 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. Section 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?

29 Answers Country Cousin/City Cousin 1.Students’ lists should include several social/environmental factors. 2.Students may understand that humans would evolve separately in response to different environmental pressures.

30 Section Outline 16–3The 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 Section 16-3

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


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