Presentation on theme: "Interest Grabber Yes, No, or Maybe"— Presentation transcript:
1 Interest Grabber Yes, No, or Maybe Section 16-1Yes, No, or MaybeSome 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.
2 Interest Grabber continued Section 16-1Make 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?
3 AnswersStudents’ answers will include dimplesand 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 16–1 Genes and Variation A. Darwin’s Ideas Revisited B. Gene PoolsC. Sources of Genetic Variation1. Mutations2. Gene ShufflingD. Single-Gene and Polygenic Traits
5 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.
6 Words to KnowGene pool – combined genetic information of all the members of a particular populationRelative frequency – the number of times an allele occurs in a gene pool compared with the number of times other alleles occurThe 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 environmentCan be limited to one or a few bases of DNACan affect lengthy segments of a chromosomeDo 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 phenotypeSome mutations affect fitness; others do not affect the organism’s ability to survive and reproduce
8 Words to KnowThe 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 Generic Bell Curve for Polygenic Trait Section 16-1Frequency of PhenotypePhenotype (height)
10 Figure 16–2 Relative Frequencies of Alleles Section 16-1Sample PopulationFrequency of Allelesallele for brown furallele for black fur48% heterozygous black16% homozygous black36% homozygous brown
11 Figure 16–3 Phenotypes for Single-gene Trait Section 16-110080604020Frequency of Phenotype(%)Widow’s peakNo widow’s peakPhenotype
12 Interest Grabber . . . All the Help I Can Get Section 16-2. . . All the Help I Can GetNatural 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.
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 16–2 Evolution as Genetic Change A. Natural Selection on Single-Gene TraitsB. Natural Selection on Polygenic Traits1. Directional Selection2. Stabilizing Selection3. Disruptive SelectionC. Genetic DriftD. Evolution Versus Genetic Equilibrium1. Random Mating2. Large Population3. No Movement Into or Out of the Population4. No Mutations5. No Natural Selection
15 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.
16 Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways:Directional selectionStabilizing selectionDisruptive 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 shiftsExample: An increase in the average size of the beaks in a particular species of Galapagos finches; better fitness as they competed for food
18 Figure 16–6 Graph of Directional Selection Section 16-2KeyDirectional SelectionLow mortality, high fitnessHigh mortality, low fitnessFood becomes scarce.
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 graphExample: 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 Figure 16–7 Graph of Stabilizing Selection Section 16-2Stabilizing SelectionKeyLow mortality, high fitnessHigh mortality, low fitnessSelection against both extremes keep curve narrow and in same place.Percentage of PopulationBirth Weight
21 Disruptive SelectionIndividuals at the upper and lower ends of the curve have higher fitness than individuals near the middle; the single curve splits into two curvesExample: 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 Figure 16–8 Graph of Disruptive Selection Section 16-2Disruptive SelectionLargest and smallest seeds become more common.KeyLow mortality, high fitnessPopulation splits into two subgroups specializing in different seeds.Number of Birds in PopulationNumber of Birds in PopulationHigh mortality, low fitnessBeak SizeBeak Size
23 Genetic DriftA random change in allele frequencyIn 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 EffectA situation in which the allele frequencies change as a result of the migration of a small subgroup of a populationExample: Evolution of several hundred species of fruit flies on the Hawaiian Islands (All descended from the same mainland)
25 Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population AFounding Population B
26 Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population AFounding Population B
27 Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population AFounding Population B
28 Interest Grabber Country Cousin/City Cousin Section 16-3Country Cousin/City CousinWhat 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?
29 AnswersCountry Cousin/City CousinStudents’ lists should include severalsocial/environmental factors.2. Students may understand that humans would evolve separately in response to different environmental pressures.
30 Section Outline 16–3 The Process of Speciation A. Isolating Mechanisms 1. Behavioral Isolation2. Geographic Isolation3. Temporal IsolationB. Testing Natural Selection in Nature1. Variation2. Natural Selection3. Rapid EvolutionC. Speciation of Darwin’s Finches1. Founders Arrive2. Separation of Populations3. Changes in the Gene Pool4. Reproductive Isolation5. Ecological Competition6. Continued Evolution
31 Reproductive Isolation Concept MapSection 16-3Reproductive Isolationresults fromIsolating mechanismswhich includeBehavioral isolationTemporal isolationGeographic isolationproduced byproduced byproduced byBehavioral differencesDifferent mating timesPhysical separationwhich result inIndependently evolving populationswhich result inFormation of new species