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The evolution of Populations chapter 23 Population Genetics Gene PoolHardy-Weinberg Theorem The Cambrian Explosion Allele Frequencies Founder EffectGenetic.

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Presentation on theme: "The evolution of Populations chapter 23 Population Genetics Gene PoolHardy-Weinberg Theorem The Cambrian Explosion Allele Frequencies Founder EffectGenetic."— Presentation transcript:

1 The evolution of Populations chapter 23 Population Genetics Gene PoolHardy-Weinberg Theorem The Cambrian Explosion Allele Frequencies Founder EffectGenetic DriftSympatric Speciation Vs. Allopatric Speciation immigrationPopulationPopulation BottleneckPolyploidy emmigrationAdaptive RadiationArtificial SelectionPunctuated Equilirbrium polymorphismsPrezygotic BarriersIntersexual selectionHeterozygote Advantage Selection pressure Postzygotic BarriersIntrasexual selectionBiological Species Concept Warm UP: Generate Questions or Tasks to complete. Match… Distinguish… Cite Evidence… Differentiate… Critique… Connect… Create… Who? What? Where? When? Why? Which? How?

2 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Natural selection is a major mechanism of evolution. a.According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations. b.Evolutionary fitness is measured by reproductive success. c.Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment. d.Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation. e.An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment.

3 Big Idea 1: The process of ___1.___ drives the diversity and unity of life. Change in the __2._____ makeup of a ___3.____ over time is evolution. ________ 4. _________ is a major mechanism of evolution. a.According to Darwin’s theory of ________4. ________, ___5.______ for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more ___6.____, thus passing traits to subsequent generations. b.Evolutionary fitness is measured by ____7._____ success. c.Genetic variation and mutation play roles in natural selection. A __8._____ gene pool is important for the survival of a species in a changing environment. d.Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation. e.An _____9._____ is a genetic variation that is favored by selection and is manifested as a trait that provides an ____10._____ to an organism in a particular environment.

4 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Natural selection is a major mechanism of evolution. a.According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations. b.Evolutionary fitness is measured by reproductive success. c.Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment. d.Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation. e.An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment.

5 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Biological evolution is supported by scientific evidence from many disciplines, including mathematics. a.Scientific evidence of biological evolution uses information from geographical, geological, physical, chemical and mathematical applications. b.Molecular, morphological and genetic information of existing and extinct organisms add to our understanding of evolution. Provide 4 examples of evidence from many disciplines, including mathematics, that support biological evolution. Explain how each supports the theory of evolution. 1) 2) 3) 4)

6 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Biological evolution is supported by scientific evidence from many disciplines, including mathematics. 1.Fossils can be dated by a variety of methods that provide evidence for evolution. These include the age of the rocks where a fossil is found, the rate of decay of isotopes including carbon-14, the relationships within phylogenetic trees, and the mathematical calculations that take into account information from chemical properties and/or geographical data. The details of these methods are beyond the scope of this course and the AP Exam. 2.Morphological homologies represent features shared by common ancestry. Vestigial structures are remnants of functional structures, which can be compared to fossils and provide evidence for evolution. 3.Biochemical and genetic similarities, in particular DNA nucleotide and protein sequences, provide evidence for evolution and ancestry. 4.Mathematical models and simulations can be used to illustrate and support evolutionary concepts. Ex. Construction of phylogenetic trees based on sequence data (cladogram lab)

7 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Natural selection is a major mechanism of evolution. f.In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations. g.Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are: (1) a large population size, (2) absence of migration, (3) no net mutations, (4) random mating and (5) absence of selection. These conditions are seldom met. h.Mathematical approaches are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population. Hardy-Weingberg Lab will give us practice w/: Graphical analysis of allele frequencies in a population Application of the Hardy-Weinberg equilibrium equation

8 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Natural selection acts on phenotypic variations in populations. a. Environments change and act as selective mechanism on populations. –Flowering time in relation to global climate change –Peppered moth b. Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations. c. Some phenotypic variations significantly increase or decrease fitness of the organism and the population. –Sickle cell anemia –Peppered moth –DDT resistance in insects d. Humans impact variation in other species. –Overuse of antibiotics –Artificial selection –Loss of genetic diversity within a crop species

9 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Evolutionary change is also driven by random processes. a.Genetic drift is a nonselective process occurring in small populations. b.Reduction of genetic variation within a given population can increase the differences between populations of the same species. Life continues to evolve within a changing environment. Speciation and extinction have occurred throughout the Earth’s history. a.Speciation rates can vary, especially when adaptive radiation occurs when new habitats become available. b.Species extinction rates are rapid at times of ecological stress. Ex. Five major extinctions Human impact on ecosystems and species extinction rates

10 Big Idea 1: The process of evolution drives the diversity and unity of life. Change in the genetic makeup of a population over time is evolution. Speciation may occur when two populations become reproductively isolated from each other. a. Speciation results in diversity of life forms. Species can be physically separated by a geographic barrier such as an ocean or a mountain range, or various pre-and post- zygotic mechanisms can maintain reproductive isolation and prevent gene flow. b.New species arise from reproductive isolation over time, which can involve scales of hundreds of thousands or even millions of years, or speciation can occur rapidly through mechanisms such as polyploidy in plants. Populations of organisms continue to evolve. a.Scientific evidence supports the idea that evolution has occurred in all species. b.Scientific evidence supports the idea that evolution continues to occur. Ex. Chemical resistance (mutations for resistance to antibiotics, pesticides, herbicides or chemotherapy drugs occur in the absence of the chemical), Emergent diseases, Observed directional phenotypic change in a population (Grants’ observations of Darwin’s finches in the Galapagos), immune system.

11 Big Idea 1: The process of evolution drives the diversity and unity of life. Essential knowledge 4.C.1: Variation in molecular units provides cells with a wider range of functions. a. b. Variations within molecular classes provide cells and organisms with a wider range of functions. [See also ␣␣␣␣␣, ␣␣␣␣␣, ␣␣␣␣␣, ␣␣␣␣␣ ] To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ Different types of phospholipids in cell membranes ␣␣ Different types of hemoglobin ␣␣ MHC proteins ␣␣ Chlorophylls ␣␣ Molecular diversity of antibodies in response to an antigen Multiple copies of alleles or genes (gene duplication) may provide new phenotypes. [See also ␣␣␣␣␣, ␣␣␣␣␣ ] Evidence of student learning is a demonstrated understanding of each of the following: 1.A heterozygote may be a more advantageous genotype than a homozygote under particular conditions, since with two different alleles, the organism has two forms of proteins that may provide functional resilience in response to environmental stresses. 2.Gene duplication creates a situation in which one copy of the gene maintains its original function, while the duplicate may evolve a new function. To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ The antifreeze gene in fish Learning Objective: ␣␣␣␣␣␣␣␣ The student is able to construct explanations based on evidence of how variation in molecular units provides cells with a wider range of functions. [See ␣␣␣␣␣␣ ] Essential knowledge 4.C.2: Environmental factors influence the expression of the genotype in an organism. a.Environmental factors influence many traits both directly and indirectly. [See also ␣␣␣␣␣, ␣␣␣␣␣ ] Return to the Table of Contents 94 © 2012 The College Board.AP Biology Curriculum Framework b. To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ Height and weight in humans ␣␣ Flower color based on soil pH ␣␣ Seasonal fur color in arctic animals ␣␣ Sex determination in reptiles ␣␣ Density of plant hairs as a function of herbivory ␣␣ Effect of adding lactose to a Lac + bacterial culture ␣␣ Effect of increased UV on melanin production in animals ␣␣ Presence of the opposite mating type on pheromones production in yeast and other fungi An organism’s adaptation to the local environment reflects a flexible response of its genome. To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ Darker fur in cooler regions of the body in certain mammal species ␣␣ Alterations in timing of flowering due to climate changes Learning Objectives: ␣␣␣␣␣␣␣␣ The student is able to construct explanations of the influence of environmental factors on the phenotype of an organism. [See ␣␣␣␣␣␣ ] ␣␣␣␣␣␣␣␣ The student is able to predict the effects of a change in an environmental factor on the genotypic expression of the phenotype. [See ␣␣␣␣␣␣ ] Essential knowledge 4.C.3: The level of variation in a population affects population dynamics. a.Population ability to respond to changes in the environment is affected by genetic diversity. Species and populations with little genetic diversity are at risk for extinction. [See also ␣␣␣␣␣, ␣␣␣␣␣, ␣␣␣␣␣ ] Return to the Table of Contents © 2012 The College Board. To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ California condors ␣␣ Black-footed ferrets ␣␣ Prairie chickens ␣␣ Potato blight causing the potato famine 95 AP Biology Curriculum Framework b. c. ␣␣ Corn rust affects on agricultural crops ␣␣ Tasmanian devils and infectious cancer Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions. To foster student understanding of this concept, instructors can choose an illustrative example such as: ␣␣ Not all animals in a population stampede. ␣␣ Not all individuals in a population in a disease outbreak are equally affected; some may not show symptoms, some may have mild symptoms, or some may be naturally immune and resistant to the disease. Allelic variation within a population can be modeled by the Hardy- Weinberg equation(s). [See also ␣␣␣␣␣ ] Learning Objectives: ␣␣␣␣␣␣␣␣ The student is able to use evidence to justify a claim that a variety of phenotypic responses to a single environmental factor can result from different genotypes within the population. [See ␣␣␣␣␣␣ ] ␣␣␣␣␣␣␣␣ The student is able to use theories and models to make scientific claims and/or predictions about the effects of variation within populations on survival and fitness.

12 MODERN MODIFICATIONS TO DARWIN’S THEORY Recall that Darwin did not know that DNA was the unit of heredity. He did not understand genetics. Genetic Engineering Punctuated Equilibrium Hardy-Weinberg Equilibrium

13 Population genetics deals with microevolution or evolution on a small scale. It can be defined as the change in the allele frequencies of a population. It also examines how genetic variation in a population, specifically- quantitative characters, is required for natural selection to work.

14 3 causes of variation: 1.Mutation 2.Recombination 3.Random fusion of gametes Superior variations that increase fitness are called adaptations.

15 Allele Frequencies within a Population The gene pool: the sum total of genes in a population at any one time. It consists of all alleles at all gene loci in all individuals of the population. If allele frequencies change with each new generation natural selection is acting on the population and the population is evolving. If the allele frequencies do not change with each new generation no natural selection is acting on the population and the population is not evolving. Allele frequency is often given as a decimal ex) P = 0.8 (A dominant) q = 0.2 (a recessive). The gene pool is not altered by Mendelian segregation alone. The relative frequencies of alleles or genotypes remain the same between one generation and the next.

16 Figure 23.3b The Hardy-Weinberg theorem Hardy-Weinberg Theorem: States that under certain (unobtainable) conditions, allele frequencies will remain constant over generations. Therefore, Evolution is NOT occurring.

17 To achieve genetic equilibrium, a population must: 1)Be large, so the effect of chance on changes in allele frequencies is thereby greatly reduced. Smaller population tend to see more chance fluctuation in their gene pool. Why? A sampling error can give funky results ex) coin toss 10 vs. 1000… should get 50% heads Differential reproduction can significantly alter the gene pool. Small numbers means they are more susceptible to change. This is called genetic drift.

18 genetic drift

19 2) Be mutation free By changing one allele into another, mutations alter the gene pool. Mutations are the original source of variation that serves as the raw material for natural selection. 3)Not have a change in allele frequency due to immigration or emigration Most populations experience gene migration, called gene flow. Some isolated populations could eventually become a single population if extensive gene flow occurs.

20 4)Have random mating (with respect to genotype) Seldom achieved (sexual selection) 5)No natural selection All individuals are equal in their ability to survive and produce viable, fertile offspring. This differential success in reproduction is what Darwin meant as “Natural Selection”. Selection results in alleles being passed along to the next generation in numbers disproportionate to frequencies in the present generation. Why? Differential survival. So… there is always (natural) selection pressure acting to disturb the Hardy-Weinberg equilibrium within a population.

21 Some sources of natural variation within a population 1. Polymorphisms Two or more distinct forms of a discrete character in a population. Allows for natural selection to act on phenotypes changing the allele frequencies of the gene pool. It gives the heterozygous individual the ability to have a better reproductive success. Called the Heterozygote advantage. This maintains both alleles in the population. Reflected in RFLP’s… DNA Fingerprint

22 Heterozygote Advantage “Carriers” have higher survival rate thus keep the gene in the gene pool. This maintains polymorphisms Cystic Fibrosis Allele- in heterozygotes offers protection from cholera (prevent dehydration) possibly TB. Sickle Cell Anemia Allele- in heterozygotes offers protection from Malaria.

23 Heterozygote Advantage A situation in which a single disadvantageous allele is not selected out of a population, because, when a person is heterozygous for that allele (the person has one disadvantagoues allele and one normal allele), the person gains some sort of local advantage by having the disadvantageous allele. For example, the allele for sickle-cell anemia offers resistance to malaria. If a person in an area high in malaria is heterozygous for sickle-cell anemia, the resistance they gain to malaria outweighs the disadvantage of having heterozygous sickle-cell anemia. A person with two sickle-cell anemia cells in such a region is will not have a greater advantage, even if they are completely resistant to malaria. What's the use of being resistant to malaria if you're blood can't carry oxygen?

24 Figure Mapping malaria and the sickle-cell allele

25 2. Most Species Exhibit Geographic variations, variations in gene pools between populations Cline = a gradual change in a trait corresponding to a graded change in some geographic axis. Some of the variation has a genetic basis. Ex) yarrow plants in the Sierra Nevada Mountains decrease in size with increasing elevation.

26 3. Mutation and Sexual recombination generate genetic variation

27 What can cause microevolution in a population? A)Genetic Drift: is the change in allele frequency of a small population, due to chance. Two events that can reduce the size of a population: 1)Bottleneck = an environmental crisis may reduce the size of the original population and the small surviving population may not be representative of the original population’s gene pool. Genetic drift may continue change the gene pool for many generations until the population is again large enough for sampling errors to be less significant.

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29 Ex) cheetah- ice age 10,000 years ago & 19th century hunting

30 2) Founder Effect = occurs when a few members of a population colonize an isolated island, lake, or some other new habitat. The allele frequency may not represent the gene pool of the larger population they left. Genetic drift will continue to affect the frequency until the population is large enough for sampling errors from generation to generation to be minimal. Ex) relatively high frequency of certain inherited disorders among human populations established by a small number of colonists.

31 Ellis-Van Creveld Syndrome AMISH Other examples: Fumarase deficiency MORMON sect.

32 Figure Modes of selection

33 B) Natural Selection natural selection acts against certain phenotypes, changing the allele frequency of the population. If the environmental or artificial conditions cause a change in the allele frequency, that 1)Shifts the overall makeup of the population by favoring variants of one extreme, it is called DIRECTIONAL SELECTION ex. Peppered moth (industrial melanism- change in color (more melanin) due to pollution in environment), insecticide resistance, artificial selection: dog/cat/ plant breeding 2)Favors variants of opposite extremes, it is called DISRUPTIVE or DIVERSIFYING SELECTION ex. bird beaks of the Galapagos Finches (adaptive radiation) 3)Acts against the individuals at the ends of the distribution, it is called STABILIZING SELECTION. ex. Human birth weight (before incubators/csection)

34 Figure Modes of selection

35 Figure Directional selection for beak size in a Galápagos population of the medium ground finch

36 Figure Diversifying selection in a finch population

37 C) Sexual Selection leads to sexual dimorphisms (different appearances between males & females) Two varieties of sexual selection: 1)contests: intrasexual selection (competition between the same sex- male vs. male usually) ex) deer or rams butting heads antlers, horns, large stature or musculature 2) Choices: intersexual selection (female mate choice based on appearance or behavior of males) ex) peacock plumage, elaborate mating behaviors

38 Figure 23.16x1 Sexual selection and the evolution of male appearance

39 Figure 23.16x2 Male peacock

40 e) Artificial Selection Humans making mating or breeding choices. Ex. Animal husbandry, agriculture, breeding of cats, dogs, horses etc.

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