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AP Biology Big Idea #1 - Evolution
Adapted from Rebecca Rehder Wingerden ©
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Simply put… Evolution is simply a change in the genetic makeup of a population over time.
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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 traits are more likely to survive and produce more offspring, - Thus passing traits to subsequent generations (Descent With Modification). “I think” Darwin’s 1837 sketch
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Natural Selection is a major mechanism of evolution
b. Evolutionary fitness is measured by reproductive success (aka: Darwinian Fitness). (a) Cactus-eater (c) Seed-eater Beak Variation in Galapagos Finches (b) Insect-eater
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Natural Selection is a major mechanism of evolution
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. Variation in a population
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Natural Selection is a major mechanism of evolution
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.
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Natural Selection is a major mechanism of evolution
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. f. In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations. PBS Evolving Ideas - How Does Evolution Really Work?(7:00 min.) - Bozeman Biology: Natural Selection (11:00 min.) -
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In the absence of evolutionary influence…
a. …a population is said to be in Hardy-Weinberg equilibrium. Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are: - 1. A large population size (No Genetic Drift – PopGen Sim) - 2. Absence of migration (No Gene Flow) - 3. No net Mutations - 4. Random mating (No Sexual Selection) - 5. Absence of environmental selection (No Natural Selection) • These conditions are seldom met in real populations.
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Hardy-Weinberg h. Mathematical approaches are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population. – The Hardy-Weinberg principle describes a population that is not evolving. – If a population does not meet the criteria of the Hardy- Weinberg principle, then it can be concluded that the population is evolving.
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Hardy-Weinberg • By convention, IF there are only 2 alleles at a locus, p and q are used to represent their frequencies. - p = A, and q = a • The frequency of all alleles in a population will add up to 1 (= 100%) – For example, p + q = 1 • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – where p2 (AA) and q2 (aa) represent the frequencies of the homozygous genotypes and 2pq (Aa) represents the frequency of the heterozygous genotype.
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Selecting alleles at random from a gene pool
2) If all of these alleles could be placed in a large bin, 80% would be CR and 20% would be CW. 3) Assuming mating is random, each time two gametes come together, there is an 80% chance the egg carries a CR allele and a 20% chance it carries a CW allele. 1) The allele frequencies of the population are 0.8 (80%) and 0.2 (20%). Alleles in a population Frequencies of alleles Gametes produced p = frequency of CR allele = 0.8 CW allele = 0.2 Each egg: Each sperm: 80% % % % chance chance chance chance
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Selecting alleles at random from a gene pool
80% CR ( p = 0.8) % CW (q = 0.2) Gametes for each generation are drawn at random from the gene pool of the previous generation. Sperm CR (80%) CW (20%) CR (80%) Egg 64% -p2 CRCR 16% -pq CRCW If the gametes come together at random, then the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: CW (20%) 16% -pq CRCW 4% -q2 CWCW 64% CRCR / 32% CRCW / 4% CW CW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of genotypes in the next generation 64% CRCR / 32% CRCW / 4% CWCW plants
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Applying the Hardy-Weinberg Principle
We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: – The PKU gene mutation rate is low – Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele! • PKU metabolic disorder resulting from homozygosity for a recessive allele: – left untreated leads to mental retardation – newborns are tested for PKU at birth – symptoms can be lessened with a phenylalanine-free diet
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Applying the Hardy-Weinberg Principle
• If the occurrence of PKU is 1 per 10,000 births…what is the frequency of heterozygote carriers? • The frequency of the recessive PKU allele is: – q2 = – q = 0.01 • The frequency of the normal allele is: – p = 1 – q = 1 – 0.01 = 0.99 • The frequency of heterozygous carriers is: – 2pq = 2 x 0.99 x 0.01 = – or approximately 2% of the U.S. population Bozeman Biology: Solving Hardy Weinberg Problems (11:00 min.) Bozeman Biology: Population Genetics and Evolution (6:00 min.) Bozeman Biology: Microevolution (7:00 min.)
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