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Measuring Evolution of Populations AP Biology 2007-2008 Populations & Gene Pools Concepts a population is a localized group of the same species that can interbreed gene pool is collection of alleles in the population Gene pool is all of the different genes in that population remember difference between alleles & genes! AP Biology Variation & Gene Pool allele frequency is the number of times that the allele occurs in the population how many A vs a in whole population Expressed in percentage (%) AP Biology i.e. 40% Black & 60% Brown Evolution of populations Evolution = change in allele frequencies in a population hypothetical: what conditions would cause allele frequencies to not change? non-evolving population REMOVE all agents of evolutionary change 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit) AP Biology 5 Agents of evolutionary change Mutation Gene Flow Genetic Drift AP Biology Non-random mating Selection Sources of Variation Mutations Caused by error in replication, radiation, chemicals in the environment Only some mutations change the phenotype & affect fitness Gene Shuffling: results from sexual reproduction 23 pairs of chromosomes can make 8.4 million gene combinations Crossing over causes differences in genes Gene shuffling doesn’t change the allele frequency Still have same # of alleles in population, but recombined AP Biology Single Gene vs Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait Single Gene Trait: controlled by a single gene (2 alleles) Polygenic Trait: traits controlled by two or more genes AP Biology Offers more variation Evolution as Genetic Change evolutionary fitness is an organism’s success in passing genes to the next generation an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individuals ability to pass along its genes Remember that evolution is any change over time in the relative frequency of alleles in a population. This reminds us that it is populations, not individual organisms that can evolve overtime AP Biology Natural Selection on Single Gene Trait Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution AP Biology i.e. Lizards Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways 1. 2. 3. AP Biology Stabilizing Selection Disruptive Selection Directional Selection Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve AP Biology Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Can create 2 distinct phenotypes AP Biology Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end AP Biology Genetic Drift Genetic Drift: the random change in allele frequency Occurs in small populations that break away from larger groups In small populations, an allele can become more or less common by chance Caused by individuals entering & leaving (migrating) Ex. Founder’s Effect When the allele frequency changes as a result of migration of a small group AP Biology Hardy-Weinberg equilibrium Hypothetical, non-evolving population preserves allele frequencies Serves as a model (null hypothesis) natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a population measuring evolutionary change G.H. Hardy AP mathematician Biology W. Weinberg physician Non-Evolving Population Needs 5 conditions for equilibrium: 1. 2. 3. 4. 5. AP Biology Random Mating Large Population (No Genetic Drift) No Migration In or Out No Mutation No Natural Selection Hardy-Weinberg theorem Counting Alleles assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add to 1 (100%), so: p+q=1 BB AP Biology Bb bb Hardy-Weinberg theorem Counting Individuals frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 BB AP Biology Bb bb H-W formulas Alleles: p+q=1 B Individuals: p2 + 2pq + q2 = 1 BB BB AP Biology b Bb Bb bb bb Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p2=.36 BB q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 2pq=.48 Bb q2=.16 bb Must What assume are thepopulation genotype frequencies? is in H-W equilibrium! AP Biology Using Hardy-Weinberg equation p2=.36 Assuming H-W equilibrium 2pq=.48 q2=.16 BB Bb bb p2=.20 =.74 BB 2pq=.64 2pq=.10 Bb q2=.16 bb Null hypothesis Sampled data How do you explain the data? AP Biology Speciation Speciation: formation of a new species Reproductive Isolation: As new species evolve, populations become more reproductively isolated from each other. Isolation Mechanisms: Temporal Isolated Two species reproduce at different times Behaviorally Isolated Can breed, but have different courtship behaviors Geographically Isolated Barriers such as rivers, mountains, bodies of water AP Biology Temporal Isolation Rana aurora - breeds January March AP Biology Rana boylii - breeds late March May Behavioral Isolation Eastern & Western Meadowlark AP Biology Geographic Isolation Albert & Kaibab Squirrels AP Biology