Measuring Evolution of Populations AP Biology Microevolution Change

Measuring Evolution of Populations AP Biology

Microevolution § Change in allele frequencies within a population over generations. Natural selection* u Genetic drift u Gene flow u AP Biology

5 Agents of evolutionary change Mutation Gene Flow Genetic Drift AP Biology Non-random mating Selection

Variation is Key to NS § Genetic Variation – without it, § there would be no natural selection. u Genetic variation produces phenotypic variation, but NOT vice versa. Geographic Variation – same species, but different genetic compositions between separate populations. AP Biology

Sources of Genetic Variation § Mutations forming new alleles u Altering gene number or position u § Deletion of gene segments (if non-harmful) § Duplication of gene segment u Rapid reproduction more chances for mutations § Sexual reproduction AP Biology

Variation in Population § Concepts a population is a localized group of interbreeding individuals u gene pool is collection of alleles in the population u § remember difference between alleles & genes! u allele frequency is how common is that allele in the population § how many A vs. a in whole population AP Biology

Evolution of populations § Evolution = change in allele frequencies in a population u u 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

Hardy-Weinberg equilibrium § Hypothetical, non-evolving population u preserves allele frequencies § Serves as a model (null hypothesis) u u natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a population § measuring evolutionary change G. H. Hardy APmathematician Biology W. Weinberg physician

Hardy-Weinberg theorem § Counting Alleles assume 2 alleles = B, b u frequency of dominant allele (B) = p u frequency of recessive allele (b) = q u § frequencies must add to 1 (100%), so: p+q=1 BB AP Biology Bb bb

Hardy-Weinberg theorem § Counting Individuals u u u frequency of homozygous dominant: p x p = p 2 frequency of homozygous recessive: q x q = q 2 frequency of heterozygotes: (p x q) + (q x p) = 2 pq § frequencies of all individuals must add to 1 (100%), so: p 2 + 2 pq + q 2 = 1 BB AP Biology Bb bb

H-W formulas § Alleles: p+q=1 B § Individuals: p 2 + 2 pq + q 2 = 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? p 2=. 36 BB q 2 (bb): 16/100 =. 16 q (b): √. 16 = 0. 4 p (B): 1 - 0. 4 = 0. 6 2 pq=. 48 Bb q 2=. 16 bb What assume Must are the genotype population frequencies? is in H-W equilibrium! AP Biology

Using Hardy-Weinberg equation p 2=. 36 Assuming H-W equilibrium 2 pq=. 48 q 2=. 16 BB Bb bb p 2=. 20 =. 74 BB 2 pq=. 64 2 pq=. 10 Bb q 2=. 16 bb Null hypothesis Sampled data How do you explain the data? AP Biology

Application of H-W principle § Sickle cell anemia u inherit a mutation in gene coding for hemoglobin § oxygen-carrying blood protein § recessive allele = Hs. Hs w normal allele = Hb u low oxygen levels causes RBC to sickle § breakdown of RBC § clogging small blood vessels § damage to organs u AP Biology often lethal

Sickle cell frequency § High frequency of heterozygotes 1 in 5 in Central Africans = Hb. Hs u unusual for allele with severe detrimental effects in homozygotes u § 1 in 100 = Hs. Hs § usually die before reproductive age Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous… AP Biology

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells Malaria 1 2 AP Biology 3

Heterozygote Advantage § In tropical Africa, where malaria is common: u homozygous dominant (normal) § die or reduced reproduction from malaria: Hb. Hb u homozygous recessive § die or reduced reproduction from sickle cell anemia: Hs. Hs u heterozygote carriers are relatively free of both: Hb. Hs § survive & reproduce more, more common in population Hypothesis: In malaria-infected cells, the O 2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. AP Biology Frequency of sickle cell allele & distribution of malaria

Any Questions? ? AP Biology 2005 -2006