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Measuring Evolution of Populations

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1 Measuring Evolution of Populations

2 Genetic variations in populations
Genetic variation and evolution are both studied in populations. Because members of a population interbreed, they share a common group of genes called a gene pool. Gene pools consist of all the genes, including the different alleles for each gene, that are present in a population Allele Frequency – the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene EVOLUTION, IN GENETIC TERMS, INVOLVES A CHANGE IN THE FREQUENCY OF ALLELES IN A POPULATION OVER TIME!!!

3 Genetic Drift In small populations, individuals that carry a particular allele may leave more descendant than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population. This kid of random change in allele frequency is called genetic drift. Tends to reduce genetic variation Tends to take place in smaller populations Two different models for genetic drift: bottleneck effect & founders effect Figure 23.7 CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

4 The Bottleneck Effect A sudden change in the environment (ex: natural disaster) may drastically reduce the size of a population Wipes out a random part of the population The gene pool may no longer be reflective of the original population’s gene pool Figure 23.8 A (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Original population Bottlenecking event Surviving population

5 The Founder Effect The founder effect
Occurs when a few individuals become isolated from a larger population Migrate to another area Can affect allele frequencies in a population

6 5 Agents of evolutionary change
Mutation Gene Flow Non-random mating Genetic Drift Selection

7 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 very large population size (no genetic drift) no migration (no gene flow in or out) no mutation (no genetic change) random mating (no sexual selection) no natural selection (everyone is equally fit)

8 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 (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?" G.H. Hardy mathematician W. Weinberg physician

9 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 Bb bb

10 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 Bb bb

11 H-W formulas Alleles: p + q = 1 Individuals: p2 + 2pq + q2 = 1 B b BB

12 Using Hardy-Weinberg equation
population: 100 cats 84 black, 16 white How many of each genotype? q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): = 0.6 p2=.36 2pq=.48 q2=.16 BB Bb bb Must assume population is in H-W equilibrium! What are the genotype frequencies?

13 Using Hardy-Weinberg equation
p2=.36 2pq=.48 q2=.16 Assuming H-W equilibrium BB Bb bb Null hypothesis p2=.20 p2=.74 2pq=.10 2pq=.64 q2=.16 q2=.16 Sampled data 1: Hybrids are in some way weaker. Immigration in from an external population that is predomiantly homozygous B Non-random mating... white cats tend to mate with white cats and black cats tend to mate with black cats. Sampled data 2: Heterozygote advantage. What’s preventing this population from being in equilibrium. bb Bb BB Sampled data How do you explain the data? How do you explain the data?

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

15 Sickle cell frequency High frequency of heterozygotes
1 in 5 in Central Africans = HbHs unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs usually die before reproductive age Sickle Cell: In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite. Cystic fibrosis: Cystic fibrosis carriers are thought to be more resistant to cholera: 1:25, or 4% of Caucasians are carriers Cc Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous…

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

17 Heterozygote Advantage
In tropical Africa, where malaria is common: homozygous dominant (normal) die of malaria: HbHb homozygous recessive die of sickle cell anemia: HsHs heterozygote carriers are relatively free of both: HbHs survive more, more common in population Hypothesis: In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele & distribution of malaria

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