Measuring Evolution of Populations Genetic variations in populations

Measuring Evolution of Populations

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!!!

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 CW CW CRCR Only 5 of 10 plants leave offspring CRCW CW CW CRCR CRCW CRCR CW CW CRCR CRCW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3 Only 2 of 10 plants leave offspring CRCR CRCR CRCW CRCR Generation 2 p = 0. 5 q = 0. 5 Figure 23. 7 CRCR Generation 3 p = 1. 0 q = 0. 0

The Bottleneck Effect u 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 u The gene pool may no longer be reflective of the original population’s gene pool 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. (a) Figure 23. 8 A Original population Bottlenecking event Surviving population

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

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

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)

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 mathematician 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 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 Bb bb

H-W formulas § Alleles: p+q=1 B § Individuals: p 2 + 2 pq + q 2 = 1 BB BB 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 Must are assume population is in H-W equilibrium! What the genotype frequencies?

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?

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 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…

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

Heterozygote Advantage § In tropical Africa, where malaria is common: u u u homozygous dominant (normal) die of malaria: Hb. Hb homozygous recessive die of sickle cell anemia: Hs. Hs heterozygote carriers are relatively free of both: Hb. Hs § survive 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. Frequency of sickle cell allele & distribution of malaria