Unformatted text preview: Feb 6 Problem of the day Population Genetics The Hardy-Weinberg Equilibrium Hardy- A population is at Hardy Weinberg equilibrium for two traits, each with two alleles. For the first trait (A/a), p = 0.9, q=0.1. For the second trait (B/b), p = 0.4, q=0.6 The traits sort independently. What is the frequency of individuals that are homozygous recessive for both traits (aabb)? 1) 0.1, 2) 0.01, 3) 0.24, 4) 0.024, 5) 0.0024 6) There is not sufficient information to figure this out The Hardy-Weinberg Theorem: In diploid organisms, allele frequencies & Hardygenotypic ratios in large biparental populations reach an equilibrium in one equilibrium generation, and remain constant thereafter, unless disturbed by... by... Mutation Gene flow Genetic drift Non-random mating NonNatural Selection Population Genetics Drivers of Evolution Population Genetics Drivers of Evolution Deviation from Hardy- Weinberg equilibrium is a clear indication that populations are evolving. , Evolution is defined as a change in gene frequencies through time. H-W Eq is the distribution of genotypes indicative of stasis Deviation from HWeq does not dictate evolution, it is a pre-requisite What we know so far: Mutation Mutation Gene flow Genetic drift Non-random mating NonNatural Selection Movement of alleles into or out of a population (the population is not closed) Gene flow Rare on a per locus basis Often deleterious Point mutation or chromosomal Causes of Evolution C. Genetic Drift The random change in allele frequencies and loss of
alleles, due to chance Genetic drift occurs because populations are not infinitely large: The larger the population, the LESS the importance of drift The smaller the population, the GREATER the importance of drift Imagine a new habitat is colonized by just a few individuals. Allele frequencies will likely be changed simply by chance events. In addition, rare alleles are likely to be lost (why rare ones?).
"New" population Established population Drift happens to some extent in all real populations (though we will ignore it in very large ones), but there are 2 demographic processes that can make drift extremely strong and important: important: 1) 2) Founder effects Population bottlenecks
These are becoming more common as we make populations very small! e.g., Founder effect 1 Causes of Evolution C. Genetic Drift, 1. Founder Effects Hall of Fame
1890-91 (100 birds) 18902009 (200,000,000 birds) The Eurasian round goby European starlings introduced to N. America in 1890 (n=60) & 1891 (n=40) by Eugene Schieffelin, of the Acclimation Society of N. America Schieffelin, Current population in N. America 200,000,000 birds Source population carries >31 alleles at 11 loci N. American population now carries 18 alleles at those same loci 42% loss relative to native populations Introduced in 1990 in ship ballast water. Spreading rapidly. Source populations identified. No apparent founder effects Mitochondrial DNA Conservation biologists also worry about reduced integrity of re-introduced species and potential founder effects that might be created Causes of Evolution C. Genetic Drift, 2. Population Bottlenecks
Population bottlenecks are very similar to founder effects, but occur when populations are greatly reduced in size Bottlenecks occur when species are overharvested by humans, or when when their habitats are reduced or fragmented extensively The Result: non-random assortment of genotypes. Spatial bottlenecks: small bottlenecks isolated habitats support very few individuals (fragmented populations). Temporal bottlenecks: a bottlenecks population passing through a period of very low population size can suffer from a reduction, or change in allele frequencies through chance events (exploited species). Can also be a selective filter 2 Causes of Evolution Bottleneck Hall of Fame Before 1884, populations estimated between 200,000 & 1,000,000 seals By 1892, hunting reduced the size of the populations to 8-20 individuals 8 In 1991, populations estimated to have recovered to >125,000 individuals Causes of Evolution Bottleneck Hall of Fame Speke's Speke' Gazelle
From Somalia / Ethiopia St Louis Zoo; 1969 4 founding animals (1 male, 3 females). ** We have yet to see the full consequences of these various near extinctions.
Go see them in the winter at Ao Nuevo State Park!! Causes of Evolution Bottleneck Hall of Fame
Both American and European Bison suffered severe population bottlenecks Florida Panther ~100, up from ~6 19945 genetic rescue Jan 2009 Causes of Evolution C. Genetic Drift, 2. Population Bottlenecks
Greater prairie chickens in Illinois were reduced by hunting & habitat loss to about habitat 50 birds in the 1990s; they lost an estimated > 75% of allelic diversity they Causes of Evolution D. Non-Random Mating NonNonrandom mating occurs when individuals choose mates with particular genotypes or phenotypes; there are 2 major types: 1. INBREEDING (or, more correctly, positive assortative mating) occurs when individuals preferentially mate with the same genotype as themselves. 2. OUTBREEDING (negative assortative mating) occurs when individuals avoid mating with similar genotypes (or close relatives); relatives); 3. Sexual Selection 4. Breeding Disparity (Giants of the Gene Pool). 3 Sexual selection Causes of Evolution D. Non-Random Mating, 1. Inbreeding NonInbreeding (positive assortative mating) occurs when individuals preferentially mate with the same genotype as themselves: A HYPOTHETICAL EXERCISE:
a. Start with 3 genotypes: A1A1, A1A2, A2A2 p (A1) = 0.5 q (A2) = 0.5 What are the EXPECTED genotype frequencies at H-W equilibrium? H- Carrying traits that would likely otherwise reduce fitness in order to attract mating opportunities
Fitness disparity Giants of the gene pool; not just for humans Effective population size (Ne) b. Mating rule: Each genotype only mates with genotypes like itself (i.e. perfect inbreeding) Causes of Evolution D. Non-Random Mating, 1. Inbreeding NonPerfect within type mating: Option 1. A1A1 homozygotes only mate with A1A1 homozygotes: homozygotes: result: A1A1 homozygotes: no change homozygotes: Option 2: A2A2 homozygotes only mate with A2A2 homozygotes result: A2A2 homozygotes: no change homozygotes:
Neither of these kinds of inbred matings will have any effect on allelic and genotypic frequencies in the next generation What matters is heterozygote matings... matings... Causes of Evolution D. Non-Random Mating, 1. Inbreeding NonOption 3. Heterozygote inbred matings... matings...
1st generation: F1's : A1A2 A1A1(25%) A1A2 A1A1(25%) x x A1A2 A1A2(50%) A1A2 A1A2(50%) A2A2(25%) A2A2(25%) All hets 50% hets 2nd generation F2's : All hets 50% hets The number of heterozygotes keeps declining by 50% per generation, generation, until they're gone they' Reality is not as extreme because inbreeding isn't "perfect", but you isn' perfect" get the idea. Causes of Evolution D. Non-Random Mating, 1. Inbreeding NonEffects of Inbreeding on Fitness: PKU (phenylketonuria) (phenylketonuria) Inbreeding increases the likelihood that deleterious recessive alleles will alleles be present in the homozygous state:
PKU is caused by a recessive mutation in a gene we'll we' call the R-locus (mutant allele = r) RRR and Rr (CARRIER) genotypes can convert phenylalanine to tyrosine rr genotypes cannot do this, so a byproduct of phenylalanine accumulates in nervous tissue and causes severe brain damage The observed frequency of the r allele is about 0.01 in human populations, so the EXPECTED frequency of rr homozygotes (diseased) is (0.01)2 = 1/10,000 In Ireland, p(r) is also about 0.01, yet until recently, the p(r) incidence of PKU was closer to 1/4500 births. How can this be? Inbreeding Depression A reduction in population fitness as a consequence of positive assortative mating (inbreeding). Mechanism: the expression of deleterious recessive alleles in the homozygous state. Genetic Load: a measure of the reduction in fitness experienced by a population as a consequence of deleterious alleles. 4 Causes of Evolution Bottleneck Hall of Fame Speke's Speke' Gazelle Zoo studbooks Causes of Evolution D. Non-Random Mating, 2. Outbreeding Non- Heterosis Increased fitness among individuals with higher heterozygosity Hybrid vigor The pin type has a stigma at the top; the thrum type is reversed Insects get pollen on their bodies and transfer it between pin and thrum flowers, or vice versa, to pollinate them Nectar at bottom Fig 22.11 in text Outbreeding depression Reduced fitness as a consequence of nonrandom negative assortative mating (outbreeding). Mendelian problem
When crossing two individuals that are heterozygous for each of two traits, the expected phenotype ratio is: 1:2:1 3:1 9:3:3:1 4:4:4:4 A. B. C. D. 5 Two ways to figure that out Count the Punnett Square, or Probabilities: (AaBb x AaBb)
P(A_) = P(aa) = P(B_) = P(bb) = P(A_B_) = x = 9/16 P(A_bb) = x = 3/16 P(aaB_) = x = 3/16 P(aabb) = x = 1/16 Hence 9:3:3:1 6 ...
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