Biology'1B'–'Evolution'Lecture'4''–' Hardy‐Weinberg,'genetic'drift,'mutat ion'Hardy (Castle) Weinberg Equilibrium: The allele frequency for any characteristic in a stable, non-evolving population will remain the same. For HWE to apply, there are three main assumptions: 1. Random mating 2. No mutation, selection, or migration. 3. A large population (no genetic drift is occurring) Given two alleles in a population, the frequencies of each (p and q respectively) are p2 + 2pq + q2 for any population in Hardy-Weinberg Equilibrium (HWE). This can be used as a “null model” for evolution: if the population is in Hardy-Weinberg Equilibrium, that trait is not evolving (being selected for/against). But if there are deviations, then that means that something interesting is occurring, possibly evolution. A key result is that if an allele is rare (eg. p < 0.1), it occurs most often in heterozygotes rather than homozygotes – i.e. 2pq >> p2 The Hardy-Weinburg Equilibrium can also be used to predict the frequency of heterozygotes in a given population, given the proportion of homozygous recessive phenotype individuals in the population. The heterozygotes are indistinguishable from the dominant homozygous individuals: however, the recessive homozygous individuals are uniquely identifiable. Therefore, q = square-root(frequency homozygous recessive). Since there are only two alleles, p = (1-q). Once you know p and q, you can solve for the frequency of heterozygotes (2pq). Genotype Phenotype AA A (p2+ 2pq) AB BB B (q2) Frequency q = square-root of frequency of B Frequency (AB) = 2*p*q Example - Cystic Fibrosis: Affected individuals in Caucasian population (homozygote recessives) = 4/10000 = 0.0004 Assuming HWE, the estimated frequency of the CF- allele = p = 0.02. Therefore, q = 0.98 The frequency of heterozygotes is 2pq = 2*(.02)*(.98) = .0392 ~~ .04 What does this tell us? Compare the probabilities. In general, recessive alleles are more likely to be present in heterozygous individuals than in homozygous individuals. Deviations from Hardy-Weinberg Equilibrium Inbreeding - How does it affect a population? Inbreeding is when two relatives produce offspring. Since the inbreeders are related, it’s more likely than random mating that for one characteristic they will have matching alleles. This includes recessive alleles. Therefore, the frequency of homozygotes increases (as the genes the offspring receives from both related parents are more likely to be the same) and the frequency of heterozygotes decreases. Consider the following case of self-fertilization, an extreme form of inbreeding: AA * AA Homozygote frequency increases AB * AB Heterozygote frequency decreases as AB*AB produces both homozygotes and heterozygotes BB * BB Homozygote frequency increasesBiology'1B'–'Evolution'Lecture'4''–' Hardy‐Weinberg,'genetic'drift,'mutat ion'Note that inbreeding changes the proportion of genotypes (increasing homozygotes) but does not in itself change allele frequencies Small Population Sizes: Genetic Drift In a small population, the sampling of gametes and fertilization to create zygotes causes random error in allele frequencies. This results in a deviation from the Hardy-Weinberg Equilibrium. This deviation is larger at small sample sizes and smaller at large sample sizes. Think of it like tossing coins - the average result for tossing two coins might be 100% heads. The average for tossing four coins might be 75% heads. But if you take a sample of 10,000 coin tosses, then you are more likely to be close to 50% heads. The direction of this change is random: the dominant or recessive allele might be over or under represented in the next generation relative to the predicted HWE values. This effect is called genetic drift, or that the amplitude of allele frequency fluctuation from one generation to the next increases in small populations. In a small population, genetic drift can result in a loss of variation across the entire genome over time. This even can result in a loss of polymorphism (alternate alleles) and driving the frequency of one allele to 1. How is this relevant to evolution? The fluctuation of allele frequencies in a small isolated population might lead to novel genetic combinations that would not be possibly merely through selection. There are several different situations, which are described below. Alternatively, genetic drift may just reduce genetic diversity (evolutionary potential). Small Population Sizes: Population Bottlenecks A population bottleneck occurs when a population undergoes a severe decrease in size. The effect of genetic drift on this new population is much higher than on the previous population. The new population will be much reduced in genetic diversity: consider the diagram below. Of a population with equal proportions of blue, yellow, and white marbles amongst a population numbering in the 100s, the new population has 5 blue marbles, 1 white marble, and no yellow marbles. This rapid change in allele frequencies could lead to divergence and a new species forming. Note that a population bottleneck does not involve migration (that’s comes later). The most likely causes of a population bottleneck are disease, habitat loss, overharvesting leading to insufficient resources, or climate change. An example is the prairie chickens of the Illinois plains. Their habitat loss resulted in many smaller populations, rather than a large unified population. The smaller population sizes led to a decrease in diversity and a decrease in the viability (i.e. the number of hatching chicks). Figure 1: Population Bottlenecks Campbell 8th edition. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings. (Pg 477)Biology'1B'–'Evolution'Lecture'4''–' Hardy‐Weinberg,'genetic'drift,'mutat ion'Small Population Sizes: Founder Events In this case, a small population of a species moves to a new habitat. The effects are mostly similar to population bottlenecks (reduced genetic diversity in the new population, rapid change in allele frequency, high potential for divergence and speciation) with the exception that there would be more selection occurring due to the new habitat. An example would be human colonization. African Homo sapiens have the most genetic diversity of any human species, while Europe, Asia, and the Americas have lower genetic diversity. This implies that the wellspring of the human species is in Africa (where the
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