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Variation in Natural Populations Overview of Evolutionary Change • Natural Selection: variation among individuals in heritable traits lead to variation among individuals in reproductive success • Evolution: change in genetic composition of a population over time Sooo, understanding evolution reduces to understanding how gene frequencies change over time Where does the genetic variation that natural selection acts on come from? • Mutation is ultimate source of new alleles • Types of Mutations – Point mutations – Chromosome alterations Point mutations QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Base substitutions could be: 1) Missense mutations 2) Silent mutations 3) Neutral mutations Chromosome Alterations Inversions: QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Crossing-over is reduced in heterozygotes for inversions: A C D E F A E D C F Alleles in an inversion are “locked together” and may be selected together as one Selection for Inversions QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Drosophila subobscura: same inversions are found in similar frequencies in similar locations along an environmental cline New genes can arise from gene duplications QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. “gene families”: genes that have arisen from gene duplications Measuring Genetic Variation in Natural Populations • Population genetics incorporates Mendelian Genetics into the study of Evolution • The goal of population genetics is to understand the genetic composition of a population and the forces that determine and change that composition So what exactly is a population? • A population = a group of interbreeding individuals of the same species living within a prescribed geographical area • A Gene Pool = the complete set of genetic information contained within all the individuals in a population Describing the genetic composition of a population • Genotypic frequencies: the proportion of individuals in a population with a given genotype Example: Gene A with two alleles, A and a Frequency of the AA genotype # of individuals with AA genotype = total # of individuals in the population Genotypic frequencies aa Aa aa Frequency (AA) = 2/10 = 0.2 = 20% AA Aa Aa Frequency (Aa) = 5/10 = 0.5 = 50% aa Frequency (aa) = 3/10 = 0.3 = 30% AA Aa Aa Note: The total = 1.0 or 100% Describing the genetic composition of a population • Allelic frequencies: the proportion of alleles of a particular gene locus in a gene pool that are of a specific type Example: Gene A with two alleles, A and a # of copies of the a allele Frequency of the a allele = total # of copies of the A gene Allelic frequencies aa Aa aa Frequency (A) = 9/20 = 0.45 = 45% AA Aa Aa Frequency (a) = 11/20 = 0.55 = 55 % aa AA Aa Aa Note: The total = 1.0 or 100% aa Allele frequencies can also be calculated from genotypic frequencies Aa aa Frequency (A) = f(AA) + 1/2 f(Aa) = 0.2 + 1/2(0.5) = 0.45 AA Aa Aa Frequency (a) = f(aa) + 1/2 f(Aa) = 0.3 + 1/2(0.5) = 0.45 aa AA Aa Aa Note: The total = 1.0 or 100% Measures of Genetic Diversity A genetic locus is said to be polymorphic if that locus has more than one allele occurring at a frequency greater than 5% (for example: if for gene A, f(A) = 0.06, f(a) = 0.94 Heterozygosity: the fraction of individuals in a population that are heterozygotes QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Most species show considerable genetic diversity QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Why do we have polymorphic loci? Shouldn’t dominant alleles replace recessive ones? Shouldn’t natural selection eliminate genetic variation? The Hardy-Weinberg Principle • Allele frequencies and genotypic frequencies will remain constant from generation to generation as long as: – – – – – The population size is large Mating is random No mutation takes place There is no migration in or out of the population There is no natural selection • If these conditions are met, the population is said to be in Hardy-Weinberg Equilibrium How does it work?-Allelic frequencies • By convention, for a given gene the frequency of the dominant allele is symbolized by p, the frequency of the recessive allele is represented by q • So for our previous example, p = f(A) = 9/20=0.45 q = f(a) = 11/20=0.55 • If these are the only two alleles for the gene in the population then p + q = 1.0 How does it work? -Genotypic frequencies Imagine a population in which p = 0.2, q = 0.8 a A a a a a a a a A a a a A a a a a a A The gene pool of this population can be pictured as a container full of gametes. The frequency of gametes carrying the A allele = 0.2 The frequency of gametes carrying the a allele = 0.8 How does it work? -Genotypic frequencies When gametes fuse to produce offspring: a (freq.=q) A (freq.=p) Sperm (generation 0) Eggs (generation 0) A (freq.=p) a (freq.=q) Genotypic frequency (we’ll call this generation 1) Freq (AA) = pxp Freq (Aa) = pxq f(AA) = p2 f(Aa) = 2pq f(aa) = q2 Freq (aA) = qxp Freq (aa) = qxq Since these are all the possible genotypes: p2 + 2pq + q2 = 1 The next generation… Gametes of Generation 0: f(A) = p f(a) = q Genotype frequencies in Generation 1: f(AA) = p2 f(Aa) = 2pq Allele frequencies in Generation 1? p’ = f(A) in generation 1 p’ = f(aa) = q2 What’s the point? • Hardy-Weinberg tells us that if certain conditions are met, there will be no change in gene frequencies--> no evolution – The population size is large – Mating is random – No mutation takes place – There is no migration in or out of the population – There is no natural selection • If one or more of these assumptions is violated, gene frequencies will change --> evolution occurs Other consequences of H-W • Genotypic/ phenotypic frequencies depend on allele frequencies, not on which allele is dominant or recessive Example: Achondroplasia gene: D =dwarfism, d= normal height p = f(D) = 0.00005; q = f(d) = 0.99995 Frequency of dwarfs = p2 + 2pq =0.0001 (one in ten thousand) • For rare recessive alleles, most individuals with the allele will be heterozygotes, and will not express it Example:Cystic fibrosis: C = normal allele, c = cystic fibrosis p = f(C) = 0.978; q = f(c) = 0.022 Freq. of cc individuals = q2 = 0.00048 (1 in 2000) Freq.of Cc individuals = 2pq = 0.043 (almost 1 in 25)