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 Base substitutions could be: 1)Missense mutations 2)Silent mutations 3)Neutral mutations

Chromosome Alterations Inversions: Crossing-over is reduced in heterozygotes for inversions: A CDEFCDEF A EDCFEDCF Alleles in an inversion are “locked together” and may be selected together as one

Selection for Inversions Drosophila subobscura: same inversions are found in similar frequencies in similar locations along an environmental cline

New genes can arise from gene duplications “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

Genotypic frequencies AA Aa aa Frequency (AA) = 2/10 = 0.2 = 20% Frequency (Aa) = 5/10 = 0.5 = 50% Frequency (aa) = 3/10 = 0.3 = 30% 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

Allelic frequencies AA Aa aa Frequency (A) = 9/20 = 0.45 = 45% Frequency (a) = 11/20 = 0.55 = 55 % Note: The total = 1.0 or 100%

Allele frequencies can also be calculated from genotypic frequencies AA Aa aa Frequency (A) = f(AA) + 1/2 f(Aa) = /2(0.5) = 0.45 Frequency (a) = f(aa) + 1/2 f(Aa) = /2(0.5) = 0.45 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

Most species show considerable genetic diversity

Why do we have polymorphic loci? Shouldn’t dominant alleles replace recessive ones? Shouldn’t natural selection eliminate genetic variation?

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 The Hardy-Weinberg Principle

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 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 AAAAaaaaaaaaaaaaaaaa

How does it work? -Genotypic frequencies When gametes fuse to produce offspring: Eggs (generation 0) A (freq.=p) a (freq.=q) A (freq.=p) a (freq.=q) Sperm (generation 0) Freq (AA) = p x p Freq (Aa) = p x q Freq (aA) = q x p Freq (aa) = q x q Genotypic frequency (we’ll call this generation 1) f(AA) = p 2 f(Aa) = 2pq f(aa) = q 2 Since these are all the possible genotypes: p 2 + 2pq + q 2 = 1

The next generation… Genotype frequencies in Generation 1: f(AA) = p 2 f(Aa) = 2pq f(aa) = q 2 Allele frequencies in Generation 1? p’ = f(A) in generation 1 p’ = Gametes of Generation 0: f(A) = pf(a) = q

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 What’s the point?

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) = ; q = f(d) = Frequency of dwarfs = p 2 + 2pq = (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) = Freq. of cc individuals = q 2 = (1 in 2000) Freq.of Cc individuals = 2pq = (almost 1 in 25)