Evolutionary Change in Populations

A population’s gene pool Includes all the alleles for all the loci present in the population Diploid organisms have a maximum of two different alleles at each genetic locus Typically, a single individual therefore has only a small fraction of the alleles present

Evolution of populations is best understood in terms of frequencies: Genotype Phenotype Allele

Genotype frequencies for all 1000 individuals of a hypothetical population

Phenotype frequencies for all 1000 individuals of a hypothetical population

Allele frequencies for all 1000 individuals of a hypothetical population

Hardy-Weinberg Principle Explains stability of successive generations in populations at genetic equilibrium Essential to understanding mechanisms of evolutionary change

Genetic equilibrium requires Random mating No net mutations Large population size No migration No natural selection

Hardy-Weinberg principle Shows that if population is large, process of inheritance alone does not cause changes in allele frequencies Explains why dominant alleles are not necessarily more common than recessive alleles

Hardy-Weinberg equation p = frequency of dominant allele q = frequency of the recessive allele: p + q = 1

The genotype frequencies of a population are described by the relationship p2 + 2pq + q2 = 1 p2 is frequency of homozygous dominant genotype 2pq is frequency of heterozygous genotype q2 is frequency of homozygous recessive genotype

(a) Genotype and allele frequencies

(b) Segregation of alleles and random fertilization

Microevolution Intergenerational changes in allele or genotype frequencies within a population Often involves relatively small or minor changes, usually over a few generations

Changes in allele frequencies of a population caused by microevolutionary processes: Nonrandom mating Mutation Genetic drift Gene flow Natural selection

Nonrandom mating Inbreeding Assortative mating Inbreeding depression Assortative mating Both of these increase frequency of homozygous genotypes

Mutation Source of new alleles Increases genetic variability acted on by natural selection

Genetic drift Random change in allele frequencies of a small population Decreases genetic variation within a population Changes it causes are usually not adaptive

Genetic drift Bottleneck is a sudden decrease in population size caused by adverse environmental factors Founder effect is genetic drift occurring when a small population colonizes a new area

Gene flow Movement of alleles caused by migration of individuals between populations Causes changes in allele frequencies

Natural selection Causes changes in allele frequencies leading to adaptation Operates on an organism’s phenotype Changes genetic composition of a population favorably for a particular environment

Modes of selection Stabilizing Directional Disruptive Favors the mean Favors one phenotypic extreme Disruptive Favors two or more phenotypic extremes

Modes of selection (a) No selection (b) Stabilizing selection

Modes of selection (c) Directional selection (d) Disruptive selection

Genetic variation in populations caused by Mutations Sexual reproduction Allows new phenotypes

Methods of evaluating genetic variation Diploidy – to have two alleles; hides genetic variation from selection in heterozygote's but maintains diversity Genetic polymorphism (when two or more morphs are present in noticeable freq.) Balanced polymorphism (ability to maintain diversity in a population) Heterozygous advantage (sickle-cell anemia – Ss – resistant to malaria Neutral variation – no selective advantage/disadvantage Little impact on reproductive success Geographic variation

Balanced polymorphism: two or more alleles persist in a population over many generations Heterozygote advantage Frequency-dependent selection

Clinal variation in yarrow (Achillea millefolium)