Population Genetics Microevolution, Natural Selection & The Hardy Weinberg Equation Packet #14 Chapter #11 12/5/2018 2:34 AM

Population Genetics The study of genetic variability within the population and of the forces that act on it. Gene Pool Consists of all alleles, of all genes, present within a freely interbreeding population. Can be calculated, mathematically, using various frequencies Genotype frequency Phenotype frequency Allele frequency 12/5/2018 2:34 AM

Important Vocabulary Allele Genotype frequency Phenotype frequency Different form of a particular gene Genotype frequency The proportion of a particular genotype in the population. Phenotype frequency The proportion of a particular phenotype in a population. Allele frequency The proportion of a specific allele in a population. 12/5/2018 2:34 AM

Hardy-Weinberg Godfrey Hardy Wilhelm Weinberg British mathematician Wilhelm Weinberg German physician In 1908, both individuals independently reported a mathematical model that describes allele frequencies in a population at any given time. The Hardy-Weinberg Equation 12/5/2018 2:34 AM

The Hardy-Weinberg Equation 12/5/2018 2:34 AM

Variables Letters are used to represent specific variables. p q Represents the frequency of the dominant allele Ranges from 0 to 1 q Represents the frequency of the recessive allele 12/5/2018 2:34 AM

Hardy-Weinberg Equation Binomial Equation (p + q)2 p2 + 2pq + q2 p2 = frequency of homozygous dominant individuals q2 = frequency of homozygous recessive individuals 2pq = frequency of hetereozygous individuals For a population segregating two alleles at a particular time, in which p represents the dominant allele and q represents the recessive allele, the total frequency of all alleles will always equal to 1. p + q = 1.0 Any sexually reproducing population in which the allele frequencies conform to this equation is at genetic equilibrium. 12/5/2018 2:34 AM

Examples See Course Website 12/5/2018 2:34 AM

Assumptions for the Hardy-Weinberg Equation Random mating of all genotypes No net mutations Large population size due to statistical constraints No migration (gene flow) No exchange of alleles with other populations No natural selection 12/5/2018 2:34 AM

Microevolution 12/5/2018 2:34 AM

Microevolution Change in a population’s allele, or genotype, frequencies over successive generations Occurs when a population does not meet all of the assumptions of the Hardy-Weinberg principle The small changes are referred to as microevolution. 12/5/2018 2:34 AM

Microevolution II There are five micro-evolutionary forces: - Non-random mating Mutations Genetic drift Bottleneck effect Founder effect Gene flow Natural selection Stabilizing selection Directional selection Disruptive selection Microevolution Non-Random Mating Mutations Genetic Drift Gene Flow Natural Selection 12/5/2018 2:34 AM

Non-Random Mating Microevolution 12/5/2018 2:34 AM

Non-random Mating Assortative Mating Individuals select mates on the basis of phenotype—indirectly selecting a corresponding genotype. This may lead to interbreeding Leads to an increased homozygous allele composition May lead to interbreeding depression and lowered “fitness” in the population Commonly seen in plants Fitness Ability to pass on genes to the next generation 12/5/2018 2:34 AM

Mutations Microevolution 12/5/2018 2:34 AM

Mutations Unpredictable changes in DNA resulting in the production of new alleles Introduces variation Cause small deviations from Hardy-Weinberg equilibrium Mutations in somatic cells are not heritable Mutations in alleles found in sex cells allow those changes to be passed to the offspring 12/5/2018 2:34 AM

Genetic Drift Microevolution 12/5/2018 2:34 AM

Genetic Drift Random events that change allele frequencies in small populations Small populations are more prone to lose alleles present in low frequencies 12/5/2018 2:34 AM

Bottleneck Effect Rapid and severe declines in population size due to an adverse environmental factor Results in an increase in different allele frequencies 12/5/2018 2:34 AM

Bottleneck Effect Example Examples Northern elephant seals Cheetah American bison Wollemi Pine 12/5/2018 2:34 AM

Founder Effect Occurs when a small population colonizes a new area. Common in island populations Finnish population exhibits much less allelic variation than the general European population Amish population in Pennsylvania has a significant number of individuals with the allele composition for a form of dwarfism. 12/5/2018 2:34 AM

Gene Flow Microevolution 12/5/2018 2:34 AM

Gene Flow The movement of alleles caused by a migration of individuals between populations Migration of breeding individuals introduces new allelic frequencies to a population Tends to counteract natural selection and genetic drift Causes populations to become more genetically similar Humans have experienced an increase in gene flow in the last few hundred years. 12/5/2018 2:34 AM

Natural Selection Microevolution 12/5/2018 2:34 AM

Natural Selection Natural selection results in changes in allele composition that lead to adaptation and is based on differential reproduction Natural selection does not act directly on an organism’s genotype but acts on the phenotype. The phenotype represents an interaction between the environment and all the alleles in the organism’s genotype. Natural selection weeds out those individuals whose phenotypes are less adapted to environmental changes Allowing the better adapted organisms to survive and pass their alleles to future generations Natural selection acts indirectly on the genotype. 12/5/2018 2:34 AM

Natural Selection There are three types of natural selection: - Stabilizing selection Directional selection Disruptive selection 12/5/2018 2:34 AM

Natural Selection Stabilizing Selection Favors intermediate phenotypes Results in reduced variation in a population Classic example is human birth weight Remember that natural selection acts on the phenotypes. 12/5/2018 2:34 AM

Natural Selection Directional Selection Favors one phenotype over another Favors one extreme of the normal distribution over the other phenotypes. Remember that natural selection acts on the phenotypes. 12/5/2018 2:34 AM

Natural Selection Disruptive Selection Favors phenotypic extremes Selects for two or more different phenotypes May result in splitting of a population into 2 or more separate species. Remember that natural selection acts on the phenotypes. 12/5/2018 2:34 AM

Review 12/5/2018 2:34 AM