Population Genetics Packet #29

Population Genetics The study of genetic variability within the population and of the forces that act on it.

Genotype, Phenotype & Allele Frequencies These frequencies can all be calculated as a populations gene pool consists of all the alleles present in the population. – 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.

Hardy-Weinberg Godfrey Hardy – British mathematician Wilhelm Weinberg – German physician Both individuals independently reported a mathematical model, that describes allele frequencies in a population at any given time, in 1908.

Hardy-Weinberg Equation Letters are used to represent specific variables. – p Represents the frequency of the dominant allele Range from 0 to 1 – q Represents the frequency of the recessive allele Range from 0 to 1 Binomial Equation – (p + q) 2 p 2 + 2pq + q 2 – p 2 = frequency of homozygous dominant individuals – q 2 = 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 equalibrium.

Hardy-Weinberg Equation Assumptions Random mating of all genotypes No net mutations Large population size due to statistical constraints No migration – No exchange of alleles with other populations No natural selection

MICROEVOLUTION

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. There are five micro-evolutionary forces

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

Mutations Description – Unpredictable changes in DNA resulting in the production of new alleles Mutations – Introduces variation – Are unpredictable, permanent changes in DNA – Mutations in somatic cells are not heritable – Are not directed – Cause small deviations from Hardy-Weinberg equilibrium

Genetic Drift Random events that change allele frequencies in small populations – Small populations are more prone to lose alleles present in low frequencies

Genetic Drift Bottleneck Effect Rapid and severe declines in population size due to an adverse environmental factor Results in an increase in different allele frequencies Examples – Northern elephant seals – Cheetah – American bison – Wollemi Pine

Genetic Drift 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.

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.

Natural Selection Causes changes in allele composition that lead to adaptation Based on differential reproduction Natural selection does not act directly on an organism’s genotype. Instead, it acts on the phenotype. – The phenotype represents an interaction between the environment and all the alleles in the organism’s genotype. Natural selection acts indirectly on the genotype.

Natural Selection Stabilizing Selection Favors intermediate phenotypes Results in reduced variation in a population – Classic example is human birthweight

Natural Selection Directional Selection Favors one phenotype over another Favors one extreme of the normal distribution over the other phenotypes.

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.

Necessities for Natural Selection Genetic variation is necessary for natural selection – Variation occurs because of mutation, crossing over, independent assortment and random fertilization of the eggs by sperm. – Genetic polymorphism exists among alleles and the proteins for which they code Presence of two or more alleles, for a given locus, within a population. Can be estimated by comparing proteins using gel electrophoresis Can be directly estimated by determining DNA sequences – Balanced polymorphism can exist for long periods of time Heterozygote advantage in carriers of the sickle cell allele – Can produce normal and abnormal hemoglobin – Genetic variation may be maintained by frequency-dependent selection Some phenotypes may be more advantageous if they are rare Prey with uncommon phenotypes may be ignored by predators. – Neutral variation may give no selective advantage or disadvantage Random mutations in DNA that do not alter protein structure do not alter the phenotype – Nonsense mutation? – Geographic variation is genetic variation that exists among different populations within the same specie Cline – Gradual change in a species’ phenotype and genotype frequencies through a series of geographically separate populations