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Chapter 23~ • Chapter 23~ The Evolution of Populations Population genetics • Population: a localized group of individuals belonging to the same species • Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring • Gene pool: the total aggregate of genes in a population at any one time • Population genetics: the study of genetic changes in populations • • Modern synthesis/neo-Darwinism “Individuals are selected, but populations evolve.” Hardy-Weinberg Theorem • Serves as a model for the genetic structure of a nonevolving population (equilibrium) • Evolution = change in allele frequencies in a population – hypothetical: what conditions not would cause allele frequencies to change? – non-evolving population REMOVE all agents of evolutionary change 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit) Hardy-Weinberg Equation • p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p) • p2=frequency of AA genotype; 2pq=frequency of Aa genotype; q2=frequency of aa genotype; • frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p2=.36 BB q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 2pq=.48 Bb q2=.16 bb What assume Must are the population genotype frequencies? is in H-W equilibrium! 5 Agents of evolutionary change Gene Flow Genetic Drift Mutation Non-random mating Selection Microevolution, I • A change in the gene pool of a population over a succession of generations • 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability) Microevolution, II: type of genetic drift • The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population Conservation issues Peregrine Falcon • Bottlenecking is an important concept in conservation biology of endangered species – loss of alleles from gene pool – reduces variation – reduces adaptability Breeding programs must consciously outcross Golden Lion Tamarin Microevolution, III type of genetic drift • Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population – just by chance some rare alleles may be at high frequency; others may be missing – skew the gene pool of new population • human populations that started from small group of colonists • example: colonization of New World Microevolution, IV • 2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) • seed & pollen distribution by wind & insect • migration of animals Microevolution, V • 3- Mutations: • Mutation creates variation a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) Microevolution, VI • 4- Nonrandom mating: • Sexual selection • inbreeding and assortive mating (both shift frequencies of different genotypes) Sexual selection QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. It’s FEMALE CHOICE, baby! Microevolution, VII 5.NaturalSelection • differential success in reproduction; • climate change • food source availability • predators, parasites, diseases • toxins • only form of microevolution that adapts a population to its environment • combinations of alleles that provide “fitness” increase in the population Natural Selection • Selection acts on any trait that affects survival or reproduction – predation selection – physiological selection – sexual selection Variation & natural selection • Variation is the raw material for natural selection – there have to be differences within population – some individuals must be more fit than others Where does Variation come from? • Mutation • errors in mitosis & meiosis • environmental damage Beak depth – random changes to DNA Wet year Dry year Dry year 1977 • Sex Dry year 1980 1982 1984 – mixing of alleles • recombination of alleles – new arrangements in every offspring • new combinations = new phenotypes – spreads variation • offspring inherit traits from parent Beak depth of offspring (mm) 11 10 9 8 Medium ground finch 8 9 10 11 Mean beak depth of parents (mm) Population variation • Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population • Geographical variation: differences in genetic structure between populations (cline) Variation preservation • Prevention of natural selection’s reduction of variation • Diploidy 2nd set of chromosomes hides variation in the heterozygote • Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sicklecell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host) Natural selection • Fitness: contribution an individual makes to the gene pool of the • • • • next generation 3 types: A. Directional B. Diversifying C. Stabilizing • Effects of Selection Changes in the average trait of a population DIRECTIONAL SELECTION giraffe neck horse size STABILIZING SELECTION human birth weight DISRUPTIVE SELECTION rock pocket mice Sexual selection • Sexual dimorphism: secondary sex characteristic distinction • Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism Any Questions??