# Chapter 23~ Chapter 23~ The Evolution of Populations

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• Chapter 23~Chapter 23~ The Evolution of Populations

• Population geneticsPopulation: a localized group of individuals belonging to the same speciesSpecies: a group of populations whose individuals have the potential to interbreed and produce fertile offspringGene pool: the total aggregate of genes in a population at any one timePopulation genetics: the study of genetic changes in populationsModern synthesis/neo-DarwinismIndividuals are selected, but populations evolve.

• Hardy-Weinberg TheoremServes as a model for the genetic structure of a nonevolving population (equilibrium)Evolution = change in allele frequencies in a populationhypothetical: what conditions not would cause allele frequencies to change?non-evolving populationREMOVE all agents of evolutionary changevery large population size (no genetic drift)no migration (no gene flow in or out)no mutation (no genetic change)random mating (no sexual selection)no natural selection (everyone is equally fit)

• Hardy-Weinberg Equationp=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

G.H. HardymathematicianW. Weinbergphysician

• What are the genotype frequencies?Using Hardy-Weinberg equationq2 (bb): 16/100 = .16q (b): .16 = 0.4p (B): 1 - 0.4 = 0.6population: 100 cats84 black, 16 whiteHow many of each genotype?bbBbBBp2=.362pq=.48q2=.16Must assume population is in H-W equilibrium!

• 5 Agents of evolutionary change

• Microevolution, IA change in the gene pool of a population over a succession of generations1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

• Microevolution, II: type of genetic driftThe 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 issuesBottlenecking is an important concept in conservation biology of endangered speciesloss of alleles from gene poolreduces variationreduces adaptabilityBreeding programs must consciously outcrossPeregrine FalconGolden Lion Tamarin

• Microevolution, III type of genetic driftFounder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent populationjust by chance some rare alleles may be at high frequency; others may be missingskew the gene pool of new populationhuman populations that started from small group of colonistsexample: colonization of New World

• Microevolution, IV2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)seed & pollen distribution by wind & insectmigration of animals

• Microevolution, V3- Mutations: Mutation creates variation a change in an organisms DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

• Microevolution, VI4- Nonrandom mating:Sexual selection inbreeding and assortive mating (both shift frequencies of different genotypes)

• Sexual selectionIts FEMALE CHOICE, baby!

• Microevolution, VII5.NaturalSelectiondifferential success in reproduction; climate changefood source availabilitypredators, parasites, diseasestoxins only form of microevolution that adapts a population to its environmentcombinations of alleles that provide fitness increase in the population

• Natural SelectionSelection acts on any trait that affects survival or reproductionpredation selectionphysiological selectionsexual selection

• Variation & natural selection Variation is the raw material for natural selectionthere have to be differences within populationsome individuals must be more fit than others

• Where does Variation come from?Mutationrandom changes to DNAerrors in mitosis & meiosisenvironmental damageSex mixing of allelesrecombination of allelesnew arrangements in every offspring new combinations = new phenotypesspreads variationoffspring inherit traits from parent

• Population variationPolymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same populationGeographical variation: differences in genetic structure between populations (cline)

• Variation preservationPrevention of natural selections reduction of variationDiploidy 2nd set of chromosomes hides variation in the heterozygoteBalanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

• Natural selectionFitness: contribution an individual makes to the gene pool of the next generation3 types:A. DirectionalB. DiversifyingC. Stabilizing

• Effects of SelectionChanges in the average trait of a populationDIRECTIONAL SELECTIONSTABILIZING SELECTIONDISRUPTIVE SELECTIONgiraffe neckhorse sizehuman birth weightrock pocket mice

• Sexual selectionSexual dimorphism: secondary sex characteristic distinctionSexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

• Any Questions??

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