2 Introductory Questions #1 Define what a gene pool is.What are the three aspects in a population we examine in order to understand how evolution is occurring in a population.If a population had 2500 individuals that are diploid, how many total alleles would be present?In a population of 1000 humans, 840 possess the ability to roll their tongues (dominant trait) and 160 cannot. Determine the frequency of the dominant and recessive alleles in the population.What is happening if the population is in “genetic equilibrium”What is the significance of the Hardy Weinberg principle?
3 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-Darwinism“Individuals are selected, but populations evolve.”
4 Hardy-Weinberg Principle Model proposed in 1908Represents an ideal situationSeldom occurs in natureMathematical model is used to compare populationsAllows biologists to calculate allele frequencies in a populationServes as a model for the genetic structure of anon-evolving population (equilibrium)Represents “genetic equilibrium”If the allele frequencies deviate from the predicted values of HW then the population is said to be evolving.
5 Hardy-Weinberg Theorem 5 conditions for Equilibrium-Very large population size- No migration- No net mutations- Random mating- No natural selection**when all these are met then a population is not evolving
6 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 genotype2pq=frequency of Aaq2=frequency of aa genotype;p2 + 2pq + q2 = 1.0
7 Solving & Analyzing HW Principle Problem: If you had 90 individuals that possessed the recessive condition in a population of 1000 individuals, determine the frequency of dominant and recessive alleles present in the population as well as the genotypic and phenotypic frequencies.Always start with the # of homozygous recessive alleles- aa = 90 and q2 = 90/1000 which is 0.09- a = square root of 0.09 which is 0.3- A = (1 – 0.3) which is 0.7- AA = (0.7) which is 0.49- Aa = ???**Remember that p2 + 2pq + q2 = 1(AA) (Aa) (aa)
8 Introductory Questions #1 Define what a gene pool is.What are the three aspects in a population we examine in order to understand how evolution is occurring in a population.If a population had 2500 individuals that are diploid, how many total alleles would be present?In a population of 1000 humans, 840 possess the ability to roll their tongues (dominant trait) and 160 cannot. Determine the frequency of the dominant and recessive alleles in the population.What is happening if the population is in “genetic equilibrium”What is the significance of the Hardy Weinberg principle?
9 Introductory Questions #2 How can allele frequencies change in a population and increase variation? Give three examples. What do we call this when this is happening?Does natural selection operate directly on the phenotype or genotype of organisms? Briefly explain your choice.Name the three modes of selection. Explain how each mode is different and draw a graph representing each mode.Define what genetic polymorphism is and why balanced polymorphism is unique. Give the two mechanisms observed for balanced polymorphism.
10 MicroevolutionInvolves small or minor changes in the allele frequencies within a populationFive processes have been identified:Nonrandom mating (inbreeding & assortative mating)Gene flow (migration between populations)Genetic drift (bottleneck effect)Mutations (unpredictable change in DNA)Natural selection (differential reproduction)**certain alleles are favored over others in nature
11 MicroevolutionA change in the gene pool of a population over a succession of generationsGenetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)
12 MicroevolutionThe 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
13 MicroevolutionFounder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population
14 Microevolution Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)
15 Microevolution Mutations: A Change in the DNA - source of new alleles - genetic variation- “raw materials of natural selection)-unpredictable in nature-Doesn’t determine the direction of evolution-causes small changes in allele frequencies
16 MicroevolutionNonrandom mating: inbreeding and assortative mating (both shift frequencies of different genotypes)Mates are chosen according to desired charachterisitics
17 MicroevolutionNatural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment
18 Natural selectionFitness: refers to the contribution an individual makes to the gene pool of the next generation3 types of Selection:A. DirectionalB. DiversifyingC. Stabilizing
20 Three modes of Selection Stabilizing Selection:-well adapted to the environment-observed in many plants-selection eliminates extreme phenotypes-intermediate form is favoredDirectional Selection:-one phenotype extreme is favored-bell shaped curve is shifted (genetic drift)-Examples: Darwin’s Finches & Peppered mothDisruptive Selection:-causes divergence; splitting apart of the extreme phenotypes-extreme traits are favored-intermediate traits become elimanated
21 Natural Selection in a Population Selects only favorable phenotypic traitsUnfavorable alleles are eliminatedCan maintain genetic diversity-heterozygous advantage (sickle cell anemia) Pg. 399-frequency-dependent selection: rarer phenotypes aremaintained, most common phenotypes eliminated anddecrease in numberNeutral Variations: offers no selective advantage or disadvantage Examples ???Geographical variations and Clines (Clinal variation)Pg. 401
22 Population VariationPolymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same populationGeographical variation: differences in genetic structure between populations (cline)
23 Preserving Variations in a Population Prevention of natural selection’s reduction of variationDiploidy nd set of chromosomes hides variation in the heterozygoteBalanced Polymorphism heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)
24 Sexual selectionSexual dimorphism: secondary sex characteristic distinctionSexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism