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Chapter 23

Chapter 23. The Evolution of Populations. Overview: The Smallest Unit of Evolution. One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve

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Chapter 23

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  1. Chapter 23 The Evolution of Populations

  2. Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but only populations evolve • Genetic variations in populations contribute to evolution

  3. 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?

  4. 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

  5. Hardy-Weinberg Principle • Model proposed in 1908 • Represents an ideal situation • Seldom occurs in nature • Mathematical model is used to compare populations • Allows biologists to calculate allele frequencies in a population • Serves as a model for the genetic structure of a non-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.

  6. 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

  7. 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 • q2=frequency of aa genotype; p2 + 2pq + q2 = 1.0

  8. 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) 2 which is 0.49 - Aa = ??? **Remember that p2 + 2pq + q2 = 1 (AA) (Aa) (aa)

  9. Microevolution • Involves small or minor changes in the allele frequencies within a population • Five 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

  10. Microevolution 1. Nonrandom mating: inbreeding and assortative mating (both shift frequencies of different genotypes) Mates are chosen according to desired characterisitics

  11. Microevolution 2. Gene Flow: genetic exchange due to the migration of fertile individuals or Gametes between populations (reduces differences between populations)

  12. Microevolution 3. Genetic drift: changes in the gene pool of a small population due to chance (usually reduces geneticvariability) Examples: Bottleneck Effect Founder Effect

  13. Microevolution • 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

  14. Microevolution • Founder Effect:a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population

  15. Microevolution 4. Mutations: A Change in the DNA - source of new alleles - genetic variation - “raw materials of naturalselection) -unpredictable in nature -Doesn’t determine the direction of evolution -causes small changes in allele frequencies http://www.arn.org/blogs/index.php/literature/2008/09/12/hairless_dogs_as_an_example_of_deleterio

  16. Microevolution 5. Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

  17. Natural selection • Fitness: refers to the contribution an individual makes to the gene pool of the next generation 3 types of Selection: • Directional • Diversifying • Stabilizing

  18. Three Types of Selection

  19. Three modes of Selection • Stabilizing Selection: -well adapted to the environment -observed in many plants -selection eliminates extreme phenotypes -intermediate form is favored • Directional Selection: -one phenotype extreme is favored -bell shaped curve is shifted (genetic drift) -Examples: Darwin’s Finches & Peppered moth • Disruptive Selection: -causes divergence; splitting apart of the extreme phenotypes -extreme traits are favored -intermediate traits become elimanated

  20. Natural Selection in a Population • Selects only favorable phenotypic traits • Unfavorable alleles are eliminated • Can maintain genetic diversity -heterozygous advantage (sickle cell anemia) Pg. 399 -frequency-dependent selection: rarer phenotypes are maintained, most common phenotypes eliminated and decrease in number • Neutral Variations: offers no selective advantage or disadvantage Examples ??? • Geographical variations and Clines (Clinal variation) Pg. 401

  21. 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)

  22. Preserving Variations in a Population Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced 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)

  23. Sexual selection • Sexual dimorphism: secondary sex characteristic distinction • Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

  24. 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.

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