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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. : Population genetics: foundation for studying evolution • Microevolution is change in the genetic makeup of a population from generation to generation • Population genetics is the study of how populations change genetically over time • Population genetics integrates Mendelian genetics with the Darwinian theory of evolution by natural selection • This modern synthesis focuses on populations as units of evolution

  4. Gene Pools and Allele Frequencies • A population is a localized group of individuals capable of interbreeding and producing fertile offspring • The gene pool is the total aggregate of genes in a population at any one time • The gene pool consists of all gene loci in all individuals of the population

  5. The Hardy-Weinberg Theorem • The Hardy-Weinberg theorem describes a population that is not evolving • It states that frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work • Mendelian inheritance preserves genetic variation in a populationHardy-Weinberg equilibrium describes a population in which random mating occurs • It describes a population where allele frequencies do not change

  6. Preservation of Allele Frequencies • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change. • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then • p2 + 2pq + q2 = 1 • And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

  7. Gametes for each generation are drawn at random from the gene pool of the previous generation: LE 23-5 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 pq CR (80%) Eggs 64% CRCR 16% CRCW 4% CWCW 16% CRCW CW (20%) qp q2

  8. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over timeThe five conditions for non-evolving populations are rarely met in nature: • Extremely large population size • No gene flow • No mutations • Random mating • No natural selection

  9. Population Genetics and Human Health • We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease

  10. Mutation • Mutation and sexual recombination produce the variation that makes evolution possible • Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals • Mutations are changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise Animation: Genetic Variation from Sexual Recombination

  11. Mutations • A point mutation is a change in one base in a gene • It is usually harmless but may have significant impact on phenotype • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Gene duplication is nearly always harmful • Mutation rates are low in animals and plants • The average is about one mutation in every 100,000 genes per generation • Mutations are more rapid in microorganisms

  12. Sexual Recombination • Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible

  13. Natural selection, genetic drift, and gene flow can alter a population’s genetic composition • Three major factors alter allele frequencies and bring about most evolutionary change: • Natural selection • Genetic drift • Gene flow • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

  14. Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change

  15. LE 23-7 CWCW CRCR CRCR CRCR CRCR CRCW CRCW CRCR CRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCR CRCR CRCR CWCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CWCW CRCR CRCW CRCR CRCR CRCR CRCW CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Generation 3 p = 1.0 q = 0.0 Generation 2 p = 0.5 q = 0.5

  16. The Bottleneck Effect • The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population • The resulting gene pool may no longer be reflective of the original population’s gene pool • Understanding the bottleneck effect can increase understanding of how human activity affects other species

  17. LE 23-8 Original population Bottlenecking event Surviving population

  18. The Founder Effect/Gene Flow • The founder effect occurs when a few individuals become isolated from a larger population • It can affect allele frequencies in a population • Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes • Gene flow causes a population to gain or lose alleles • It tends to reduce differences between populations over time

  19. Natural selection is the primary mechanism of adaptive evolution • Natural selection accumulates and maintains favorable genotypes in a population

  20. Genetic Variation • Genetic variation occurs in individuals in populations of all species • It is not always heritable • Both discrete and quantitative characters contribute to variation within a population • Discrete characters can be classified on an either-or basis • Quantitative characters vary along a continuum within a population

  21. LE 23-9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white

  22. Polymorphism • Phenotypic polymorphism describes a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable • Genetic polymorphisms are the heritable components of characters that occur along a continuum in a population

  23. Measuring Genetic Variation • Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels • Average heterozygosity measures the average percent of loci that are heterozygous in a population. • Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups

  24. LE 23-10 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 9.10 11.12 13.17 15.18 XX

  25. Heights of yarrow plants grown in common garden • Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Seed collection sites

  26. A Closer Look at Natural Selection • From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations

  27. Evolutionary Fitness • The phrases “struggle for existence” and “survival of the fittest” are commonly used to describe natural selection but can be misleading • Reproductive success is generally more subtle and depends on many factors • Fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Relative fitness is the contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus

  28. Directional, Disruptive, and Stabilizing Selection • Selection favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection: • Directional=Directional selection favors individuals at one end of the phenotypic range • Disruptive-Disruptive selection favors individuals at both extremes of the phenotypic range • Stabilizing-Stabilizing selection favors intermediate variants and acts against extreme phenotypes

  29. LE 23-12 Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Directional selection Disruptive selection Stabilizing selection

  30. The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population • Diploidy maintains genetic variation in the form of hidden recessive alleles • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • Balancing selection leads to a state called balanced polymorphism • Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes=HETEROZYGUS ADVANTAGE-SICKLE CELL AMEMIA • Natural selection will tend to maintain two or more alleles at that locus

  31. LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7.5–10.0% 10.0–12.5% >12.5%

  32. Selection • In frequency-dependent selection, the fitness of any morph declines if it becomes too common in the population • Neutral variation is genetic variation that appears to confer no selective advantage • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

  33. On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). LE 23-14 Parental population sample 0.6 Experimental group sample 0.5 Phenotypic variation 0.4 Frequency- independent control 0.3 0.2 0 20 40 60 80 100 Generation number Plain background Patterned background

  34. Intrasexual selection is competition among individuals of one sex for mates of the opposite sex • Intersexual selection occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex • Selection may depend on the showiness of the male’s appearance

  35. The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap” • Sexual reproduction produces genetic variation that may aid in disease resistance

  36. LE 23-16 Asexual reproduction Sexual reproduction Generation 1 Female Female Generation 2 Male Generation 3 Generation 4

  37. Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations

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