23.3 23.4

1. 23.3 23.4 Natural selection, genetic drift, and gene flow can alter a population’s genetic composition

2. Concept 23.3: 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

3. Genetic Drift Unpredictable fluctuation in alleles frequency from one generation to the next because of a population finite size. • Statistically, the smaller a sample • The greater the chance of deviation from a predicted result

4. Natural Selection • Differential success in reproduction • Results in certain alleles being passed to the next generation in greater proportions

5. CWCW CRCR CRCR CRCR CRCR Only 2 of 10 plants leave offspring Only 5 of 10 plants leave offspring CRCR CRCW CRCR CRCW CRCR CRCR CWCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCR CWCW CRCR CRCW CRCR CRCR CRCR CRCR CRCW CRCW CRCW Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Figure 23.7 • Genetic drift • Describes how allele frequencies can fluctuate unpredictably from one generation to the next • Tends to reduce genetic variation

6. Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Figure 23.8 A The Bottleneck Effect • A sudden change in the environment may drastically reduce the size of a population • The gene pool may no longer be reflective of the original population’s gene pool Bottlenecking event Surviving population Original population

7. Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Figure 23.8 B • Understanding the bottleneck effect • Can increase understanding of how human activity affects other species

8. The Founder Effect • Occurs when a few individuals become isolated from a larger population • Can affect allele frequencies in a population

9. Gene Flow • Results from the movement of fertile individuals or gametes • Causes a population to gain or lose alleles • Tends to reduce differences between populations over time

10. Concept 23.4:Natural selection is the primary mechanism of adaptive evolution • Natural selection • Accumulates and maintains favorable genotypes in a population

11. (a) Map butterflies thatemerge in spring:orange and brown (b) Map butterflies thatemerge in late summer:black and white Figure 23.9 A, B Genetic Variation • Occurs in individuals in populations of all species • Is not always heritable Seasonal differences in hormones

12. Variation Within a Population • Both discrete and quantitative characters Contribute to variation within a population • Discrete characters • Can be classified on an either-or basis • The different forms can be called morphs. • Quantitative characters • Vary along a continuum within a population

13. Polymorphism • Phenotypic polymorphism • Describes a population in which two or more distinct morphs for a character are each 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 • Alleles of several loci affect the character • height

14. Measuring Genetic Variation • Population geneticists measure the number of polymorphisms in a population by determining the amount of heterozygosity • At the gene level • Average heterozygosity: measures the average percent of loci that are heterozygous in a population • At the molecular level:nucleotide variability Average heterozygosity tends to be greater than nucleotide variability

15. 3.14 2.4 5.18 7.15 1 6 19 XX 9.12 10.16 13.17 8.11 1 2.19 3.8 4.16 5.14 6.7 13.17 XX 11.12 15.18 9.10 Figure 23.10 Variation Between Populations • Most species exhibit geographic variation • Differences between gene pools of separate populations or population subgroups

16. Heights of yarrow plants grown in common garden EXPERIMENTResearchers observed that the average sizeof yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants. Mean height (cm) CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates therole of genetic as well as environmental differences. RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. Atitude (m) Sierra NevadaRange Great BasinPlateau Seed collection sites Figure 23.11 Some examples of geographic variation occur as a cline: a graded change in a trait along a geographic axis (parallels to the gradient in the environment)

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

18. Evolutionary Fitness • The phrases “struggle for existence” and “survival of the fittest” • Are commonly used to describe natural selection • Can be misleading • Reproductive success • Is generally more subtle and depends on many factors

19. Fitness • Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • In a more quantitative way: Relative fitness • the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

20. Directional, Disruptive, and Stabilizing Selection • Selection • Favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection are • Directional • Disruptive • Stabilizing

21. Directional selection • Favors individuals at one end of the phenotypic range • Disruptive selection • Favors individuals at both extremes of the phenotypic range • Stabilizing selection • Favors intermediate variants and acts against extreme phenotypes

22. The three modes of selection Disruptive selection Stabilizing selection Directional selection

23. Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. • In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals themfrom predators.

24. Disruptive selection • favors variantsat both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

25. Stabilizing selection removesextreme variants from the populationand preserves intermediate types. If the environment consists of rocks ofan intermediate color, both light anddark mice will be selected against.

26. The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population • Diploidy (Eukaryotes are diploid) • Maintains genetic variation in the form of hidden recessive alleles

27. Balancing Selection • Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • Leads to a state called balanced polymorphism

28. Heterozygote Advantage • Some individuals who are heterozygous at a particular locus • Have greater fitness than homozygotes • Natural selection • Will tend to maintain two or more alleles at that locus

29. Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Figure 23.13 The sickle-cell allele • Causes mutations in hemoglobin but also confers malaria resistance • Exemplifies the heterozygote advantage

30. Frequency-Dependent Selection • The fitness of any morph declines if it becomes too common in the population

31. On pecking a moth imagethe blue jay receives afood reward. If the bird does not detect a mothon either screen, it pecksthe green circle to continueto a new set of images (anew feeding opportunity). Parental population sample Experimental group sample 0.06 0.05 0.04 Frequency-independent control 0.03 0.02 60 20 40 80 100 0 Generation number Figure 23.14 Plain background Patterned background • An example of frequency-dependent selection Phenotypic diversity

32. Neutral Variation • Is genetic variation that appears to confer no selective advantage

33. Sexual Selection • Is natural selection for mating success • Sexual dimorphism: • marked differences between the sexes in secondary sexual characteristics

34. Intrasexual selection • Is a direct competition among individuals of one sex for mates of the opposite sex

35. Figure 23.15 Intersexual selection • Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex • May depend on the showiness of the male’s appearance

36. Sexual reproduction Asexual reproduction Generation 1 Female Female Generation 2 Male Generation 3 Generation 4 Figure 23.16 The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction • Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap

37. If sexual reproduction is a handicap, why has it persisted? • It produces genetic variation that may aid in disease resistance

38. Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Each species has a legacy of descent with modification. (birds 4 legs & wins?) • Adaptations are often compromises • Seals swim well, walk not as well • Chance and natural selection interact • Selection can only edit existing variations