1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 20 Genes Within Populations 1

2. 2 Genetic variation –Differences in alleles of genes found within individuals in a population –Produces phenotypic variation in population –Raw material for natural selection Genetic Variation and Evolution http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecEvolution/Evolution2PopGen.html http://en.wikipedia.org/wiki/File:Coquina_variation3.jpg

3. 3 Evolution –Change in a population over time –How this change occurs –Development of modern concept traced to Charles Darwin “Descent with modification” Genetic Variation and Evolution

4. “Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones.” –Charles Darwin (On the Origin of Species) 4 http://images3.makefive.com/images/debate/history/all-time-greatest-visionaries/charles-darwin-7.jpg

5. Darwin was not the first to propose a theory of evolution –Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution Rival theory of Jean-Baptiste Lamarck was evolution by inheritance of acquired characteristics 5 http://www.macroevolution.net/images/jean-baptiste-lamarck-239-224-11.jpg

6. 6 Reproduction Stretching a. Lamarck’s theory: acquired variation is passed on to descendants. Proposed ancestor of giraffes has characteristics of modern-day okapi. The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lamarck’s hypothesis was incorrect…not supported by data

7. 7 Reproduction Individuals pass on their traits to next generation. b. Darwin’s theory: natural selection on genetically-based variation leads to evolutionary change. Over many generations, longer-necked individuals are more successful, perhaps because they can feed on taller trees, and pass the long-neck trait on to their offspring. Natural selection Some individuals born happen to have longer necks due to genetic differences. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pre-existing natural variation

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9. Population genetics –Study of properties of genes in a population –Evolution results in a change in the genetic composition of a population Occurs at population level, not individual level –Genetic variation is the raw material for selection In nature, genetic variation is the rule 9 http://fc01.deviantart.net/fs70/i/2011/077/d/8/genetic_variation__by_ravenousaddict-d3bwfna.jpg

10. Polymorphic variation –More than one allele at frequencies greater than mutation alone Single nucleotide polymorphisms - SNPs –Used to assess patterns in human and natural populations 10

11. Hardy–Weinberg principle Predicting changes in allele frequency Hardy–Weinberg equilibrium –Proportions of genotypes do not change in a population if… 1.No mutation takes place 2.No genes are transferred to or from other sources (no immigration or emigration) 3.Random mating is occurring 4.The population size is very large 5.No selection occurs 11

12. H-W principle can be written as a binomial expansion for two alleles –Used to calculate allele frequencies –For 2 alleles, p and q p = B for black coat color Black cat is BB or Bb q = b for white coat color White cats are bb p 2 + 2pq + q 2 = 1 BB + Bb + bb = 1 12

14. 14 Eggs Sperm p 2 + 2 pq + q 2 = 1 b q = 0.40 Bb pq = 0.24 bb q 2 = 0.16 B p = 0.60 BB p 2 = 0.36 Bb pq = 0.24 B p = 0.60 b q = 0.40 Generation Two Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15. If all 5 assumptions for Hardy- Weinberg equilibrium are true, allele and genotype frequencies do not change from one generation to the next –In reality, most populations will not meet all 5 assumptions –To determine this, look for changes in allele frequency –Suggest hypotheses about what process or processes are at work to cause changes to the frequencies 15 http://campus.murraystate.edu/academic/faculty/tderting/bio116/hardy_weinberg.jpg

16. Five agents of evolutionary change (violations of the 5 assumptions) Mutation –Rates generally low –Other evolutionary processes usually more important in changing allele frequency –Ultimate source of genetic variation –Makes evolution possible 16 MutagenDNA T A G G G G C C Mutation a. The ultimate source of variation. Individual mutations occur so rarely that mutation alone usually does not change allele frequency much. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17. Gene flow –Movement of alleles from one population to another –Several sources Animal physically moves into new population Drifting of gametes or immature stages into an area Mating of individuals from adjacent populations 17 Gene Flow b. A very potent agent of change. Individuals or gametes move from one population to another. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18. Nonrandom mating –Assortative mating Phenotypically similar individuals mate Increases proportion of homozygous individuals –Disassortative mating Phenotypically different individuals mate Produces excess of heterozygotes 18 Self-fertilization Nonrandom Mating c. Inbreeding is the most common form. It does not alter allele frequency but reduces the proportion of heterozygotes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

19. Genetic drift –In small populations, allele frequency may change by chance alone –Magnitude of genetic drift is negatively related to population size –Change in alleles is random –Founder effect –Bottleneck effect 19 http://evolution.berkeley.edu/evosite/evo101/images/beetles_mech3.gif

20. Founder effect Alters allele frequencies in small populations –Large “parent” population –Small number of individuals “drift” from population –Can lead to the random loss of alleles in isolated populations  loss of genetic diversity 20 Genetic Drift d. Statistical accidents. The random fluctuation in allele frequencies increases as population size decreases. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

21. Bottleneck effect If organisms do not move from place to place their populations may be drastically reduced (reduction of numbers “in place”) Survivors may constitute a random genetic sample of the original population Results in loss of genetic variability We see this happen with habitat destruction causing species to become isolated 21

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24. Northern Elephant Seal –Bottleneck case study –Nearly hunted to extinction in 19 th century –As a result, species has lost almost all of its genetic variation –Population has rebounded, now numbers in tens of thousands, but still has high homozygosity 24 UNITED STATES Guadalupe MEXICO population in 1890, reduced to inhabiting Guadalupe only current population Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Florida panther is another example of a bottleneck

25. Selection (5 th agent of evolutionary change) –Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior –Artificial selection (human influenced) –Natural selection 25 http://evolution.berkeley.edu/evolibrary/images/interviews/naturalselection1.gif

26. Three conditions for natural selection to occur and to result in evolutionary change 1.Genetic variation must exist among individuals in a population 2.Differential fitness – variation among individuals must result in differences in the number of offspring surviving in the next generation 3.Heritability – variation must be genetically inherited 26 http://www.historyrv.com/blog/wp-content/uploads/2011/12/natural_selection.png

27. Natural selection and evolution are not the same –Natural selection is a process (mechanism) Only one of several processes that can result in evolution –Evolution is the historical record, or outcome, of change through time –Result of evolution driven by natural selection is that populations become better adapted to their environment 27

28. Three agents of selection 28 Light coat color pocket mouse is vulnerable on lava rock Light coat color favored by natural selection because it matches sand color Dark coat color favored by natural selection because it matches black lava color Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Selection to avoid predators –Pocket mice come in different colors Dark-colored mice living on rocks are favored Light-colored mice living on sand are favored

29. Three agents of selection Selection to match climate conditions –Alleles coding for enzymes that differ in their optimal temperature vary over latitude in many species –In mummichogs, the frequency of lactate dehydrogenase-B allele changes with temperature & latitude in the Chesapeake Bay 29 http://www.nicerweb.com/bio3400/Locked/media/ch26/geographic_variation.html http://snhs-plin.barry.edu/Research/MUMMICHOG_FUNDULUS_HETEROCLITUS_files/Mummichog.jpg

30. Three agents of selection Selection for pesticide and microbial resistance –Antibiotic resistance is common among many pathogenic bacteria MRSA (Methicillin-resistant Staphylococcus aureus) MDR-TB (Multi-drug-resistant tuberculosis) 30 http://www.niaid.nih.gov/topics/tuberculosis/understanding/ whatistb/visualtour/pages/xdr-tb.aspx

31. –Houseflies have evolved pesticide resistant alleles at… pen gene decreases insecticide uptake kdr and dld-r genes decrease target sites for insecticide 31 Pesticide molecule Resistant target siteInsect cell membrane b. Insect cells with resistance allele at kdr gene: decreased number of target sites for the pesticide. a. Insect cells with resistance allele at pen gene: decreased uptake of the pesticide. Target site Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

32. Fitness and its measurement Fitness –Individuals with one phenotype leave more surviving offspring in the next generation than individuals with alternative phenotypes –Differential fitness is a “relative” concept the most fit phenotype is simply the one that produces, on average, the greatest number of offspring 32 Image by B. Tyler

33. Fitness has many components –Survival –Sexual selection – some individuals more successful at attracting mates –Number of offspring per mating –Traits favored for one component may be a disadvantage for others Selection favors phenotypes with the greatest fitness –Phenotype with greater fitness usually increases in frequency 33 Image by B. Tyler

35. Interactions Mutations and genetic drift may counter selection –In nature, mutation rates are rarely high enough to counter selection –Selection is nonrandom but genetic drift is random Drift may decrease an allele favored by selection Selection usually overwhelms drift except in small populations 35

36. Gene flow can be –Constructive Spread beneficial mutation to other populations –Constraining Can impede adaptation by continual flow of inferior alleles from other populations 36 Endangered Florida panther may exemplify both of these processes…Texas panthers were brought in to increase genetic diversity to counter male infertility, but since Texas panthers are adapted to a different environment, there is concern of “diluting” adaptive genes in the Florida population. http://www.floridapanther.com/articles/FLORIDA%20PANTHER.htm

37. Oscillating selection –Selection favors one phenotype at one time and another phenotype at another time –Effect will be to maintain genetic variation in the population –Medium ground finch of Galápagos Islands Birds with big bills favored during drought Birds with smaller bills favored in wet conditions 37

38. Heterozygote advantage –Heterozygotes are favored over homozygotes –Works to maintain both alleles in the population –Sickle cell anemia Hereditary disease affecting hemoglobin Causes severe anemia Homozygotes for sickle cell allele usually die before reproducing (without medical treatment) 38

39. Why is the sickle cell allele not eliminated? Leading cause of death in central Africa is malaria Heterozygotes for sickle cell allele do not suffer anemia and are much less susceptible to malaria 39 Normal red blood cells Sickled red blood cells Geographic distribution of P. falciparum Sickle cell allele in Africa 1–5% 5–10% 10–20% Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

40. Selection Many traits affected by more than one gene Selection operates on all the genes for the trait Changes the population depending on which genotypes are favored Types of selection –Disruptive –Directional –Stabilizing 40

41. Disruptive selection Acts to eliminate intermediate types Different beak sizes of African black- bellied seedcracker finch –Available seeds fall into 2 categories –Favors bill sizes for one or the other 41 0255010012575 Body Size (g) Number of Individuals 0255010012575 Body Size (g) Selection for small and large individuals a. Disruptive selection Two peaks form Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

42. Birds with intermediate-sized beaks are at a disadvantage with both seed types – they are unable to open large seeds and too clumsy to efficiently process small seeds 42

43. 43 Directional selection Acts to eliminate one extreme Often occurs in nature when the environment changes In Drosophila, artificially selected flies that moved toward the light –Now fewer have that behavior Peak shifts 0255010012575 Body Size (g) 0255010012575 Selection for larger individuals b. Directional selection Number of Individuals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

44. Stabilizing selection Acts to eliminate both extremes Makes intermediate more common by eliminating extremes In humans, infants with intermediate weight at birth have the highest survival rate 44 0255010012575 Body Size (g) 0255010012575 Body Size (g) Selection for mid-size individuals c. Stabilizing selection Distribution gets narrower Number of Individuals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

45. 45 Stabilizing selection for birth weight in humans 20 15 10 5 20 30 50 70 100 5 7 3 2 2345678910 Birth Weight in Pounds Percent of Births in Population Percent Infant Mortality births in population infant mortality Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

46. Experimental studies To study evolution, biologists have traditionally investigated what has happened in the past –Fossils or DNA evidence Laboratory studies on fruit flies common for more than 50 years Only recently started with lab and field experiments 46

47. Guppy (Poecilia reticulata) Pikecichlid (Crenicichla alta) Guppy (Poecilia reticulata) Killifish (Rivulus hartii) 47 Pike cichlid (predator) rarely found above waterfall –The killifish rarely eats guppies –Guppy males larger and gaudier (more colorful) Predator common below waterfall –Individuals more drab (less colorful) and reproduce earlier Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

48. Limits of selection Multiple phenotypic effects of alleles –Larger clutch size leads to thinner shelled eggs Lack of genetic variation –Gene pool of thoroughbreds limited and performance times have not improved for more than 50 years –Phenotypic variation may not have genetic basis Interactions between genes – epistasis –Selective advantage of an allele at one gene may vary from one genotype to another 48

49. 49 Selection for increased speed in racehorses is no longer effective Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 110 115 120 125 130 Year 200019801960194019201900 Kentucky Derby Winning Time (seconds)