Genes Within Populations Chapter 19 1 Introduction Natural

Genes Within Populations Chapter 19 1

Introduction • Natural populations exhibit variation – Species accumulate differences over time – Descendants differ from their ancestors – New species arise from existing ones 2

Genetic Variation and Evolution • Genetic variation – Differences in alleles of genes found within individuals in a population – Raw material for natural selection • Evolution – How an entity changes through time – Development of modern concept traced to Darwin • “Descent with modification” 3

• “Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones. ” – Charles Darwin 4

• 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

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Stretching Proposed ancestor of giraffes has characteristics of modern-day okapi. Stretching The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring. Reproduction a. Lamarck’s theory: acquired variation is passed on to descendants. 6

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Some individuals born happen to have longer necks due to genetic differences. Reproduction Individuals pass on their traits to next generation. Natural selection Reproduction 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. b. Darwin’s theory: natural selection or genetically-based variation leads to evolutionary change. 7

• Population genetics – Study of properties of genes in a population – Evolution results in a change in the genetic composition of a population – Genetic variation is the raw material for selection – In nature, genetic variation is the rule 8

• Human populations contain substantial amounts of genetic variation – Genes that influence blood groups – Genes that influence enzymes • Enzyme polymorphisms 9

• Polymorphic variation – More than one allele at frequencies greater than mutation alone • SNPs – DNA polymorphism – Used to assess patterns in human and natural populations 10

Hardy–Weinberg principle • 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

• Principle can be written as an equation • 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 + 2 pq + q 2 = 1 BB + Bb + bb = 1 12

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Generation One Phenotypes 84% 16% Genotypes BB Bb bb Frequency of genotype in population 0. 36 0. 48 0. 16 Frequency of gametes 0. 36 + 0. 24 = 0. 60 B 0. 24 + 0. 16 = 0. 40 b 13

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Generation Two B p = 0. 60 b q = 0. 40 B p = 0. 60 BB p 2 = 0. 36 Bb pq = 0. 24 b q = 0. 40 Bb pq = 0. 24 bb q 2 = 0. 16 Eggs Sperm p 2 + 2 pq + q 2 = 1 14

• 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 frequency • Suggest hypotheses about what process or processes are at work to cause changes to the frequencies 15

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

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

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

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. • Genetic drift – In small populations, allele frequency may change by chance alone – Magnitude of genetic drift is negatively related to population size – Founder effect – Bottleneck effect Genetic Drift d. Statistical accidents. The random fluctuation in allele frequencies increases as population size decreases. 19

Genetic drift • Alters allele frequencies in small populations – Population must be large – Small number of individuals drift from population – Can lead to the loss of alleles in isolated populations 20

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Parent population Bottleneck (drastic reduction in population) Surviving individuals Next generation • Genetic drift can lead to the loss of alleles in isolated populations • Alleles that initially are uncommon are particularly vulnerable 21

Bottleneck effect • If organisms do not move from place to place their populations may be drastically reduced • Survivors may constitute a random genetic sample of the original population • Results in loss of genetic variability 22

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. UNITED STATES population in 1890, reduced to inhabiting Guadalupe only current population Guadalupe MEXICO • 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 now numbers in tens of thousands 23

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http: //get. adobe. com/flashplayer. 24

• Selection – Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior – Artificial selection – Natural selection 25

• 3 conditions for natural selection to occur and to result in evolutionary change 1. Variation must exist among individuals in a population 2. Variation among individuals must result in differences in the number of offspring surviving in the next generation 3. Variation must be genetically inherited 26

• Natural selection and evolution are not the same – Natural selection is a process • 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

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 • Pocket mice come in different colors – Population living on rocks favor dark color – Populations living on sand favor light color 28

Fitness and its measurement • Fitness – Individuals with one phenotype leave more surviving offspring in the next generation than individuals with an alternative phenotype – Relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number of offspring 29

• 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 30

Selection for increased speed in racehorses is no longer effective 31

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 32

• Gene flow promotes or constrains evolutionary change – Degree of copper tolerance in grass plants on and near ancient mine sites 33

• Gene flow can be – Constructive • Spread beneficial mutation to other populations – Constraining • Can impede adaptation by continual flow of inferior alleles from other populations 34

Maintenance of variation • Frequency-dependent selection – Fitness of a phenotype depends on its frequency within the population – Negative frequency-dependent selection • Rare phenotypes favored by selection • Rare forms may not be in “search image” – Positive frequency-dependent selection • Favors common form • Tends to eliminate variation • “Oddballs” stand out 35

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Percent of Color Type Taken by Fish Predators • Negative frequencydependent selection • In water boatman, fish eat the most common color type more than they would by chance alone 100 80 60 40 Color type of water boatman dark brown medium brown light brown 20 20 40 60 100 80 Color Type Frequency in Population 36

• 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

• 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

• 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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Normal red blood cells Sickled red blood cells Sickle cell allele in Africa 1– 5% 5– 10% 10– 20% Geographic distribution of P. falciparum 39

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

– Available seeds fall into 2 categories – Favors bill sizes for one or the other 0 25 50 75 100 125 Body Size (g) Selection for small and large individuals Number of Individuals • Disruptive selection • Acts to eliminate intermediate types • Different beak sizes of African blackbellied seedcracker finch Number of Individuals Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Two peaks form 0 25 50 75 100 125 Body Size (g) a. Disruptive selection 41

• 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

– Now fewer have that behavior Number of Individuals 0 25 50 75 100 125 Body Size (g) Selection for larger individuals Number of Individuals • 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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Peak shifts 0 25 50 75 100 125 Body Size (g) b. Directional selection 43

Light Dark Average Tendency to Fly Toward Light Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 11 10 9 8 7 6 5 4 3 2 1 0 2 4 6 8 10 12 14 16 Number of Generations 18 Directional selection for negative phototropism in Drosophila 20 44

0 25 50 75 100 125 Body Size (g) Selection for mid-size individuals Number of Individuals • 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 Number of Individuals Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Distribution gets narrower 0 25 50 75 100 125 Body Size (g) c. Stabilizing selection 45

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. births in population infant mortality 100 70 15 50 30 20 10 10 7 5 Percent Infant Mortality Percent of Births in Population 20 5 3 2 2 3 4 5 6 7 8 9 10 Birth Weight in Pounds Stabilizing selection for birth weight in humans 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 47

Constraints on evolutionary change • Mutation and genetic drift – May counter or promote selection • Natural in abandoned mine sites in Great Britain – Copper tolerance in plants 48

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Index of Copper Tolerance Pollen Prevailing wind Mine Site Nonmine 60 Nonmine Bent grass (Agrostis tenuis) 40 20 0 Distance Upwind from Mine (m) 0 20 40 60 80 100 120 140 160 Distance Away from Mine (m) • Slender bent grass at copper mines – Resistance allele occurs at intermediate levels in many areas – Individuals with resistance gene grow slower on uncontaminated sites – Gene flow between sites high enough to counteract selection 49

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 50

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Kentucky Derby Winning Time (seconds) 130 125 120 115 110 1900 1920 1940 1960 1980 2000 Year Selection for increased speed in racehorses is no longer effective 51