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2 Genes Within Populations Chapter 20

3 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” Genetic Variation and Evolution

“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

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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

Polymorphic loci –More than one allele at frequencies greater than mutation alone Heterozygosity –Probability that a randomly selected gene will be heterozygous in a randomly selected individual 9

Hardy–Weinberg principle 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 takes place) 3.Random mating is occurring 4.The population size is very large 5.No selection occurs 10

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 + 2pq + q 2 = 1 BB + Bb + bb = 1 11

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If all 5 assumptions for 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 Look for changes in frequency Suggest hypotheses about what process or processes at work 14

5 agents of evolutionary change Mutation –Rates generally low –Other evolutionary processes usually more important in changing allele frequency –Ultimate source of genetic variation –Makes evolution possible 15

Gene flow –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 16

Nonrandom mating –Assortative mating Phenotypically similar individuals mate Increases proportion of homozygous individuals –Disassortative mating Phenotypically different individuals mate Produces excess of heterozygotes 17

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 18

Genetic drift can lead to the loss of alleles in isolated populations Alleles that initially are uncommon are particularly vulnerable 19

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 20

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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 22

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 23

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 24

Common sulphur butterfly –Caterpillar usually pale green –Excellent camouflage –Bright blue color morph rare and kept at low frequency by predation 25

Pocket mice come in different colors –Population living on rocks favor dark color –Populations living on sand favor light color 26

Housefly has pesticide resistance alleles at –pen gene decreases insecticide uptake –kdr and dld-r genes decrease target sites for insecticide 27

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 28

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 29

Larger female water striders lay more eggs per day Large females survive for a shorter period of time As a result, intermediate-sized females produce the most offspring over the course of their entire lives and thus have the highest fitness 30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Number of Eggs Laid per Day Length of Adult Female Water Strider (mm) Life Span of Adult Female (days) Number of Eggs Laid During Lifetime Length of Adult Female Water Strider (mm)

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 31

Gene flow can be –Constructive Spread beneficial mutation to other populations –Constraining Can impede adaptation by continual flow of inferior alleles from other populations 32

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 33

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 34

Negative frequency- dependent selection In water boatman, fish eat the most common color type more than they would by chance alone 35

36 Positive frequency-dependent selection

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 Galapagos 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 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

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

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 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

44 Directional selection for negative phototropism in Drosophila

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 45

46 Stabilizing selection for birth weight in humans

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

Guppy coloration –Found in small streams in northeastern South America and Trinidad –Some are capable of colonizing portions of streams above waterfalls Different dispersal methods Other species not able to make it upstream –Dispersal barriers create 2 different environments Predators rare above waterfall 48

49 Pike cichlid (predator) rare above waterfall –Killifish rarely eats guppies –Guppy males larger and gaudier Predator common below waterfall –Individuals more drab and reproduce earlier

Guppy lab study –Other explanations are possible for field results –10 large pools –Added pike cichlids to 4, killifish to 4, and 2 left as controls –14 months and 10 guppy generations later –Guppies in killifish and control pool – large and colorful –Guppies in pike cichlid pools – smaller and drab 50

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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 52

53 Selection for increased speed in racehorses is no longer effective

54 Differences in the number of ommatidia in fly eyes does not have a genetic basis