19.0 Introduction

1. Most Organisms Have Different Genetic Compositions
1. Genetic Variation Provides Raw Material for Selection
2. Genetic Variation Exists in Natural Populations fig 19.1

1. Gene Variation Is the Raw Material of Evolution
1. Microevolution Leads to Macroevolution
1. Macroevolution
1. Evolution of new species from old species
2. Changes occurring over long periods of time

2. Microevolution
1. Evolutionary changes within species
2. Natural selection for certain characteristics
3. Characteristics favor increased reproductive success

3. Darwin's explanation: Adaptation is the result of natural selection
4. Adaptation by natural selection results in microevolution (changes in a species)
1. Accumulated events of microevolution leads to macroevolution (new species developed)

2. The Key Is the Source of the Variation
1. Darwin proposed natural selection as the mechanism for evolution
1. New kinds of organisms evolve from existing ones
2. Some individuals have traits that enable them to produce more offspring
3. Offspring in turn carry those traits

2. Lamarck proposed evolution occurred by inheritance of acquired characteristics
1. Individuals acquire body and behavioral changes
2. Pass these changes on to their offspring
3. Example: Neck length in giraffes fig 19.2

3. According to Darwin
1. Experience does not cause variation
2. Variation already existed when acted upon by selection

2. Gene Variation in Nature
1. Genetic Variation Is the Raw Material for Selection
1. Evolution is a series of adaptive changes
1. Brought about by natural selection
2. Results in allele frequency changes

2. Examine genetic variation among individuals within a species

2. Measuring Levels of Genetic Variation
1. groups
1. Over 30 blood groups genes in humans, in addition to ABO
2. 45 other variable genes encoding other proteins in blood and plasma
3. More than 75 variable genes associated with blood system alone

2. Enzymes
1. Measure variation in alternative alleles for enzymes via electrophoresis
2. Great deal of variation at enzyme-specifying loci
3. 5% of enzyme loci in humans are heterozygous

3. Almost all people are different from one another

3. Enzyme Polymorphism
1. Polymorphic loci have more variation than can be explained by mutation fig 19.3
1. Modern study based on techniques like gel electrophoresis
2. Insect and plants polymorphic at over half of loci

2. Heterozygosity: Probability that a randomly selected gene will be heterozygous
1. About 15% in Drosophila and other invertebrates
2. Between 5 and 8% in vertebrates
3. Around 8% in outcrossing plants

3. High levels of genetic variation provide ample raw material for evolution

4. DNA Sequence Polymorphism
1. Gene technology enables examination of variation at level of DNA sequence
2. Example: Sequencing of ADH genes isolated from 11 Drosophila individuals
1. Found 43 variable sites
2. Only one was detected by electrophoresis

3. Abundant variation exists in coding regions and noncoding introns

1. Population Genetics
1. Study of the Properties of Genes Within Populations
1. Genetic Variation Was a Puzzle Before the Discovery of Meiosis

2. Genetic variation in populations puzzled Darwin and contemporaries
1. Selection should always favor an optimal form

2. The Hardy-Weinberg Principle
1. Development of Principle
1. Persistence of variation solved independently by two researchers
2. Original proportions of genotypes in populations will remain constant
3. Requires meeting five assumptions
1. Large population size
2. Random mating occurs
3. No mutation takes place
4. No genes are introduced from other sources (no migration)
5. No selection occurs

4. Dominant alleles do not replace recessive ones
5. Hardy-Weinberg equilibrium: Proportions of genotypes do not change
6. Mathematical basis: Binomial expansion of algebraic equation
1. (p + q)² = p² + 2pq + q²
1. p²= individuals homozygous for A
2. 2pq = individuals heterozygous, alleles A and a
3. q² = individuals homozygous for a

7. Frequency:
1. Proportion of individuals falling in a category compared to total
2. Specific case/total number of individuals

8. Example: Coat color in cats
1. Initial counts: Black= 84; white= 16; total = 100 cats
1. Frequency for black = 0.84 = 84%
2. Frequency for white = 0.16 = 16%
3. Frequency of A allele = p
4. Frequency of a allele = q
5. p + q = 1

2. Assume that white cats are homozygous recessive, bb
3. Black cats are therefore either BB or Bb
4. Calculate allele frequencies of each allele in population
1. Frequency of bb: q² = 0.16
2. Frequency of b: q = 0.4
3. Since 1 = p + q ; frequency of B: p = 0.6

5. Calculate genotype frequencies
1. Frequency of BB: p²= 0.6 x 0.6 = 0.36 (36 individuals out of 100)
2. Frequency of Bb: 2pq = 2 x 0.4 x 0.6 = 0.48 (48 individuals out of 100)

2. Using the Hardy-Weinberg Equation
1. Simple extension of Punnett square, alleles assigned frequencies p and q fig 19.4
1. Trace genetic reassortment during sexual reproduction
2. See how it affects frequencies of B and b genes in next generation
3. Assume union of egg and sperm is random, all combinations of b and B equally likely
4. Genetic reassortment during sexual reproduction
1. Random mating, alleles b and B randomly mixed
2. Individual chance to get B allele = 0.6
3. Individual chance to get b allele = 0.4

5. In next generation
1. Chance for BB = p² = 0.6 x 0.6 = 0.36
2. Chance for bb =q² = 0.4 x 0.4 = 0.16 = 16% =16 individuals out of 100
3. Chance for Bb = 2pq = 2 x 0.6 x 0.4 = 0.48 = 48% = 48 individuals out of 100

2. Example: Cystic fibrosis in North Americans of Caucasian descent
1. Frequency of allele: 22 per 1000 = 0.022 = q
2. Frequency of double recessive (affected) = 0.022 x 0.022 = 0.00048 = 1 per 2000
3. Frequency of dominant allele: p = 1 - 0.022 = 0.978
4. Frequency of carriers: 2pq = 2 x 0.978 x 0.022 = 0.043 = 43 per 1000

3. Calculations are accurate when populations are large and randomly mating
4. When genes do not match predicted values they say a lot about evolution

3. Why Do Allele Frequencies Change?
1. Hardy-Weinberg predicts consistency from generation to generation
1. Large, randomly mating population
2. No mutation, no migration, no selection

2. Used as convenient baseline to measure changes
3. Expressed as heterozygosity: Likelihood of individual being heterozygous at locus
4. Identify many factors that affect equilibrium
5. Five factors alter proportions enough to cause deviation from equilibrium tbl 19.1
1. Mutation
2. Migration (immigration into and emigration out of a population)
3. Genetic drift (random loss of alleles, more likely in small population)
4. Nonrandom mating
5. Selection
1. Only one that produces evolutionary change
2. Only one dependent on nature of environment

3. Five Agents of Evolutionary Change
1. Mutation
1. Change from one allele to another
2. Alters proportion of alleles in population
1. Generally low rate
2. Rare for population to be stable enough to slowly accumulate mutations
3. Ultimate source of genetic variation making evolution possible

2. Migration
1. Movement of individuals from one population to another
1. Powerful agent of change due to exchange of genetic material between populations
2. Characteristics of newcomer must differ from individuals already present
3. Newcomer must be adapted well enough to survive in new area
4. Newcomer must mate successfully for genetic composition to change

2. Includes subtle movements of drifting immature stages or gametes fig 19.2
3. Even low levels tend to homogenize allele frequency in populations, prevent divergence
4. Gene flow: Movement of genes between populations
1. Via migration
2. Via hybridization between adjacent populations
3. Degree of gene flow dependent on several factors
1. Size of unoccupied area separating populations
2. Degree of gene flow dependent on several factors
3. Distance two individuals usually travel to mate

3. Nonrandom Mating
1. Mating of certain individuals more common than expected
2. Inbreeding: Mating with relatives
1. Does not change frequency of alleles
2. Increases proportion of homozygote individuals
3. Self-fertilizing plant populations tend to be mostly homozygous individuals

3. Outcrossing: Mating with nonrelatives
1. Plants breed with individuals other than self
2. Have higher proportion of heterozygotes

4. Faults associated with inbreeding
1. Promotes occurrence of double recessive combinations
2. Increases likelihood of genetic disorders

4. Genetic Drift
1. Changes in allele frequency in small population caused by chance alone
1. Appears to be random, drifting event
2. Small, isolated populations become very different fig 19.6
3. Harmful alleles may increase in frequency, despite selective disadvantage
4. Favorable alleles may be lost, despite selective advantage
5. May be major factor in human evolution

2. Founder effect
1. Few individuals begin new, isolated population
2. Source population rare allele may be significant in new population
3. Important factor in oceanic island evolution

3. Bottle neck effect
1. Populations greatly reduced in size, due to flood, drought, earthquakes
2. Surviving individuals represent random genetic sample of original population
3. Example: Current cheetah population practically genetically identical
4. Occurs in larger populations when only a few individuals breed
1. Large deviations in genotype frequency can occur by chance
2. Leads to loss of alleles in isolated populations

4. Individual isolates may differ from one another substantially
1. Population may seem diverse if isolating barriers are not apparent
2. Population is really numerous genetically uniform isolated subpopulations

5. election
1. Comparing artificial and natural selection
1. Artificial selection
1. Breeder selects characteristics
2. Example: Pigeons fig 19.7

2. Natural selection
1. Environment determines characteristics that confer more offspring
2. Proportions of genes of future populations affected

2. Selection to match climatic conditions
1. Natural studies examine genes encoding enzymes, consequences are easy to assess
2. Enzyme allele frequencies often vary latitudinally (north/south) fig 19.8
1. Allele more common in northern location
2. Allele less common in southern location
3. Implies northern allele encodes form that functions better in colder climate

3. Such situations are called clines
4. Example: Hemoglobin variation in fish Zoarces viviparus
5. Presence of cline not always evidence that selection is working on that enzyme
1. May be case of linkage disequilibrium
2. Selection acts on gene located nearby on chromosome

6. Example: Alcohol dehydrogenase (Adh) in Drosophila fig 19.8
1. Allozyme is an allelic form of enzyme
2. Allozyme more frequent in north more active at low temperatures than allozyme more frequent in south
3. At DNA level only two allozyme alleles and another site show a cline
4. Rules out linkage disequilibrium
5. Selection clearly maintains this cline in allele frequency

3. Selection to avoid predators
1. Include many dramatic documented instances of adaptation
2. Example: Larva of common sulphur butterfly
1. Larvae exhibit color that enables them to blend in with leafy surroundings
2. Alternate bright blue color kept at low frequency by bird predation

3. Example: Shell markings of land snail fig 19.9
1. Enable snail to blend into surroundings
2. More conspicuous snails become food
3. Banding patterns different in areas due to diversity of environment
4. Different habitats conceal different morphs

4. Selection in laboratory environments
1. Action of selection assessed by artificial selection in experiments
2. Develop strains that are genetically identical except for gene studied
3. Discounts potential for linkage disequilibrium
4. Example: Hartyl's crosses with bacteria 6-PGD enzyme
1. All strains grew at same rate
2. Different alleles were selectively neutral in a normal genetic background
3. Under different conditions certain alleles were superior to others

5. Selection for pesticide resistance
1. Example: Tobacco budworm
1. Pyrethroids disable voltage-gaited sodium channels in insect neurons
2. Specimens collected and tested for paralysis by pyrethroid pesticides
3. Specimens characterized for DNA variation at sodium channel and other locus
4. Sensitivity to insecticide paralleled genetic variation at sodium channel locus

2. Example: Triazine resistance in pigweed
1. Triazine inhibits photosynthesis, binds to protein in chloroplast membrane
2. Have sequenced gene encoding the protein
3. Compare nucleotide sequence of resistant and susceptible strains
4. Single amino acid substitution, serine in susceptible to glycine in resistant

4. Identifying the Evolutionary Forces Maintaining Polymorphisms
1. The Adaptive Selection Theory
1. What evolutionary force maintains polymorphism?
1. Migration and nonrandom mating are not major influences in natural population
2. Mutation, genetic drift or selection is most likely force

2. Adaptive selection theory suggests selection is the force that maintains polymorphisms
1. Advanced by Lewontin
2. Natural populations are quite heterogeneous
3. Selection pulls gene frequencies in different directions in different microhabitats
4. Generates a condition in which many alleles persist

2. The Neutral Theory
1. Kimura suggests a balance between mutation and genetic drift maintains polymorphism
1. Demonstrated by elegant mathematics
2. Mutation rates need to be high to generate the variation
3. Population size needs to be small to promote genetic drift
4. Selection is not an acting force, differences between alleles are "neutral to selection"

2. Kimura's equation: H=1/(4Ne m + 1)
1. H (mean heterozygosity): Likelihood that a random individual will be heterozygous at a randomly selected locus
2. Value is influenced by population size (N_e) and mutation rate (m )
3. Level of polymorphism = very large number (N_e) x very small number (m )
4. Both values hard to measure with any precision
5. Theory can thus account for almost any value of H
6. Difficult to prove or disprove, source of controversy

3. Testing the Neutral Theory
1. Difficult to chose between two theories
2. Both account equally well for most data on natural populations
3. Attempt to test neutral theory by examining large-scale patterns of polymorphism
4. Complications regarding population size
1. Polymorphism (H) should be proportional to effective population size N_e
2. Assume mutation rate (m ) is constant
3. H should be greater in insects (Drosophila) than humans
1. When DNA sequence variation is measured, it is by sixfold
2. When enzyme polymorphism examined levels are similar

5. The nearly-neutral model
1. Assume many variants are slightly deleterious, not strictly neutral to selection
2. Helps explain patterns
3. Little evidence that enzyme polymorphism is slightly deleterious

6. Detailed picture of DNA sequence variation is emerging
1. Most nucleotide substitutions are disadvantageous and are eliminated
2. Protein alleles not so clear, may be neutral or advantageous

7. Level of polymorphisms at enzyme-encoding genes may depend on both
1. Adaptive selection theory: Action of selection on the gene
2. Nearly-neutral theory: Population dynamics of the species
3. Relative contribution varies from one gene to next

5. The Shifting Balance Theory
1. Adaptive Selection and Genetic Drift Play Important Role
1. Developed by Sewell Wright
1. Focuses on the way genes interact in determining phenotypes
2. Balance of alleles at different loci determine fitness
3. Compare two genes, produce series of curves representing resulting fitness
4. Graphically looks like topographical map
5. Displays called adaptive topographies fig 19.10

2. Analogy With Inverted Adaptive Topography and Marble
1. Peaks now show as holes, depressions in adaptive surface
1. Life one edge, gravity is selection
2. Marble rolls along, falls into a hole
3. Steeper incline (greater selection), faster marble rolls (more rapid the adaption)
4. Marble stays in hole, held by gravity (selection) even if deeper hole nearby
5. Optimal solution may not arise, selection drives process to first workable solution

2. Random genetic drift is the basis for adaptive improvement
1. Rock the topography model, simulates random genetic drift
2. Marble jumps around
1. Genetic drift shifts balance of between two gene loci
2. Movement is random

3. Shake hard enough,marble may jump out of hole
4. Now free to roll around, perhaps encounter a deeper hole
5. Genetic drift random perturbations enables exploration of new adaptive strategies
1. Eventually deepest, non-escapable hole found
2. Random exploration eventually leads to adaptive improvement

1. Forms of Selection
1. Selection Analogous to Skill in a Game
1. In a competitive game
1. Difficult to predict winner, chance can play important role
2. Over long season, team with most skillful players wins most games

2. In nature
1. Individuals best suited to environment win evolutionary game
2. Leave most offspring
3. Chance can play role in life of any one individual

3. Many traits in nature affected by more than one gene
1. Example: Determining human height fig 13.18
2. Selection operates on all genes
3. Strongest influence on genes making greatest contribution
4. Changes to population depends on which genotypes are favored

2. Disruptive Selection
1. Eliminates intermediate type
2. Example: Color patterns of African butterfly fig 19.11a
3. Partitions population into homozygous groups

3. Stabilizing Selection
1. Eliminates both extremes from array of phenotypes fig 19.11b
2. Increases frequency of intermediate, already the most common
3. Example: Bumpus' examination of English sparrows
4. Prevents change away from middle range
5. Example: Human infant birth weight fig 19.12

4. Directional Selection
1. Eliminates one extreme from array of phenotypes fig 19.11c
2. Decreases frequency of genes promoting extreme
3. Example: Drosophila behavior with light fig 19.13

2. Limits to What Selection Can Accomplish
1. Limits of Selection
1. Alternative alleles may interact with other genes
1. Example: Chicken clutch size
2. Strong selection results in rapid change initially
3. Change dies out as interactions between genes increase

2. Restrict maximums achieved by selective breeding

2. Selecting for Desirable Traits
1. Analysis of performance of Thoroughbred horses
2. 80% of stock today trace back to 31 individuals
3. Despite intense directional selection, times haven't improved in 50 years fig 19.14
1. Probably not due to depletion of genetic variation
2. Difficult to devise alternative explanations

3. Selection Against Rare Alleles
1. Selection acts only on phenotype
2. Only expressed characteristic can affect ability to produce progeny
1. Selection does not operate on rare recessive alleles
2. Cannot select against recessive unless present as homozygotes fig 19.15

3. Selection against undesirable traits difficult unless heterozygotes detected
1. If undesirable trait has frequency of q = 0.01
2. Would take 1000 generations (25,000 human years) to lower frequency to 0.005
3. Frequency still 1 in 40,000 (25% of initial value)