Evolution & Population Genetics
Chapter 20
Why do monarch butterflies have orange and black wings?
 Proximate answer: “Because of special pigments in their wings” (functional
process)
 Ultimate answer: “Because bright colors provide an advantage by advertising
toxicity” (evolutionary process)
Evolution
 Evolution = change
 Biological or organic evolution is the change in the properties of groups of
organisms over the course of generations
Individuals do not evolve
Populations evolve, undergoing “descent with modification”
Before Darwin
 Special creation
Species are fixed and unchanging
Before Darwin
 Lamarck’s inheritance of acquired characteristics
Darwin’s Voyage
Alfred Wallace
 Naturalist who arrived at the same conclusions Darwin did
 Prompted Darwin to finally present his ideas in 1858
What are the necessary conditions for evolution due to natural selection to
occur?
Darwin’s problem
 How are traits passed to offspring?
Darwin’s idea: Pangenesis (blending inheritance)
Mendel’s research on particulate inheritance in peas (1865; rediscovered in
1900) was key in understanding evolution
Genetics review
Individuals have 2 alleles for each gene
Population genetics & allele frequency
Hardy-Weinberg Principle
 Original proportions of genotypes in a population will remain constant from
generation to generation if:
• No mutation takes place
• No genes are transferred to or from other sources
• Random mating is occurring
• The population size is very large
• No selection occurs
Hardy-Weinberg Principle
Calculate genotype frequencies with a binomial expansion
(p+q)2 = p2 + 2pq + q2
 p = individuals homozygous for first allele
 2pq = individuals heterozygous for both alleles
 q = individuals homozygous for second allele
 because there are only two alleles:
p plus q must always equal 1
Hardy-Weinberg Principle
Evolution = Change in allelle frequencies over time
A population not in Hardy-Weinberg equilibrium indicates that one or more of the
five evolutionary agents are operating in a population
Mutation
 A random change in a cell’s DNA
 Mutation rates have little effect on H-W equilibrium
 Ultimate source of genetic variation
Gene flow
 A movement of alleles from one population to another
 Powerful agent of change
 Tends to homogenize allele frequencies
Non-random mating
 Mating with specific genotypes
Shifts genotype frequencies
 Assortative Mating: does not change frequency of individual alleles;
increases the proportion of homozygous individuals
 Disassortative Mating: phenotypically different individuals mate; produce
excess of heterozygotes
Genetic drift
 Random fluctuation in allele frequencies over time by chance
Important in small populations
• Founder effect
• Bottleneck effect
Founder effect
Few individuals found new population (small allelic pool)
Bottleneck effect
 Drastic reduction in population and gene pool size
Natural selection
 Natural selection is not evolution
 Differential survival and/or reproduction
 Differences in survival and/or reproduction are not due to chance
 Reduces variation in a population
Selection to avoid predators
Selection to match climatic conditions
 Lactate dehydrogenase in Fundulus heteroclitus varies with latitude
 Enzymes formed function differently at different temperatures
 Form of lactate dehydrogenase at northern latitudes is a better catalyst at low
temperatures
Selection for pesticide resistance
 Norway rats have developed resistance to warfarin
Fitness and its measurement
Fitness: A phenotype with greater fitness usually increases in frequency
Most fit is given a value of 1
Fitness is a combination of:
Survival
Mating success
Number of offspring per mating that survive
Interactions among evolutionary forces
 Mutation and genetic drift may counter selection
 The magnitude of drift is inversely related to population size
Interactions among evolutionary forces
 Gene flow may promote or constrain evolutionary change
Spread a beneficial mutation
Impede adaptation by continual flow of inferior alleles from other
populations
 Extent to which gene flow can hinder the effects of natural selection depends
on the relative strengths of gene flow
High in birds & wind-pollinated plants
Low in sedentary species
Copper tolerance
Maintenance of variation
 Frequency-dependent selection depends on how frequently or infrequently a
phenotype occurs in a population
Negative frequency-dependent selection - rare phenotypes are favored by
selection
Positive frequency-dependent selection - common phenotypes are favored;
variation is eliminated from the population
 Strength of selection changes through time
Negative frequency dependent selection
Positive frequency-dependent selection
Scale eating fish
Inverse-frequency dependent selection
Oscillating selection
 Selection favors one phenotype at one time and a different phenotype at
another time
 Fitness of a phenotype does not depend on its frequency
 Environmental changes lead to oscillation in selection
Oscillating selection
 Galápagos Islands ground finches
Wet conditions favor bigger bills
Dry conditions favor smaller bills
Heterozygote advantage
 Heterozygotes may exhibit greater fitness than homozygotes
 Keeps deleterious alleles in a population
Malaria
 Malaria caused by Plasmodium falciparum (a protist) that infects red blood
cells
 Heterozygote advantage arises from balance of opposing selective factors –
anemia and malaria (antagonistic selection)
Anemia/malaria
Experimental studies of natural selection
 In some cases, evolutionary change can occur rapidly
 Evolutionary studies can be devised to test evolutionary hypotheses
Guppy studies (Poecilia reticulata) in the lab and field
Endler’s guppies
 High predation environment - Males exhibit drab coloration and tend to be
relatively small and reproduce at a younger age
 Low predation environment - Males display bright coloration, a larger number
of spots, and tend to be more successful at defending territories
Laboratory experiment
 10 large pools
 2,000 guppies
 4 pools with pike cichlids (predator)
 4 pools with killifish (nonpredator)
 2 pools as control (no other fish added)
 10 generations
Field transplant experiment
 Transplanted from lower pools to upper pools and control (no predator) pools
 Re-visited 10 generations later
Limits of selection
 Some genes have multiple effects (pleiotropy)
 Evolution requires genetic variation
 Gene interactions (epistasis) may constrain selection