Chapter 21 – Genes Within Populations
1. Genetic Variation is the Raw Material of Evolution
a. Darwin: Descent with Modification
b. Through time and natural selection, species accumulate differences
(adaptations), new species form, different from ancestors
c. Microevolution (small changes with gene frequencies of populations) leads to
speciation (Macroevolution)
2. Population Genetics
a. Study of changes in gene frequencies within a population
b. Hardy-Weinberg Principle
i. Null hypothesis: when all the following assumptions are met, population
is in equilibrium, i.e. not changing, not evolving
ii. Ideal conditions that keep genotype proportions constant from generation
to generation are
1. Large population size
2. Random mating is occurring
3. No mutation occurs
4. No immigration of genes into population (no migration) (no
movement of genes into/out)
5. No selection occurs
iii. If all conditions met, Hardy-Weinberg Equilibrium exists!
iv. Can this be? No evolution?
c. Hardy-Weinberg Equation
i. Mathematical explanation and prediction of changes in gene frequencies
ii. Terminology
1. Population – geographically localized group of individuals of
same species
2. Gene Pool – all alleles in population for a particular trait
3. Gene Frequency - proportion of population with a particular
gene/allele
4. Evolution – change in gene frequency in population
iii. The Equations
1. p+q = 1 ; p = frequency of A, q = frequency of a
a. This equation represents the gene pool for a particular
trait in a population
2. p2 + 2pq + q2 = 1 ; p2 = frequency of homozygous dominant,
2pq – frequency of heterozygote, q2 = frequency of
homozygous recessives
a. This equation represents total genotype frequencies in
population. Binomial expansion of (p+q) (p+q) = 1
3. Five Agents of Evolutionary Change
a. Mutation
i. Ultimate source of variation
ii. Random, not adaptive
iii. So rare, mutation alone doesn’t change allele frequency much
b.
c.
d.
e.
iv. If selected for fitness, frequency increases, alternate allele decreases
v. If dominant, effect is more immediate
Non-Random Mating
i. Inbreeding, due to proximity, ex. Self pollinating plants
ii. Individual with same genotypes mate produce more homozygotes, less
heterozygotes
iii. Changes genotype ratios, but maybe not gene frequencies
iv. Non-adaptive
Gene Flow or Migration
i. A very potent agent of change
ii. Movement of alleles from one population to another
iii. Non-adaptive change
iv. May introduce or lose genes or blend gene pools of 2 populations
v. Insects/animals can carry pollen great distances
vi. Animals join new groups, bring their genes
Effect of Small Population
i. Genetic Drift – change in gene pool of small population due to chance
ii. Non-adaptive
iii. Smaller the population, greater chance to deviate from expected outcomes
iv. Two causes of Genetic Drift
1. Bottleneck Effect – see fig. 21.6
a. Pop, drastically reduced by natural disaster, weather,
earthquake
b. Remaining indiv. not representative of original population
c. Loss of variability
d. Elephant seals hunted
2. Founder Effects
a. Small, unrepresentative group migrates from original
population
b. New population may increase rare allele or the allele may
disappear – by chance
c. Non-adaptive
d. Polydactyly (6 fingers) in Amish population in PA., many
oceanic island populations, Galapagos Island populations,
Hawaiian Island populations.
Selection
i. Driving force of evolution – adaptive genetic change
ii. Natural selection is the process, evolution is the outcome, the historical
record of change over time
iii. Three conditions must be met for natural selection to occur
1. Variation occurs within population
2. Variation results in differential reproduction (number of
offspring surviving)
3. Variation must be genetically inherited
iv. Examples of selection on phenotype variations
1. Selection to avoid predators through coloration – camouflage
a. Pocket mice in New Mexico, two different colors, two
different environments, lava formation and desert
2. Selection to match climatic conditions
a. Frequency of different alleles for enzymes varies with
temperature/climate/latitude
b. Ex. The frequency of cold-adapted allele for enzyme in a
fish decreases with lower latitudes (warmer climates)
3. Selection for pesticide resistance
a. In insects, selection for genes that
i. Decrease uptake of pesticide or
ii. Decrease number of target sites or
iii. Enzymes that destroy pesticide
4. Measuring Fitness
a. What does survival of the fittest mean? What is fitness?
b. Combination of 3 components:
i. How long individual survives
ii. How often it mates
iii. How many offspring it produces per mating
c. Water strider body size and egg laying example – see page 442
i. Larger females lay more eggs per day BUT… survive for shorter period of
time
ii. Therefore intermediate sized females produce most offspring during
lifetime, and have HIGHEST fitness
5. Selection Interacts with Other Evolutionary Forces
a. Natural Selection and Mutation Rates
i. If mutation rate of favorable allele to unfavorable allele is greater than
selection, then that allele MAY increase,
ii. BUT THIS IS RARE
b. Natural Selection and Genetic Drift
i. Both can eliminate an allele, BUT selection is adaptive for fitness, genetic
drift is random reduction of population size
ii. Only in smaller populations, it’s more likely genetic drift will occur
c. Natural Selection and Gene Flow
i. Gene flow may counter natural selection by introducing unfavorable
genes
ii. Or it could accelerate adaptiveness by introducing favorable genes
iii. Changes in allele frequencies depends on which process is stronger –
natural selection or gene flow
iv. Wind pollinated plants higher rates of gene flow than sedentary animals.
6. Natural Selection Can Maintain Variation in Population
a. Frequency-Dependent Selection
i. Negative Frequency Dependent Selection (most common form is selected
against)
1. Most common form is preyed upon: “search image”
2. Less frequent forms NOT preyed upon, survived, and increased!
3. Resource competition another cause of negative frequency
dependent selection
a. If rare genotype that requires different resource than
common form, less competition and it will survive better IF
all resources equally available
b. More competition for common form
ii. Positive Frequency-Dependent Selection – see page 444
1. When rare form is more likely to be preyed upon (selected
against), favoring the common form, more frequent form
b. Oscillating Selection
i. Selection favors one phenotype at one time and another phenotype at
another time
ii. Climate and seed size and beak size of finches
1. In drought, less small seeds, more large seeds 0 favors large billed
finches
2. When wet conditions return, small seeds more available, favors
small beaked finches
iii. Fitness of phenotype depends on environmental conditions, no frequency
of phenotype
c. Heterozygous Advantage
i. Keeps both alleles for gene in population because being heterozygous is
an advantage for survival
ii. Diploidy keeps recessive alleles in population, maintaining variation
iii. Sickle cell anemia gene(s) is in high frequency in Africa
1. A heterozygote individual is favored in malaria infested areas in
Africa because they are resistant to malaria AND do not die from
sickle cell anemia
2. SC gene is lower in African Americans because not selected for
without malaria selection factor
7. Selection Affects Distribution of Polygenic Traits with continuous Variation –
handout #437
a. Disruptive Selection
i. Increase extreme phenotypes, eliminating intermediate forms
ii. Balanced polymorphism
iii. Beak sizes and seed sizes, banded snails vs. solid colored snails &
camouflage
b. Directional Selection
i. Elimination of one extreme, favors other extreme
ii. Transient polymorphism
iii. Peppered moths, drug resistant bacteria, giraffe neck length
c. Stabilizing Selection
i. Selection against extremes of phenotypes
ii. Increases intermediate form or common form
iii. Results in uniformity
iv. Birth weights, clutch size of eggs
8. Limitations on selection
a. Pleiotropy
i. Alleles may have multiple effects on other traits
ii. Ex: Large clutch sizes of eggs produce thin shells
b. Epistasis
i. If selection for one gene (enzyme) effect is hindered by another gene
required in pathway
c. Can’t eliminate variation
i. Evolution requires genetic variation