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Introductory Questions #3
1) Define what a gene pool is.
2) What are the three aspects in a population we
examine in order to understand how evolution is
occurring in a population.
3) If a population had 2500 individuals that are diploid,
how many total alleles would be present?
4) In a population of 1000 humans, 840 possess the
ability to roll their tongues (dominant trait) and 160
cannot. Determine the frequency of the dominant and
recessive alleles in the population.
5) What is happening if the population is in “genetic
6) What is the significance of the Hardy-Weinberg
Introductory Questions #4
1) How can allele frequencies change in a population and
increase variation? Give three examples. What do we
call this when this is happening?
2) Does natural selection operate directly on the
phenotype or genotype of organisms? Briefly explain
your choice.
3) Name the three modes of selection. Explain how each
mode is different and draw a graph representing each
4) Define what genetic polymorphism is and why
balanced polymorphism is unique. Give the two
mechanisms observed for balanced polymorphism.
Chapter 23-Microevolution
Population genetics
• Population:
a localized group of individuals
belonging to the same species
• Species:
a group of populations whose
individuals have the potential to
interbreed and produce fertile offspring
• Gene pool:
the total aggregate of genes in a
population at any one time
• Population genetics:
the study of genetic changes in
• Modern synthesis/neo-Darwinism
• “Individuals are selected, but populations
Hardy-Weinberg Principle
Model proposed in 1908
Represents an ideal situation
Seldom occurs in nature
Mathematical model is used to compare populations
Allows biologists to calculate allele frequencies in a
• Serves as a model for the genetic structure of a
non-evolving population (equilibrium)
Represents “genetic equilibrium”
If the allele frequencies deviate from the predicted values of HW then
the population is said to be evolving.
Hardy-Weinberg Theorem
5 conditions for Equilibrium
-Very large population size
- No migration
- No net mutations
- Random mating
- No natural selection
**when all these are met then
a population is not
Hardy-Weinberg Equation
• p=frequency of one allele (A);
q=frequency of the other allele (a)
(p=1-q & q=1-p)
P2=frequency of AA genotype
2pq=frequency of Aa
q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
Solving & Analyzing HW Principle
Problem: If you had 90 individuals that possessed the recessive
condition in a population of 1000 individuals, determine the
frequency of dominant and recessive alleles present in the
population as well as the genotypic and phenotypic frequencies.
(1) Always start with the # of homozygous recessive alleles
- aa = 90 and q2 = 90/1000 which is 0.09
- a = square root of 0.09 which is 0.3
- A = (1 – 0.3) which is 0.7
- AA = (0.7) 2 which is 0.49
- Aa = ???
**Remember that p2 + 2pq + q2 = 1
• Involves small or minor changes in the allele
frequencies within a population
• Five processes have been identified:
Nonrandom mating (inbreeding & assortative mating)
Gene flow
(migration between populations)
Genetic drift
(bottleneck effect)
(unpredictable change in DNA)
Natural selection
(differential reproduction)
**certain alleles are favored over others in nature
A change in the gene pool
of a population over a
succession of
Genetic drift: changes
in the gene pool of a
small population due to
chance (usually
reduces genetic
• The Bottleneck
Effect: type of
genetic drift resulting
from a reduction in
population (natural
disaster) such that the
surviving population is
no longer genetically
representative of the
original population
• Founder Effect:
a cause of genetic drift
attributable to
colonization by a
limited number of
individuals from a
parent population
Gene Flow:
genetic exchange due to the
migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
A Change in the DNA
- source of new alleles
- genetic variation
- “raw materials of natural
-unpredictable in nature
-Doesn’t determine the direction
of evolution
-causes small changes in allele
Nonrandom mating: inbreeding and assorative mating
(both shift frequencies of different genotypes)
Mates are chosen according to desired characteristics
• Natural Selection:
– differential success in reproduction
-only form of microevolution that adapts
a population to its environment
Natural selection
• Fitness: refers to the contribution an individual
makes to the gene pool of the next generation
3 types of Selection:
• A. Directional
• B. Diversifying
• C. Stabilizing
Three Types of Selection
Three modes of Selection
• Stabilizing Selection:
-well adapted to the environment
-observed in many plants
-selection eliminates extreme phenotypes
-intermediate form is favored
• Directional Selection:
-one phenotype extreme is favored
-bell shaped curve is shifted (genetic drift)
-Examples: Darwin’s Finches & Peppered moth
• Disruptive Selection:
-causes divergence; splitting apart of the extreme phenotypes
-extreme traits are favored
-intermediate traits become elimanated
Natural Selection in a Population
• Selects only favorable phenotypic traits
• Unfavorable alleles are eliminated
• Can maintain genetic diversity
-heterozygous advantage (sickle cell anemia) Pg. 399
-frequency-dependent selection: rarer phenotypes are
maintained, most common phenotypes eliminated and
decrease in number. (Observed in cichlids)
• Neutral Variations: offers no selective advantage or
disadvantage Examples ???
• Geographical variations and Clines (Clinal variation)
**Observed in the common yarrow wildflower in the
Sierra Nevada Mtns. (Pg. 401)
Population Variation
• Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
• Geographical
variation: differences in
genetic structure between
populations (cline)
Preserving Variations in a Population
Prevention of natural selection’s
reduction of variation
2nd set of chromosomes hides
variation in the heterozygote
Balanced Polymorphism
- heterozygote advantage (hybrid
vigor; i.e., malaria/sickle-cell
- frequency dependent selection
(survival & reproduction of any 1
morph declines if it becomes too
common; i.e., parasite/host)
Sexual selection
• Sexual dimorphism:
secondary sex
characteristic distinction
• Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism
Balanced Polymorphism
Two or more alleles persist in a population
over many generations.
Preserved by:
Heterozygote advantage
Frequency-dependent selection