Chapter 23
The Evolution of
Populations
Question?
Is
the unit of evolution the
individual or the population?
So what do we study?
Population Genetics
Modern Synthesis
Population
Species
Gene Pool
Microevolution
Hardy-Weinberg
Theorem
Basic Equation
Expanded Equation
Genotypes
Example Calculation
Let’s
look at a population
where:
A = red flowers
 a = white flowers

Starting Population
N
= 500
Red = 480 (320 AA+ 160 Aa)
White = 20
Total Genes = 2 x 500
= 1000
Dominant Allele
A
= (320 x 2) + (160 x 1)
= 800
= 800/1000
A = 80%
Recessive Allele
a
= (160 x 1) + (20 x 2)
= 200/1000
= .20
a = 20%
A and a in HW equation
Cross:
Aa X Aa
Result = AA + 2Aa + aa
Remember: A = p, a = q
Substitute the values
for A and a
p2 +
2pq + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32 + .04 = 1
Dominant Allele
A
= p2 + pq
= .64 + .16
= .80
= 80%
Recessive Allele
a
= pq + q2
= .16 + .04
= .20
= 20%
Result
Importance of
Hardy-Weinberg
Example
PKU Frequency
Dominant Allele
Expanded Equation
Final Results
Practice Problem
A
fruit fly population has a
gene with two alleles A1 &
A2. 70% of the gametes
produced in the population
carry A1. What is the
proportion of the population
that are heterozygous?
Practice Problem
In
a H-W population with two
alleles, A & a, that are in
equilibrium, the frequency of
allele a is 0.7. What is the
percentage of the population
that is heterozygous for this
allele?
AP Problems Using
Hardy-Weinberg
for q2 (% of total).
Solve for q (equation).
Solve for p (1- q).
H-W is always on the national
AP Bio exam (but no
calculators are allowed).
Solve
Hardy-Weinberg
Assumptions
If H-W assumptions
hold true:
Microevolution
Causes of
Microevolution
Genetic Drift
By Chance
Bottleneck Effect
Result
Importance
Founder's Effect
Result
Importance
Gene Flow
Result
Mutations
Result
Nonrandom Mating
Causes
Result
Natural Selection
Comment
Result
Genetic Basis of
Variation
Polymorphism
Examples
Garter Snakes
Gaillardia
Human Example
Other examples
Quantitative Characters
Yarrow and Altitude
Sources of Genetic
Variation
Preserving Genetic
Variation
Example
Result
Comment
Fitness - Darwinian
Relative Fitness
Rate of Selection
Modes of Natural
Selection
Stabilizing
Directional Selection
Diversifying
Comment
Sexual Mate selection
Result
Comments
Question
Does
evolution result in
perfect organisms?
Answer - No
Evolution is limited by
historical constraints
Adaptations are often
compromises.
Chance, Natural selection
and the environment
all interact
Selection can only act on
existing variations
Summary
Know
the difference between
a species and a population.
Know that the unit of
evolution is the population
and not the individual.
Summary
Know
the H-W equations and
how to use them in
calculations.
Know the H-W assumptions
and what happens if each is
violated.
Summary
Identify
various means to
introduce genetic variation
into populations.
Know the various types of
natural selection.