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The Genetic Analysis of
Populations and How They
Evolve
Chapter 24 p. 659
Population Genetics
 Population: a local group belonging to a single
species within which mating can occur
 Gene pool: The set of genetic information carried by
all interbreeding members of a population
What is the focus?
 In the study of population genetics, the focus is on
the groups rather than individuals


Also on the measurement of allele and genotype frequency
rather than on the distribution of genotypes from a single
mating
Allele frequency: represents the freq of alleles in contrast to
genotype freq.
Populations are dynamic
 They grow/expand or diminish based on
 birth and death rates
 Migration
 Merging with other populations
 Over time, this can lead to changes in the genetic structure of
the population
How to measure Allele Frequency:
Refer to page 661 for this problem
 In a population of 100, 36 are type M(M), 48 are
type MN and 16 are type N(N)
 36 M is actually 72 ( additional M)
 16 N is actually 32 (additional N)
 48MN will include an additional 48M alleles

Therefore there are 72+48 or 120M alleles out of 200
alleles (100 in population with 2 alleles)
 120/200= 60% of M allele in population
 48+32= total number of N alleles 80/200= 40% of
population
Outline of Chapter 24
 The Hardy-Weinberg law
 A model for understanding allele, genotype, and phenotype
frequencies for single gene traits in a genetically stable
population
 Calculations beyond Hardy-Weinberg
 Measuring how selection and mutations change allele
frequencies over time
3 Tenets of the Hardy-Weinberg Law
 1. allele frequencies predict genotype frequencies
 2. at equilibrium, allele and genotype frequencies do
not change from generation to generation
 3. equilibrium is reached in one generation of
random mating
The Hardy-Weinberg law clarifies the relations between genotype and allele frequency
within a generation and from one generation to the next
 Five assumptions
 Infinitely large population
 Individuals mate at random
 No new mutations appear in gene pool
 No migration into or out of population
 No genotype-dependent differences in ability to survive and
reproduce
 If all assumptions hold, population is in Hardy-
Weinberg equilibrium
Hardy-Weinburg Equilibrium
 p2 +2pq+q2=1
 In the random combination of gametes in the
population, the prob that sperm and egg both
contain the R allele (RR) is pxp=p2
 Then chance that gametes will carry unlike alleles
(Rr)

(pxq) + (pxq)=2pq
 And the chance that a homozygous recessive (rr)
individual will result is qxq=q2
Step 2
Fig. 20.3
 Use gamete allele frequency to calculate genotype
frequencies in the zygotes of next generation
Two steps in translating the genotype frequencies from one generation to the next
Step 1
Fig. 20.2
 Calculate allele frequency of gametes – same as
adults
 p2 is the probability that both gametes in a
fertilization event will carry the R allele, it is also the
measure of the freq of RR homozygotes in the next
generation
 2pq describes the freq of Rr
 q2 is a measure of the freq of homozygous recessive
(rr) zygotes
 p2+ 2pq +q2=1
 Allele frequencies do not change from generation to
generation in a population at Hardy-Weinberg
equilibrium

Equilibrium is equal to 1
 A Hardy-Weinberg population achieves the genotype
frequencies of p2, 2pq, and q2 in just one generation
and maintains them in subsequent generations
Measuring how mutation and selection cause changes in allele
frequency
 Evolution – sometimes defined as change in allele
frequency over multiple generations
 Macroevolution – changes that occur through
geologic time among species
 Microevolution – changes that occur from
generation to generation within a species
Can use Hardy-Weinberg law to examine microevolution
 Violations of assumptions to Hardy-Weinberg can be
used to analyze evolutionary forces causing deviations in
allele frequencies

Natural selection acts on differences in fitness to
alter allele frequencies
 Fitness – individual’s relative ability to survive and
transmit genes to next generation

Viability and reproductive success
 Natural selection – individuals with higher fitness
survive and reproduce more than individuals with
lower fitness
Summary of evolutionary equilibrium
between mutation and selection
 New alleles arise in populations by mutation
 When the allele affects fitness, selection will drive
frequency towards an equilibrium with wild-type
allele
 Equilibrium value is determined by relative selection
coefficients for heterozygous and homozygous
individuals for new allele
 If new allele has no effect on fitness, genetic drift will
determine its frequency
Changes in allele and genotype frequency when selection acts on
genotype-dependent differences in fitness
Fig. 20.4
Frequency of q allele in next generation where q’ is
frequency of q allele in generation after selection
Fig. 20.6
How a recessive genetic condition influences allele
frequency of a population
 rr genotype has
decreased fitness
 Fitness or RR and Rr
same
 WRR = 1, WRr = 1, Wrr =
1-s

Fig. 20.7
s = selection coefficient
against rr which varies from
0 (no affect) to 1 (lethal)
 P. 678
 1, 2,