Download Ch. 23 HW_Populations

The Evolution of
Populations
Chapter 23
What you must know:
How mutation and sexual reproduction
each produce genetic variation.
 The conditions for Hardy-Weinberg
equilibrium.
 How to use the Hardy-Weinburg equation
to calculate allelic frequencies and to test
whether a population is evolving.
 What effects genetic drift, migration, or
selection may have on a population, and
analyze data to justify your predictions.

Smallest unit of evolution
Microevolution: change in the allele
frequencies of a population over
generations
Darwin did not know how
organisms passed traits to
offspring
 1866 - Mendel published his
paper on genetics
 Mendelian genetics supports
Darwin’s theory  Evolution
is based on genetic
variation

Review:
Sources of Genetic Variation
Point mutations: changes in one base
(eg. sickle cell)
 Chromosomal mutations: delete,
duplicate, disrupt, rearrange  usually
harmful
 Sexual recombination: contributes to
most of genetic variation in a population
1. Crossing Over (Meiosis – Prophase I)
2. Independent Assortment of
Chromosomes (during meiosis)
3. Random Fertilization (sperm + egg)

Hardy-Weinberg Equilibrium
Five Fingers of Evolution:
Paul Anderson/TED Talks
To watch the video use this link:
http://ed.ted.com/lessons/five-fingers-of-evolution
Population genetics: study of how
populations change genetically over
time
Population: group of individuals
that live in the same area and
interbreed, producing fertile offspring
Gene pool: all of the alleles for all genes
in a population
 Diploid species: 2 alleles for a gene
(homozygous/heterozygous)
 Fixed allele: all members of a population
only have 1 allele for a particular trait
 The more fixed alleles a population has,
the LOWER the species’ diversity

Hardy-Weinberg Theorm
Evolution does NOT occur if the gene pool
remains constant (in equilibrium) from
one generation to the next.
 Outside forces must act on a population
for there to be change

NO Evolution = genetic equilibrium
Evolution = NO genetic equilibrium
Conditions for Hardy-Weinberg equilibrium
1.
2.
3.
4.
5.
No mutations.
Random mating.
No natural selection.
Extremely large population size (no
genetic drift)
No gene flow (no migration).
If at least one of these conditions is NOT
met, then the population is EVOLVING!
Counting alleles
Gene with 2 alleles : B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q
 frequencies must = 1 (100%), so:

p+q=1
(therefore: 1 – p = q and 1 – q = p)
Counting individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%),

2
p
+ 2pq +
2
q
=1
Hardy-Weinberg Equilibrium Equation
p2 + 2pq + q2 = 1
 p + q = 1 (1 – p = q and 1 – q = p)
 p = dominant allele
 q = recessive allele
 p2 = homozygous dominant
 2pq = heterozygous
 q2 = homozygous recessive

Strategies for solving H-W Problems:
If you are given the genotypes (AA, Aa,
aa), calculate p and q by adding up the total
# of A and a alleles.
2. If you know phenotypes, then use “aa” to
find q2, and then q. (p = 1-q)
3. To find out if population is evolving,
calculate p2 + 2pq + q2.
 If in equilibrium, it should = 1.
 If it DOES NOT = 1, then the population is
evolving!
1.
Using Hardy-Weinberg Equation
Population: 100 cats
84 black, 16 white
How many of each
genotype?
p2 = .36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq = 2(.4)(.6)
2pq = .48
Bb
q2 = .16
bb
Must
assume
population
in H-W equilibrium!
What
are the
genotypeis
frequencies?
HARDY WEINBERG PRACTICE
One in 1700 US Caucasian newborns have cystic
fibrosis.
 C for normal is dominant
 c for cystic fibrosis
Calculate the allele frequencies for C and c
HARDY WEINBERG PRACTICE
1/1700 have cystic fibrosis
q2 = 1/1700
q=
0.00059
q = 0.024
p = 1 – 0.024 = 0.976
Frequency of C = 97.6%
Frequency of c = 2.4%
Now find the genotype frequencies.
HARDY WEINBERG PRACTICE
cc = q2 = 1/1700 = 0.00059
CC = p2 = (0.976)2 = .953
Cc = 2pq = 2 (0.976) (0.024) = 0.0468
cc = .06% of population
CC = 95.3% of population
Cc = 4.68% of population
Now you can answer questions about the
population:
How many people in this population are heterozygous?
0.0468 (1700) = 79.5 ~ 80 people are Cc
It has been found that a carrier is better able to survive
diseases with severe diarrhea. What would happen to the
frequency of the "c" if there was a epidemic of cholera or
other type of diarrhea producing disease?
Cc more likely to survive than CC.
c will increase in population
The gene for albinism is known to be a recessive allele.
In Michigan, 9 people in a sample of 10,000 were found to
have albino phenotypes. The other 9,991 had normal skin
pigmentation.
Assuming hardy-Weinberg equilibrium, what is the allele
frequency for the dominant pigmentation allele in this
population?
q2 = 9/10000 = .0009
q=
0.0009
q = 0.03
p = 1 – 0.03 = 0.97
Frequency of C = 97%
Frequency of c = 3%
Frequency of CC and Cc
CC = p2 = (.97)2 = .9409
Cc = 2pq = 2(.97)(.03) = .0582
CC = 94.09% = 9409
Cc = 2.9% = 582
II. Causes of Evolution
.
Violations to H-W Equilibrium –
cause evolution
Minor Causes of Evolution:
#1 - Mutations
 Rare, very small changes in allele
frequencies
#2 - Nonrandom mating
 Affect genotypes, but not allele
frequencies
Major Causes of Evolution:
 Natural selection, genetic drift, gene flow
(#3-5)
Major Causes of Evolution
#3 – Natural Selection
 Individuals with variations better suited
to environment pass more alleles to
next generation
Major Causes of Evolution
#4 – Genetic Drift
 Small populations have greater chance
of fluctuations in allele frequencies from
one generation to another
 Examples:
 Founder Effect
 Bottleneck Effect
Genetic Drift
Founder Effect
A few individuals isolated from larger
population
 Certain alleles under/over represented

Polydactyly in Amish population
Bottleneck Effect

Sudden change in environment drastically
reduces population size
Northern elephant seals hunted
nearly to extinction in California
Major Causes of Evolution
#5 – Gene Flow
 Movement of fertile
individuals between
populations
 Gain/lose alleles
 Reduce genetic
differences between
populations
How does natural
selection bring about
adaptive evolution?
Fitness : the contribution an individual makes
to the gene pool of the next generation
Natural selection can alter frequency
distribution of heritable traits in 3 ways:
1. Directional selection
2. Disruptive (diversifying) selection
3. Stabilizing selection
Directional Selection:
eg. larger black bears
survive extreme cold
better than small ones
Disruptive Selection:
eg. small beaks for
small seeds; large
beaks for large seeds
Stabilizing Selection:
eg. narrow range of
human birth weight
Sexual selection


Form of natural selection – certain individuals
more likely to obtain mates
Sexual dimorphism: difference between 2
sexes
 Size, color, ornamentation, behavior
Sexual selection


Intrasexual – selection within same sex (eg.
M compete with other M)
Intersexual – mate choice (eg. F choose
showy M)
Preserving genetic variation


Diploidy: hide recessive alleles that are less
favorable
Heterozygote advantage: greater fitness
than homozygotes
 eg. Sickle cell disease
Natural selection cannot fashion perfect
organisms.
1.
2.
3.
4.
Selection can act only on existing variations.
Evolution is limited by historical constraints.
Adaptations are often compromises.
Chance, natural selection, and the
environment interact.