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Chapter 23-The Evolution of
Populations
Populations Evolve
Individuals do not.
• A population is defined as a group of individuals
belonging to the same species for which there are no
restrictions to random mating among its members.
• gene pool, the total of all alleles in the population
• Allele frequencies- the relative abundance of
different alleles carried by the individuals in that
population
There is a stability in the allele frequencies of a
population.
In the absence of disturbing factors, the frequency of different
genotypes in a population will reach an equilibrium and will remain
stable from generation to generation.
This observation is based on the Hardy Weinberg Principle.
Nonevolving Populations
• Nonevolving populations are said to be in HardyWeinberg equilibrium. The Hardy-Weinberg
theorem is a theory maintaining that the sexual
shuffling of genes alone cannot alter the overall
genetic make-up of a population.
• The five conditions that are required for a population
to be in Hardy-Weinberg equilibrium are:
1. A very large population size
2. Isolation from other populations
3. No mutations
4. Random mating
5. No natural selection
Hardy Weinberg Principle
p2 + 2pq + q2 = 1
p=frequency of dominant allele
q=frequency of recessive allele
p2=frequency of homozygous dominant genotype
2pq=frequency of the heterozygous genotype
q2=frequency of the homozygous recessive genotype.
ALSO,
p+q=1
HWP can be used to detect if random mating is taking place and
if evolutionary changes are taking place in a given population.
Wildflower Population in Hardy-Weinberg Equilibrium
Figure 23.3a The Hardy-Weinberg theorem
• One of the HardyWeinberg equations is p
+ q = 1, where p is the
frequency of the
dominant allele and q is
the frequency of the
recessive allele.
• Here is the genetic
structure of the parent
population of a
population of
wildflowers that are in
Hardy-Weinberg
equilibrium. Notice that
p (.8) + q (.2) = 1.
Wildflower Population in Hardy-Weinberg Equilibrium
(continued)
• Another equation used in
when populations are in
Hardy-Weinberg equilibrium
is p2+ 2pq + q2 = 1. Here,p2
is the frequency of the
homozygous dominant
genotype, 2pq is the
frequency of the
heterozygous genotype, and
q2 is the frequency of the
homozygous recessive
genotype.
• Notice that the frequencies
of the alleles of these
offspring are the same as
their parents (p = .8 and q =
.2).
Figure 23.3b The Hardy-Weinberg theorem
Why does evolution take place?
Evolution occurs when a population moves away form HWE.
This is due to changes in the allele frequencies in a gene pool.
What causes these allele frequencies to change?
MICROEVOLUTION!
http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter
20/animation_-_mechanisms_of_evolution.html
Causes of Microevolution
Figure 23.4 Genetic drift
https://www.youtube.com/watch?v
=PQuZkMjQn6c
• Microevolution is a change
in the gene pool of
population over a succession
of generations.
• There are five causes of
microevolution:
1. Genetic drift
2. Gene flow
3. Mutation
4. Nonrandom mating
5. Natural selection
• The picture at the left
illustrates genetic drift,
which is a random change in
the gene frequency of a
small breeding population.
• Genetic drift can lead to
extinction because it reduces
variation making organisms
susceptible to a changing
environment.
• Ex. The really fast zebra
gene.
Special Cases of Genetic Drift
• One type of genetic
Ex. Old order Amish of
drift is termed the
Lancaster
founder effect. The
founder effect occurs
Started out with a population of a
when a small population
few dozen. This population
of organisms colonizes
shows a large percentage of
a new area. The gene
dwarfism and polydactylism.
frequency of a
population will change
because this small
population most likely
http://highered.mcgrawwill not represent the
hill.com/sites/dl/free/0072835125/
gene pool of the larger 126997/animation45.html
population
•A second type of genetic drift is the bottleneck effect.
The bottleneck effect occurs when there is a sudden decrease in
the number of individuals of a population due to some natural
disaster.
A decrease in the # of individuals will also decrease variation.
Ex. Elephant seal
Hunted to near extinction, the elephant seal population was put
under protection. Today we have a large population but very little
variation.
Figure 23.5
The bottleneck effect: an analogy
Bottleneck Example
Nene Goose
Gene Flow
• Gene flow is the loss or gain of alleles from a
population due to emigration or immigration of fertile
individuals.
• Result-can introduce new alleles (increase variation)
or change existing allele frequencies (decrease
variation)
• Ex. Human movement around the globe gene flow
became an important agent of microevolutionary
change in isolated populations.
http://nortonbooks.co
• Ex. Bird Migration
m/college/biology/ani
mations/ch17a01.htm
Mutation Any change in the genetic material that alters the
DNA.
3 types
A change in the nucleotide base pairs
Rearrangement of genes on a chromosome
Change in the chromosome
Mutations are the source of variation in a population.
Unfavorable mutations are usually not selected.
Non Random Mating- Inbreeding or selection of mates for specific
phenotypes. (reduces the frequency of heterozygous individuals)
•A common type of nonrandom mating is assortative mating, which
occurs when individuals select partners with similar phenotypic
characters.
OR
Inbreeding- causes the relative frequencies of genotypes to deviate
from the expected HWE. (more chances for recessive traits to come
together)
Natural Selection- A process of interaction between organisms
and their environment that results in a differential rate of
reproduction.
Can result in a change in allele frequency and genotypes in a
population.
Of all the mechanisms of microevolution, natural selection is
most likely to adapt a population to its environment.
Some examples of adaptive natural selection.
Cryptic Coloration (protective coloration) color or scheme that
permits an organism to blend into its environment.
Ex Insects, Preying Mantis, Katydid, Walking Sticks
Aposematic Coloration (Warning Coloration) Any brightly colored
species that usually contains some type of toxin.
Mimicry- Two different organisms look alike.
Ex. 1 Batesian mimicry- A harmless or edible species resembles one
that is dangerous.
Viceroy and Monarch butterfly.
Ex. 2 Mullerian mimicry- Two or more unpalatable, aposematic
colored species resemble each other.
Advantage- Predators learn quickly to avoid brightly colored prey
Insects that feed on milkweed resemble the warning color of the
monarch.
http://www.youtube.com/watch?v=FvDpbjSRpuc
VICEROY
MONARCH
The Three Modes of Natural Selection
• Stabilizing selection
favors intermediate
phenotypes over
extreme phenotypes.
• Directional selection
favors one extreme
phenotype over the
other phenotypes.
• Diversifying
selection occurs when
individuals on both
extremes of a
phenotypic range are
favored over
intermediate
Figure 23.12 Modes of selection
Directional Selection
Diversifying Selection
Other Important Facts Over Chapter 23
• Polymorphism is the condition where two or more
contrasting forms of a particular trait are
represented in relatively high frequencies in a
population.
•Heterozygote advantage means that individuals who are
heterozygotes for a particular trait are more likely to survive
than homozygotes of the same trait.
•Sexual selection will select for traits that enhance an
individual’s chance of mating.
Sexual Dimorphism