Chapter 23
The Evolution of Populations
A. Population Genetics
1. Combines Darwinian selection and Mendelian
inheritance
a. Population genetics is the study of genetic variation
within a population.  Importance of quantitative
characters.
b. In the 1940s, a comprehensive theory of evolution,
called modern synthesis, was formed. Until then, many did
not accept that Darwin’s theory of natural selection could
drive evolution.
- Modern synthesis combined discoveries from different
fields including paleontology, taxonomy, biogeography,
and population genetics.
 It emphasizes the importance of populations as units
of evolution, with natural selection as the most important
mechanism of evolution, and backs up the idea of
gradualism.
2. Allele frequencies define gene pools
Say that we have 500 flowering plants  480 with red flowers,
20 with white flowers and that the alleles express themselves
by pure Mendelian inheritance. We know:
Of the red, some will be RR and some Rr; Suppose 320 red
are homozygous (RR) and 160 are heterozygous (Rr). The
white will be only rr.
We know there are 1000 copies of the genes for color (we
know this because the plants are diploid). Thus, the allele
frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
a. Remember, a population is a localized group of
individuals of the same species. A species is a group
of populations whose individuals have the ability to
breed and produce fertile offspring.
b. Individuals near a population center are, on
average, more closely related to one another than to
members of other populations.
c. A population’s gene pool is the total of all genes in the
population at any one time.
d. If all members of a population are homozygous for a
particular allele, then the allele is fixed in the gene pool.
3. The Hardy-Weinberg Theorem
a. The Hardy-Weinberg Theorem is used to describe a nonevolving population*  Shuffling of alleles by meiosis and
random fertilization have no effect on the overall gene pool.
* Natural populations are not expected to actually be in
Hardy-Weinberg equilibrium. Deviation from H-W equilibrium
usually results in evolution. Understanding a non-evolving
population, helps us to understand how evolution occurs.
b. Assumptions of the H-W Theorem:
- Large population size: small populations can have
chance fluctuations in allele frequencies (e.g. fire, storm)
- No migration: immigrants can change the frequency of an
allele by bringing in new alleles to a population
- No net mutations: if alleles change from one to another,
this will change the frequency of those alleles
- Random mating: if certain traits are more desirable, then
individuals with those traits will be selected and this will
not allow for random mixing of alleles.
- No natural selection: If some individuals survive and
reproduce at a higher rate than others, then their offspring
will carry those genes and the frequency will change for
the next generation.
c. Hardy-Weinberg Equilibrium
 The gene pool of a non-evolving population remains
constant over multiple generations; i.e., the allele frequency
does not change over generations of time.
d. Hardy-Weinberg Equation
1.0 = p2 + 2pq + q2
where p2 = frequency of RR genotype; 2pq = frequency of Rr
plus rR genotype; q2 = frequency of rr genotype
Figure 23.5 (p. 457) – The Hardy-Weinberg theorem.
But we know that evolution does occur within populations.
 What causes it?
 Microevolution refers to changes in allele frequencies in a
gene pool from generation to generation. Represents a
gradual change in a population.
1. Causes of microevolution
a. Genetic drift
- Genetic drift is the alteration of the gene pool of a small
population due to chance.
Figure 23.7 (p. 461) – Genetic drift.
 Two factors may cause genetic drift:
- Bottleneck effect may lead to reduced genetic variability
following some large disturbance that removes a large
portion of the population. The surviving population often
does not represent the allele frequency in the original
population.
Figure 23.8 (p. 461) – The bottleneck effect: an analogy.
- Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat (example, retinitis pigmentosa).
b.
Gene flow
- Gene flow is genetic exchange due to the migration of fertile
individuals or gametes between populations.
Gene flow and human evolution:
c. Mutation
- Mutation is a change in an organism’s DNA and is
represented by changing alleles.
-Mutations can be transmitted in gametes to offspring, and
immediately affect the composition of the gene pool.
-More coverage later in lecture.
The primary mechanism for adaptive evolution is: Natural
Selection
1. Genetic (heritable) variation exists within and between
populations. Exists both as what we can see (e.g. eye color)
and what we cannot see (e.g. blood type).
Remember, not everything that we see is due to the genotype,
the environment can alter an individual’s phenotype (e.g. the
hydrangea we saw before)
Fig. 23.9 – Map butterflies (color changes are due to seasonal
difference in hormones).
a. Variation within populations
 Most variations occur as quantitative characters (e.g.
height) that vary along a continuum usually indicating
polygenic inheritance. Few variations are discrete (e.g. red
versus white flower color).
- Polymorphism is the existence of two or more forms of a
character, in high frequencies, within a population. This
applies only to discrete characters. An example would be the
red versus white flower color.
b. Variation between populations
- Geographic variations are differences between gene pools
due to environmental factors. Natural selection may
contribute to geographic variation. It often occurs when
populations are located in different areas, but may also
occur in populations with isolated individuals.
Figure 23.12 (p. 465) – Geographic variation between
isolated populations of house mice. Normally house mice
are 2n = 40. However, chromosomes fused in the mice in
the example, so that the diploid number has gone down.
Note that populations are separated by mountains and that
the populations evolved differently from each other!
- Cline, a type of geographic variation, is a graded variation
in individuals that correspond to gradual changes in the
environment. (Example: Body size of North American birds
tends to increase with increasing latitude. Can you think of
a reason for the birds to evolve differently?)
Figure 23.11 (p. 464) – Clinal variation in a plant. Growth
height has some genetic basis. Can you think of a reason
for the plants to evolve differently?
2. Mutation and sexual recombination generate genetic
variation (p. 459)
a. New alleles originate only by mutations (changes in the
nucleotide sequence of DNA).
- In stable environments, mutations often result in little or no
benefit to an organism.
- Mutations are more beneficial in changing
environments. (Example: HIV resistance to antiviral drugs)
b. Sexual recombination is the source of most genetic
differences between individuals in a population.
- Vast numbers of recombination possibilities result in
varying genetic make-up. FILM!
3. How is genetic variation preserved? (p. 466)
a. Diploidy often hides genetic variation from selection in the
form of recessive alleles (i.e. the dominant allele is expressed
and the recessive allele can be maintained as a silent gene.)
Dominant alleles “hide” recessive alleles in heterozygotes.
b. Balanced polymorphism is the ability of natural selection
to maintain stable frequencies of at least two phenotypes.
Includes:
- Heterozygote advantage is one example of a balanced
polymorphism, where the heterozygote has greater survival
and reproductive success than either homozygote (Example:
Sickle cell anemia where heterozygotes are resistant to
malaria) Fig 23.13 (p. 466) – Mapping malaria and the sicklecell allele.
-Frequency-dependent selection = survival of one phenotype
declines if that form becomes too common
(Example: Parasite-Host relationship. Co-evolution occurs,
so that if the host becomes resistant, the parasite changes to
infect the new host. Over the time, the resistant phenotype
declines and a new resistant phenotype emerges.)
Figure 23.14 (p. 467) – Frequency-dependent selection in a
host-parasite relationship.
c. Neutral variation is genetic variation that results in no
competitive advantage to any individual
- Example: human fingerprints
D. A Closer Look at Natural Selection as the Mechanism of
Adaptive Evolution
1. Natural Selection increases the frequencies of certain
alleles over a period of time that includes many generations.
The way that natural selection works is twofold:
a. Evolutionary (Darwinian) fitness  Contribution of an
individual to the gene pool, relative to the contributions of
other individuals: the number of offspring may be greater or
less than the number of offspring produced by others..
And,
b. Relative fitness
- Contribution of a genotype to the next generation,
compared to the contributions of alternative genotypes for
the same locus.
-Survival doesn’t necessarily increase relative fitness;
relative fitness is zero (0) for a sterile plant or animal.
There are three ways (modes of selection) in which
natural selection can affect the contribution that a genotype
makes to the next generation. These are (Figure 23.12; p.
465 – Modes of selection.):
a. Directional selection favors individuals at one end of the
phenotypic range - Most common during times of
environmental change or when moving to new habitats.
b. Diversifying selection favors extreme over
intermediate phenotypes.
- Occurs when environmental change favors an extreme
phenotype.
c. Stabilizing selection favors intermediate over extreme
phenotypes.
- Reduces variation and maintains the current average.
3. Natural selection maintains sexual reproduction, p. 469
a. Sex generates genetic variation during meiosis and
fertilization. This is the advantage of using sexual
reproduction as opposed to asexual reproduction.
-Generation-to-generation variation may be of greatest
importance to the continuation of sexual reproduction.
-However, there are disadvantages to using sexual
reproduction.  Asexual reproduction produces many more
offspring. Figure 23.16 demonstrates what happens when all
individuals are female, versus half female/half male. Because
males don’t reproduce, the overall output is lower for sexual
reproduction.
The variation produced during meiosis greatly outweighs this
disadvantage, so sexual reproduction is here to stay.
4. Sexual selection leads to differences between sexes,
p. 468
a. Sexual dimorphism is the difference in appearance
between males and females of a species.
- Intrasexual selection is the direct competition between
members of the same sex for mates of the opposite
sex. This gives rise to males most often having
secondary sexual equipment such as antlers that are
used in competing for females.
- In intersexual selection (mate choice) one sex is
choosy when selecting a mate of the opposite sex. This
gives rise to often amazingly sophisticated secondary
sexual characteristics, e.g. peacock feathers.
5. Natural selection does not produce perfect organisms, p.
469
a. Evolution is limited by historical constraints (e.g. humans
have back problems because our ancestors were 4-legged).
 The legacy of natural selection, our evolutionary history
constrains what we can achieve.
b. Adaptations are compromises. Humans are athletic due to
flexible limbs, which often dislocate or suffer torn ligaments.
c. Not all evolution is adaptive. Chance probably plays a
huge role in evolution and not all changes are for the best.
d. Selection edits existing variations. New alleles cannot
arise as needed, but most develop from what already is
present.  Evolution adaptation is random and not goal-