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Chapter 17:
Evolution
of
Populations
Evolution of Populations
1.
When Darwin developed his theory of
evolution, he did not understand:
• how heredity worked.
This left him unable to explain two things:
a. source of variation
b. how inheritable traits pass
from one generation to the next
Evolution of Populations
In the 1940’s, Mendel’s work on genetics was
“rediscovered” and scientists began to combine
the ideas of many branches of biology to
develop a modern theory of evolution. When
studying evolution today, biologists often focus
on a particular population. This evolution of
populations is called microevolution.
Vocabulary:
Population: group of individuals of the same
species living in the same area that breed with each
other.
gene pool
combined genetic
info. for all
members
of a population
Allele:
one form of a gene
2. relative frequency of an allele: # times an
allele occurs in the gene pool compared to
other alleles (percent)
Example
Relative Frequency:
70% Allele B
30% Allele b
3. Sources of Variation:
a. mutations: any change in DNA sequence
♦
Can occur because of:
♦mistakes in replication
♦ environmental chemicals
♦
May or may not affect an organism’s
phenotype
3. Sources of Variation
b. Gene Shuffling: recombination of genes that
occurs during production of gametes
♦
♦
♦
♦
Cause most inheritable differences between relatives
Occurs during meiosis
As a result, sexual reproduction is a major source of
variation in organisms.
Despite gene shuffling, the frequency of alleles does
not change in a population. Explain why this is true.
Similar to a deck of cards – no matter how many
times you shuffle, same cards (alleles) are always
there.
4. Gene Traits:
A) Single gene trait: controlled by single gene
with two alleles
♦
Examples: widow’s peak, hitchhiker’s thumb, tongue
rolling
(4. Gene Traits:)
B) Polygenic trait: controlled by 2 or more genes,
each with 2 or more alleles
♦
Examples: height, hair color, skin color, eye color
Most human traits are polygenic.
Do the following graphs show the distribution of
phenotypes for single-gene or polygenic traits?
Explain.
type: single gene
type: polygenic
why? Only two
phenotypes possible
why? Multiple (many)
phenotypes possible
Example: tongue roller
or non-tongue roller
Example: height range 4feet
to 9 feet all
5. Natural selection acts on phenotypes, not
genotypes.
Example: in a forest covered in brown leaves, dirt and
rocks which mouse will survive better brown or
white?
Brown, more hidden.
5. If brown is dominant can a predator tell the
difference between:
BB
Bb
?
Mouse with highest fitness will have the most alleles
passed on to the next generation.
White mouse will have low fitness
5. Which mouse will have the lowest fitness?
White, bb (recessive)
BB
Bb
?
Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness
advantage of being brown
Natural Selection
Three ways in which natural selection
affects polygenic traits.
•Directional Selection
•Stabilizing Selection
•Disruptive Selection
Directional Selection: individuals at one end of
the curve have higher fitness so evolution
causes increase in individuals with that trait
Key
Food becomes scarce.
Low
mortalit
High
y,
high
mortalit
fitness
y, low
fitness
♦
♦
Individuals with highest fitness: those at one end
of the curve
Example: Galapagos finches – beak size
Directional Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection: individuals at the center of
the curve have highest fitness; evolution keeps
center in the same position but narrows the
curve
Stabilizing Selection
Key
Individuals with
highest fitness: near
the center of the
curve (average
phenotype)
Example: human
birth weight
Low
mortality,
High
high fitness
mortality, low
fitness
Birth Weight
Selection
against both
extremes
keep curve
narrow and
in same
place.
Disruptive Selection: individuals at both ends
of the curve survive better than the middle
of the curve.
Disruptive Selection
High mortality,
low fitness
♦
specializing in
different seeds.
Number of Birds
in Population
Individuals with highest fitness: both ends of
Key
Low mortality,
Population splits
curve
high fitness
into two subgroups
Number of Birds
in Population
♦
Largest and smallest seeds become more common.
Beak Size
Beak Size
Example: birds
where seeds are either
large
or small
Stabilizing Selection
Stabilizing Selection
Key
Low mortality, high
fitness
High mortality, low
fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Disruptive Selection
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
However:
No
examples ever
observed in animals
A couple examples that
may demonstrate
speciation exist in plants
and some insects.
Genetic Drift
random change in allele
frequency that occurs in small
populations

Genetic Drift
Two phenomena that result
in small
populations and cause genetic
drift
Founder Effect
2. Bottleneck Effect
1.
Genetic Drift
The results of genetic crosses can usually be predicted
using the laws of probability.
In small populations, however, these predictions are not
always accurate.
Founder effect
Allele frequencies change due to
migration of a small subgroup
of a population
Founder Effect
Two groups from a large, diverse population could produce new
populations that differ from the original group.
2. Bottleneck effect
Major change in allele frequencies
when population decreases
dramatically due to catastrophe
♦
Example: northern elephant seals
decreased to 20 individuals in 1800’s, now 30,000
no genetic variation in 24 genes
Bottleneck Effect:
Northern Elephant Seal Population
♦
♦
♦
Hunted to near extintion
Population decreased to 20
individuals in 1800’s, those
20 repopulated so today’s
population is ~30,000
No genetic variation in
24 genes
Bottleneck Effect
Original
population
Bottleneck Effect
Catastrophe
Original
population
Bottleneck Effect
Catastrophe
Original
population
Surviving
population
Evolution Versus Genetic Equilibrium
What conditions are required to maintain genetic
equilibrium?
According to the Hardy-Weinberg principle, five
conditions are required to maintain genetic
equilibrium:
(1) The population must be very large
(2) there can be no mutations
(3) there must be random mating
(4) there can be no movement into or out of the
population
(5) no natural selection can occur
Genetic equilibrium = no evolution
A population is in genetic equilibrium if allele
frequencies in the population
remain the same. If allele frequencies don’t change,
the population will not evolve.
The Hardy-Weinberg principle describes the
conditions under which evolution does not occur.
The Hardy-Weinberg principle states that allele
frequencies in a population remain constant
unless one or more factors cause those
frequencies to change.
Hardy-Weinberg principle
1. Large Population
Genetic drift can cause changes in allele frequencies in small
populations.
Genetic drift has less effect on large populations.
Large population size helps maintain genetic equilibrium
2. No Mutations
If mutations occur, new alleles may be introduced into the
gene pool, and allele frequencies will change.
Hardy-Weinberg principle
3. Random Mating
All members of the population must have an equal opportunity to
produce offspring.
Individuals must mate with other members of the population at
random.
In natural populations, however, mating is not random.
•Female peacocks, for example, choose mates on the basis of
physical characteristics such as brightly patterned tail feathers.
•Such non-random mating means that alleles for those traits are
under selection pressure.
Hardy-Weinberg principle
4. No Movement Into or Out of the Population
Individuals who join a population may introduce new alleles into the
gene pool. (Immigration)
Individuals who leave may remove alleles from the gene pool.
(emigration)
Thus, for no alleles to flow into or out of the gene pool, there must be
no movement of individuals into or out of a population.
Hardy-Weinberg principle
5. No Natural Selection
All genotypes in the population must have equal
probabilities of surviving and reproducing. No
phenotype can have a selective advantage over
another.
Sexual Reproduction and Allele Frequency

Meiosis and fertilization do not change the
relative frequency of alleles in a
population.

The shuffling of genes during sexual
reproduction produces many different
gene combinations but does not alter the
relative frequencies of alleles in a
population.
The Process of Speciation
 The
formation of new
biological species, usually by
the division of a single
species into two or more
genetically distinct one.
Three Isolating Mechanisms:
Isolate species forming subspecies and
perhaps causing speciation.
1.
2.
3.
Geographic Isolation
Behavioral Isolation
Temporal Isolation
Example: Eastern and Western
Meadowlark
Male birds sing a matting song that
females like, East and West have
different songs. Females only respond
to their subspecies song.
1. Geographic Isolation
 Two
populations separated
by a geographic barrier;
river, lake, canyon,
mountain range.
Example: 10,000 years ago the Colorado
River separated two squirrel populations.
Kaibab Squirrel
Abert Squirrel
Kaibab Squirrel
Abert Squirrel
This resulted in a subspecies, but did not result
in speciation because the two can still mate if
brought together.
Example: Eastern and Western
Meadowlark
 Eastern
and Western Meadowlark
populations overlap in the middle
of the US
2. Behavioral Isolation
Two populations are capable of
interbreeding but do not
interbreed because they have
different ‘courtship rituals’ or
other lifestyle habits that
differ.
3. Temporal Isolation
Populations reproduce at
different times
January
7
1 2
3 4 5 6
8 9
10 11 12 13
Example: Northern Leopard Frog
& North American Bullfrog
Mates in:
April
Mates in:
July
Conclusion:
Geographic, Behavioral and
Temporal Isolation are all
believed to lead to
speciation.