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Chapter 16:
Evolution
of
Populations
Students know both genetic variation and environmental factors are
causes of evolution and diversity or organisms. Students know new
mutations are constantly being generated in a gene pool.
Content Objectives:
 Describe the main sources of heritable
variation in a population?
 Explain how is evolution defined in
genetic terms.
 Explain what determines the number of
phenotypes for a given trait.
 Describe the factors involved in the
formation of new species?
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
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
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
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.
Gene Traits:
A) Single gene trait: controlled by single
gene with two alleles
♦ Examples: widow’s peak, hitchhiker’s thumb,
tongue rolling
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
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.
Three ways in which
natural selection affects
polygenic traits.
a. 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
b. 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.
c. Disruptive Selection: individuals at
both ends of the curve survive better
than the middle of the curve.
Disruptive Selection
Largest and smallest seeds become more common.
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
♦ Individuals with highest fitness: both
ends of curve
Key
Beak Size
♦ Example: birds where seeds are either
large or small
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. Geographic Isolation
2. Behavioral Isolation
3. Temporal Isolation
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
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.
Example: Eastern and
Western Meadowlark
 Eastern and Western
Meadowlark populations
overlap
the US
in the middle of
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.
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:
Mates in:
April
July
Conclusion:
 Geographic, Behavioral and
Temporal Isolation are all
believed to lead to
speciation.
Genetic Drift
 random change in allele
frequency that occurs in small
populations
The results of genetic crosses can
usually be predicted using the laws of
probability. In small
populations, however, these
predictions are not always accurate.
a. Founder effect: allele frequencies change due to
migration of a small subgroup of a population
♦ Example: fruit flies on Hawaiian islands
result
in small populations
and cause genetic
drift
Two phenomena that
1. Founder Effect
2. Bottleneck Effect
Founder effect
allele frequencies change
due to migration of a small
subgroup of a population
Founder Effect: : Fruit Flies
on Hawaiian islands
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
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
Catastrophe
Original
population
Surviving
population
Another picture to illustrate
bottleneck effect