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Evolution - Unifying Theme of Biology
Microevolution – Chapters 13 &14
New Synthesis
Natural Selection
Unequal Reproductive Success
Examples and Selective Forces
Types of Natural Selection
The New Synthesis
Ernst Mayr (1904-2005) helped develop the modern
evolutionary synthesis of Mendelian genetics and Darwinian
Natural Selection and
Evolutionary biologist Ernst Mayr
has dissected the logic of
Darwin’s theory into three
inferences based on five
• Observation #1: For any species,
population sizes would increase
exponentially if all individuals that
are born reproduced successfully
• Observation #2: Populations tend
to be stable in size, except for
seasonal fluctuations
• Observation #3: Resources are
• Inference #1: Production of more
individuals than the environment
can support leads to a struggle for
existence among individuals of a
population, with only a fraction of
their offspring surviving
• Observation #4: Members of a
population vary extensively in
their characteristics; no two
individuals are exactly alike
• Observation #5: Much of this variation is
• Inference #2: Survival depends in part on
inherited traits; individuals whose inherited
traits give them a high probability of surviving
and reproducing are likely to leave more
offspring than other individuals
• Inference #3: This unequal ability of individuals
to survive and reproduce will lead to a gradual
change in a population, with favorable
characteristics accumulating over generations
Directional, Disruptive, and
Stabilizing Selection
• Selection favors certain genotypes by acting
on the phenotypes of certain organisms
• Three modes of selection:
– Stabilizing
– Disruptive
– Directional
Stabilizing Selection
When the average individuals in the population have higher
fitness and are favored.
Directional Selection
When individuals at one end of the curve have higher fitness
and are favored more than individuals in the middle or at the
other end.
Communities spray DDT pesticide on Mosquitoes to stop the
spread of malaria and the West Nile virus. Those mosquitoes
with a high resistance to DDT will be favored over those who
have no or little resistance to DDT.
Disruptive Selection
When individuals at both of the extremes (upper and
lower ends) of a trait are both more fit and are favored
over the average individuals in the population.
Frequency of
LE 23-12
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
Speciation can take place with or
without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
Allopatric and Sympatric
• In allopatric speciation, a new species forms
while geographically isolated from its parent
• In sympatric speciation, a reproductive
barrier isolates a subset of a population
without geographic separation from the
parent species
LE 24-5
Allopatric speciation
Sympatric speciation
Allopatric (“Other Country”)
• In allopatric speciation, gene flow is
interrupted or reduced when a population
is divided into geographically isolated
• One or both populations may undergo
evolutionary change during the period of
LE 24-6
A. harrisi
A. leucurus
To determine if allopatric speciation has
occurred, reproductive isolation must have
been established
LE 24-7a
Reproductive Barriers may Evolve as Populations Diverge
Initial population
of fruit flies
Some flies
raised on
starch medium
Mating experiments
after several generations
Some flies
raised on
maltose medium
LE 24-7b
Note: Mating behavior has changed!!
Mating frequencies
in experimental group
populations population
Maltose Starch
Starch Maltose
population populations
Mating frequencies
in control group
Sympatric (“Same Country”)
• In sympatric speciation, speciation takes
place in geographically overlapping
• May occur when mating and resulting
gene flow between populations are
reduced by factors such as polyploidy,
habitat differentiation, or sexual selection.
Habitat Differentiation and
Sexual Selection
• Sympatric speciation can also
result from the appearance of new
ecological niches
• In cichlid fish, sympatric speciation
has resulted from nonrandom
mating due to sexual selection
LE 24-10
P. pundamilia
P. nyererei
Normal light
Monochromatic orange light
Allopatric and Sympatric
Speciation: A Summary
• In allopatric speciation, a new species forms
while geographically isolated from its parent
• In sympatric speciation, a reproductive
barrier isolates a subset of a population
without geographic separation from the
parent species
Adaptive Radiation
• Adaptive radiation is the evolution
of diversely adapted species from
a common ancestor upon
introduction to new environmental
• The evolution of
many diverselyadapted species
from a common
ancestor is called
an adaptive
Fig. 24.11
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
LE 22-6
Cactus eater. The long,
sharp beak of the
cactus ground finch
(Geospiza scandens)
helps it tear and eat
cactus flowers and
Seed eater. The large
ground finch (Geospiza
magnirostris) has a large
beak adapted for cracking
seeds that fall from plants
to the ground.
Insect eater. The green warbler finch
(Certhidea olivacea) used its narrow,
pointed beak to grasp insects.
Heterozygote Advantage
The case in which the heterozygote genotype has a
higher relative fitness than either the homozygote
dominant or homozygote recessive genotype. This
selection favoring the heterozygote is one of the
mechanisms that maintain polymorphism and help to
explain some kinds of genetic variability. There are
several cases in which the heterozygote conveys
certain advantages and some disadvantages while both
versions of homozygotes are only at disadvantages.
A well-established case of heterozygote advantage is
that of the gene involved in sickle cell anaemia.
• Polyploidy is presence of extra sets of
chromosomes due to accidents during cell
• It has caused the evolution of some plant
• An autopolyploid is an individual with more
than two chromosome sets, derived from
one species
LE 24-8
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
2n = 6
Gametes produced
by flowers on this
tetraploid branch
are diploid.
Offspring with
tetraploid karyotypes may be
viable and fertile—
a new biological
4n = 12
LE 24-9
An allopolyploid is a species with multiple
sets of chromosomes derived from
different species
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
Meiotic error;
Species A chromosome
number not
2n = 4
reduced from
2n to n
2n = 10
Normal gamete
Species B
2n = 6
Normal gamete
The End