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WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 8
THE EVOLUTION OF POPULATIONS
Figure 8-10, part 1 Evolution defined.
THE EVOLUTION OF POPULATIONS
TIGER POPULATION
Allele frequencies:
Proportion of orange
fur-pigment alleles in the
population
Proportion of white fur-pigment
alleles in the population
Evolution is a change in the allele frequencies of a population over time. For example, a change in the proportion of
pigment alleles in the population of tigers means that
evolution has occurred.
Figure 8-10, part 2 Evolution defined.
© 2010 W. H. Freeman & Company
CHAPTER 8 • EVOLUTION AND NATURAL SELECTION, PAGE 9
MECHANISMS OF
EVOLUTIONARY CHANGE: MUTATION
EVOLUTION
MUTATION
A mutation can create a new
allele in an individual. When this
happens, the population experiences a change in its allele
frequencies and, consequently, experiences
evolution.
Mutagen
DNA
Normal basepair sequence
Mutated basepair sequence
Normal
protein
Mutated
protein
No
Normal
enotype
phen
phenotype
Mutated
phenotype
Despite mutation’s vital role in the generation of
variation, mutations almost always cause early death
or lower the reproductive success of an organism.
Figure 8-11 Agents of evolutionary change: mutation.
© 2010 W. H. Freeman & Company
WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 10
MECHANISMS OF
EVOLUTION
GENETIC DRIFT
A population can experience
random changes in allele
frequency that do not influence
reproductive success and,
consequently, the population experiences
evolution.
GENETIC DRIFT
POPULATION BEFORE
GENETIC DRIFT
Allele frequencies:
cleft chin (dominant)
smooth chin
(recessive)
Neither allele is related to
reproductive success.
Inheritance is based
solely on chance.
REPRODUCTION
In this example, a heterozygous couple (Cc)
could have two children that are homozygous recessive (cc), causing an increase in
the proportion of recessive alleles in the
population.
POPULATION AFTER
GENETIC DRIFT
There are now more
recessive alleles in
the population than
before.
FIXATION
Genetic drift leads to fixation when an allele’s frequency
becomes 100% in a population. If this occurs, there is no
longer genetic variation for the gene.
Figure 8-12 Agents of evolutionary change: genetic drift.
MECHANISMS OF
GENETIC DRIFT: FOUNDER EFFECT
SOURCE POPULATION
Allele frequencies:
5 digits per hand
(recessive)
>5 digits per hand
(dominant)
A group of individuals may
leave a population and become
the founding members of a new,
isolated population.
NEWLY FOUNDED POPULATION
The new population will be
dominated by the genetic
features present in the founding
members.
Figure 8-13 One way that genetic drift occurs: founder effect.
© 2010 W. H. Freeman & Company
EVOLUTION
FOUNDER EFFECT
The founding members of a
new population can have
different allele frequencies than
the original source population
and, consequently, the new
population experiences
evolution.
CHAPTER 8 • EVOLUTION AND NATURAL SELECTION, PAGE 11
MECHANISMS OF
GENETIC DRIFT: BOTTLENECK EFFECT
Occasionally, famine or disease or rapid environmental
change may cause the deaths of a large, random
proportion of the individuals in a population.
SOURCE POPULATION
EVOLUTION
BOTTLENECK EFFECT
The surviving members of a
catastrophic event can have
different allele frequencies than
the source population and,
consequently, the new population experiences evolution.
EXTREME AND RAPID ENVIRONMENTAL CHANGE
NEW POPULATION
The new population will
be dominated by the
genetic features present
in the surviving members.
All cheetahs living today can trace their ancestry back to a dozen or so individuals that
happened to survive a population bottleneck
about 10,000 years ago!
Figure 8-14 Another way that genetic drift occurs: bottleneck effect.
MIGRATION (GENE FLOW)
1
BEFORE MIGRATION
Two populations of the same species exist in
separate locations. In this example, they are
separated by a mountain range.
Population 1
2
EVOLUTION
MIGRATION
After a group of individuals
migrates from one population
to another, both populations
can experience a change in
their allele frequencies and,
consequently, experience evolution.
MIGRATION
A group of individuals from Population 1 migrates
over the mountain range.
Population 1
3
Population 2
MECHANISMS OF
Population 2
AFTER MIGRATION
The migrating individuals are able to survive and
reproduce in the new population.
Population 1
Population 2
Figure 8-15 Agents of evolutionary change: migration (gene flow).
© 2010 W. H. Freeman & Company
WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 12
Figure 8-16 part 1 Necessary conditions for natural selection: 1. Variation for a trait.
Figure 8-16 part 2 Necessary conditions for natural selection: 1. Variation for a trait.
© 2010 W. H. Freeman & Company
CHAPTER 8 • EVOLUTION AND NATURAL SELECTION, PAGE 13
Figure 8-17 Necessary conditions for natural selection: 2. Heritability.
The tiniest dog in a litter has
reduced differential reproductive
success. Its more robust siblings
prevent access to the food it
needs to grow and thrive.
Figure 8-18 Necessary conditions for natural selection: 3. Differential reproductive success.
© 2010 W. H. Freeman & Company
WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 14
EVOLUTION BY NATURAL SELECTION: A SUMMARY
1
2
VARIATION FOR A TRAIT
Different traits are
present in individuals of
the same species.
HERITABILITY
Traits are passed on from
parents to their children.
3
DIFFERENTIAL REPRODUCTIVE
SUCCESS
In a population, individuals with
traits most suited to reproduction
in their environment generally
leave more offspring than individuals with other traits.
MECHANISMS OF
EVOLUTION
NATURAL SELECTION
When these three conditions
are satisfied, the population’s
allele frequencies change and,
consequently, evolution by
natural selection occurs.
Figure 8-19 Agents of evolutionary change: natural selection.
NATURAL SELECTION IN NATURE
1
VARIATION FOR A TRAIT
Running speed in rabbits can vary from one
individual to the next.
Speed
2
HERITABILITY
The trait of running speed is passed on from
parents to their offspring.
3
DIFFERENTIAL REPRODUCTIVE SUCCESS
In a population, rabbits with slower running speeds
are eaten by the fox and their traits are not passed
on to the next generation.
Figure 8-20 Removing the losers.
© 2010 W. H. Freeman & Company