Biology ECO
SC.912.L.15.13
Sophiaa Lopez, P2.
• As powerful as natural selection is, unfortunately, it is sometimes
misunderstood. A persistent misconception is that natural selection
occurs mainly through differences between organisms in death rates, or
differential mortality. Selection normally proceeds in a much more
subtle and inconspicuous way. Whenever one organism leaves more
successful offspring than another, in time its genes will come to
dominate the population gene pool. Eventually, the genotype leaving
fewer offspring must become extinct in a stable population, unless
concomitant changes confer an advantage on it as it becomes scarcer.
Ultimately, natural selection operates only by differential
reproductive success. Differential mortality can be selective but only
to the degree that it creates differences between individuals in the
number of reproductive progeny they produce. Hence, phrases such as
“the struggle for existence” and “survival of the fittest” have had an
unfortunate consequence.
Simpson Evolution
Example
Natural Selection
Individuals with favorable
traits are more likely to leave
more offspring better suited
for their environment
Example:
English peppered
moth (Biston betularia)
Peppered Moth Lab
Peppered Moth Natural Selection Simulation
MUTATION
#1
A mutation can create
a new allele in an individual. MECHANISMS
When this happens, the
OF
population experiences a
EVOLUTION
change in its allele
frequencies and,
consequently, experiences
evolution.
 Despite mutation’s vital role in
the generation of variation,
mutations almost always cause
early death or lower the
reproductive success of an
organism.
EVOLUTIONARY CHANGE: MUTATION
Mutagen
DNA
Normal basepair sequence
Mutated basepair sequence
Normal protein
Mutated protein
Normal
phenotype
Mutated
phenotype
Mutations
• Are rare because you have self correcting enzymes
• Natural Process that produces genetic diversity
• Not all mutations are bad
– Some won’t affect the body at all
• Blood types/ear lobes
– Some are advantageous
(thumb)
five
digit
s
wrist
bone
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.
POPULATION
AFTER GENETIC
DRIFT
There are now more
recessive alleles in the
population than
before.
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.
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.
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.
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.
MECHANISMS
OF 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.
AMISH
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
MECHANISMS
OF 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.
Population 2
MIGRATION (GENE FLOW)
2 MIGRATION
A group of individuals from Population 1
migrates over the mountain range.
Population 1
MECHANISMS
OF 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.
Population 2
MIGRATION (GENE FLOW)
MECHANISMS
OF 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.
3 AFTER MIGRATION
The migrating individuals are able to survive and
reproduce in the new population and they may
experience evolutionary changes from population 1.
Population 1
Population 2
Mechanism of
evolution
Natural Selection
1. VARIATION OF A TRAIT IN A POPULATION
3 Conditions that must
occur for Natural Selection
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.
2. The trait must be inheritable
3. One version of the trait must be in greater
abundance than a different version of the trait.
EVOLUTION BY NATURAL SELECTION: A SUMMARY
1 VARIATION
FOR A TRAIT
Different traits are present
in individuals of the same
species
2 HERITABILITY
Traits are passed on from
parents to their children.
MECHANISMS
OF EVOLUTION
NATURAL SELECTION
When these three conditions
are satisfied, the population’s
allele frequencies change and,
consequently, evolution by
natural selection occurs.
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.