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Changes in the genetic
makeup of populations
Natural Selection
1.Populations
tend to
overproduce
offspring.
Natural Selection
2. Genetic variation exists within populations
Natural Selection
3. The best adapted organisms survive and reproduce in
greater numbers.
Natural Selection
4. Subsequent generations
contain higher frequencies of
adapted orgs.
Adaptation
Inherited trait
 Increases chance of survival/reproduction
 Environment-specific

Natural Selection
The environment
determines success of traits
1.Overproduction
2.Genetic variation
3.Struggle for survival
4.Differential reproduction
Natural Selection
Hardy-Weinberg Conditions
Hypothetical
conditions that
must exist in a
population for
no evolution to
occur.
Population Characteristics
Hinder Evolution
Help Evolution
(Hardy-Weinberg Conditions)
(Natural Conditions)

POPULATION SIZE
Infinitely Large
 Small
 Genetic Drift
MATING

Random Mating
 Natural
Selection
 Natural
Selection
VIABILITY

Equal Survival
 Isolation

No mutations
INTERACTION W/ OTHER POPS
 Gene Flow
DNA CHANGES
 Mutations
Genetic Drift
Random fluctuations in genetic makeup
Example:
2
red, 2 blue orgs seek shelter
 50% survive. All are red
 Blues eliminated by chance (17%)
4
red, 4 blue head for shelter
 50% survive. All are red
 Blues eliminated by chance (1.4%)
Genetic Drift
Bottleneck Effect

Catastrophe cuts pop size



Decreased variety of traits
Altered genetic makeup
Genetic drift more likely

Founder Effect

Small group emigrates
to new habitat
Gene Flow
Introduction
of new traits through immigration
May be beneficial or insignificant
New traits  altered genetic makeup

Modes of Selection
Stabilizing
 Environment
is stable
 Initially adaptive
form becoming more
numerous from many
generations of
natural selection
MOST
ADAPTIVE
FORM
RANGE OF PHENOTYPES
Modes of Selection
Directional
 Environment is
changing
(or has changed)
 Formerly adaptive form is
no longer adaptive
 Pop evolves toward
newly adaptive form
MOST
ADAPTIVE
FORM
RANGE OF PHENOTYPES
Modes of Selection
Disruptive
(Diversifying)
 Intermediate
form is
undesirable and
decreases in #
RANGE OF PHENOTYPES
MOST
ADAPTIVE
FORMS
MOST
ADAPTIVE
FORMS
Evolution of the Ctukis
Ctukis
are creatures from a planet you’ve never
heard of.
You will construct a ctuki out of a single 8½x11
sheet of paper.
 Do
not add any materials.
 Put your names on the ctuki.
 Your ctuki must be able to asexually reproduce in class
(it should take no more than one minute).