MICROEVOLUTION
INVOLVES THE
EVOLUTIONARY
CHANGES WITHIN A
POPULATION.
A population is defined as
all the members of a single
species occupying a
particular area and
reproducing with one
another.
The
members of a
population vary
from one another.
Variation
is the raw
material
for evolutionary change.
WHAT CAUSES
VARIATIONS?
1.
2.
3.
4.
MUTATIONS
CROSSING OVER
INDEPENDENT ASSORTMENT
RANDOM FERTILIZATION
Mutations are the only way to
introduce new traits/alleles into a
population
Godfrey Hardy and
Wilhelm Weinberg
created a principle
that explains the
conditions necessary
for evolution NOT to
take place in a
population.
HARDY-WEINBERG
PRINCIPLE
THE
PRINCIPLE
When allele
frequencies
remain
constant, a
population is in
genetic
equilibrium.
TRANSLATION
If you don’t add or
subtract traits that
exist in a
population, then the
population won’t
evolve (change)
HARDY-WEINBERG EQUILIBRIUM
IS MAINTAINED IN A
POPULATION IF FIVE
CONDITIONS ARE MET. THIS
MEANS THAT EVOLUTION WILL
NOT HAPPEN.
1.
2.
3.
4.
5.
No mutations
No gene flow (migration of alleles
in or out of the population)
Random mating must occur
No genetic drift
No natural selection
 These
conditions are rarely if ever
met in the real world. Thus allele
frequencies continually change and
microevolution occurs.
 The
value of the Hardy-Weinberg
principle is that it describes the
factors that cause evolution.
THEREFORE FOR EVOLUTION TO
TAKE PLACE, THE FOLLOWING
HAS TO HAPPEN.
1.
2.
3.
4.
5.
Mutations
Gene flow
Genetic drift
Nonrandom mating
Natural selection
1. MUTATIONS WILL OCCUR FOR
EVOLUTION TO TAKE PLACE.
Mutations are the raw material of
evolutionary change. (alteration in the
DNA nucleotide sequence of an allele)
 Mutation
introduces new variation into
a population. They can be harmful or
beneficial.
2. GENE FLOW WILL OCCUR FOR
EVOLUTION TO TAKE PLACE.
Gene flow, or gene migration, occurs
when breeding members of a
population leave a population or new
members enter.
 Gene
migration can introduce new
alleles to populations. When new
individuals enter an existing
population, they also introduce all of
their genetic material (DNA, traits,
etc…).
3. GENETIC DRIFT WILL OCCUR FOR
EVOLUTION TO TAKE PLACE.
Chance events that cause the allele
frequency to change is called genetic
drift.
AN EXAMPLE OF GENETIC
DRIFT IS THE BOTTLENECK
EFFECT.
A
bottleneck occurs
when an event reduces
the number of
organisms in a
population.
 The
variation in that
population is reduced,
changing the allele
frequencies within the
population.



Bottleneck effect is caused by a severe
reduction in population size due to
natural disaster, predation, or habitat
reduction.
Bottleneck effect causes severe
reduction in total genetic diversity of
the original gene pool.
The cheetah bottleneck causes relative
infertility because of the intense
interbreeding when populations were
reduced in earlier times.
THE FOUNDER EFFECT IS
ANOTHER EXAMPLE OF
GENETIC DRIFT.
 The
founder effect occurs when
combinations of alleles occur at a higher
frequency in a population that has been
isolated from a larger population.
 This is due to founding individuals
containing a fraction of total genetic
diversity of original population. Which
particular alleles are carried by the
founders is dictated by chance alone.
Genetically inherited
diseases like Ellis-van
Creveld are more
concentrated among the
Amish because they
marry within their
own community, which
prevents new genetic
variation from entering
the population.
4. NONRANDOM MATING MUST OCCUR
FOR EVOLUTION TO TAKE PLACE.
 When
males and
females reproduce
together strictly by
chance it is called
random mating.
 Any
behavioral activity
that fosters the selection
of specific mates is
nonrandom mating.
5. NATURAL SELECTION MUST OCCUR
FOR EVOLUTION TO TAKE PLACE.
Natural
selection is the process
that adapts populations to the
environment.
Operates
to select certain
phenotypes, which are passed to
different generations.
THREE TYPES OF
SELECTION ARE:
1.
2.
3.
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
DIRECTIONAL
SELECTION
 When
one extreme
phenotype is favored by
natural selection, the
distribution of the
phenotype shifts in that
direction.
 This
type of selection is
therefore called
directional selection.
STABILIZING
SELECTION
• Stabilizing selection
occurs when the
intermediate phenotype
is favored.
DISRUPTIVE
SELECTION
 In
disruptive selection,
natural selection acts upon
both extremes of the
phenotype. This creates a
increasing division within the
population which may
ultimately lead to two
different phenotypes.
 Disruptive
selection is the
process that leads to
speciation (creation of a new
species).
DIRECTIONAL
DISRUPTIVE
STABILIZING
MAINTENANCE OF
VARIATIONS
 The
preservation of variation in a
population is important because it
provides a foundation on which natural
selection can act.
 Variation
is preserved by a variety of
mentioned processes.
Mutations and genetic recombination
 Gene flow
 Natural selection
