microevolution
• Microevolution – A change of allele
frequencies in a population over a short
time. The basis for all large-scale or
macroevolution.
• To fully appreciate evolution, we need to
look not just at the outcomes but also at
the processes that created those
outcomes.
Terminology review
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Population
Genotype
Phenotype
Allele
Gene pool – all the alleles that exist in a
population
The two frog
populations now have
different environmental
pressures.
Exactly how do
populations evolve?
• There are many
different alleles in a
population but a
frog can only have
two.
• These two frogs
have different
alleles for
coloration.
What causes Evolution?
• Five Agents of Change that can alter
allele frequencies
– Mutation
– Gene flow
– Genetic drift
– Non-random mating
– Natural selection
Mutation
• Any permanent alterations in the makeup of
DNA.
– They must be heritable
– Base pair, deletion, translocation, etc.
– Most do nothing, a few are harmful, rarely are they
beneficial.
– These mutations are not working to further survival
and reproduction.
– These mutations are not likely to account for a
change in allele frequency.
– These good mutations bring new genetic information
into the genetic pool.
Gene Flow
• A change in allele frequency that can
occur when mating with members from
another population occurs.
– The movement of genes from one population
to another.
– Migration – the movement of individuals from
one population into the territory of another.
– Plants migrate, too.
Gene flow
The ranges of the Bullock's and Baltimore orioles. The
species overlap in a hybrid zone, where gene flow occurs.
Migration
Genetic Drift
• The chance alteration of allele
frequencies in a population, with such
alterations having greatest impact on
small populations.
– Bottleneck effect
– Founder’s effect
• A small sample of alleles is likely to yield
a gene pool that is different from the
distribution found in the larger population.
If there is a drastic change (disease, migration or natural
catastrophe) in the survival of a large population, there is little
change in the frequency of that population's alleles. In a smaller
population, drastic change can result in loss of an entire allele.
• Bottleneck effect – The change in allele
frequency in a population due to chance
following a sharp reduction in the
population size.
• Founder effect – When a small portion of
a population migrates to another area,
starting a new population.
Bottleneck effect
Bottleneck effect
Cheetahs survived a drastic bottleneck
in the nineteenth century.
Founder effect
This Native American
woman, like all Native
Americans, is a
descendant of people
who crossed the
Bering Strait into
Alaska over 10,000
years ago. No Native
Americans have type B
blood, apparently by
chance, none of the
emigrants had this
blood type.
Nonrandom Mating
• Mating in which a given member of a
population is not equally likely to mate
with any other given member.
• Sexual selection is a form of nonrandom
mating. This is mating based on
phenotype, based on the choices made
by the female of the species.
• The male will increase the proportion of
his alleles in the next generation.
Natural Selection
• A process in which the differential
adaptation of organisms to their
environment selects those traits that will
be passed on with greater frequency from
one generation to the next.
• Traits of those who are more successful in
reproducing will become more widespread
in a population, the alleles that bring about
these traits will increase in frequency from
one generation to the next.
Artificial selection
• Artificial selection:
Humans choose
traits they prefer in
animals or crops and
breed to achieve
those “desirable”
individuals.
• Natural selection is the only one of the
five agents of microevolution that
consistently works to adapt organisms to
their environment.
– Genetic drift is random
– Mutation has a negative effect, or no effect
– Gene flow doesn’t necessarily bring in genes
that are better suited to the environment
– Non random mating doesn’t have anything to
do with matching individuals to environment
Fitness
• The success of an organism in passing
on its genes to offspring, relative to the
other members of its population.
• The allele frequencies will increase.
• Also known as “survival of the fittest”
1. Large cactus finch (Geospiza
conirostris) 2. Large ground finch
(Geospiza magnirostris) 3. Medium
ground finch (Geospiza fortis)
4. Cactus finch (Geospiza scandens)
5. Sharp-beaked ground finch
(Geospiza difficilis) 6. Small ground
finch (Geospiza fuliginosa)
7. Woodpecker finch (Cactospiza
pallida) 8. Vegetarian tree finch
(Platyspiza crassirostris) 9. Medium
tree finch (Camarhynchus pauper)
10. Large tree finch (Camarhynchus
psittacula) 11. Small tree finch
(Camarhynchus parvulus) 12.
Warbler finch (Certhidia olivacea)
13. Mangrove finch (Cactospiza
heliobates)
Three modes of
Natural Selection
• When natural selection operates on
characters that are polygenic and
continuously variable, it can proceed in
one of three ways.
• Does natural selection favor what is
average or what is extreme?
Stabilizing Selection
• Intermediate forms are
favored over extreme forms.
• Human baby birth weight.
• Infant deaths are higher at
the extremes of birth weight,
children most likely to survive
have an average birth weight.
• Natural selection is working
to make average birth
weights more common
Directional Selection
• When natural selection moves
a character towards one of its
extremes.
• Smoothness of brachiopod
shells took 10 million years to
achieve.
Disruptive Selection
• When natural selection moves a
character to both its extremes.
• Occurs much less frequently in
nature.
• Finches in West Africa
• When food is scarce, largebilled birds specialize in
cracking a large seed.
Small-billed birds feed on
soft varieties of seed.
• A bird with an intermediate
sized bill would get less
food and not survive.