3. 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.
6. 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.
7. What causes Evolution?
Five Agents of Change that can alter
allele frequencies
Mutation
Gene flow
Genetic drift
Non-random mating
Natural selection
8. Mutation
Any permanent alterations in the makeup of
DNA.
They must be heritable
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.
9. 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.
10. The ranges of the Bullock's and Baltimore orioles. The
species overlap in a hybrid zone, where gene flow occurs.
Gene
flow
12. Genetic Drift
The chance alteration of allelesin 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.
13. 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.
14. 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.
16. Cheetahs survived a drastic bottleneck
in the nineteenth century.
Bottleneck effect
17. 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.
18. 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.
19.
20. 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?
21. 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
22.
23. Directional Selection
When natural selection
moves a character towards
one of its extremes.
Smoothness of brachiopod
shells took 10 million years to
achieve.
24.
25. Disruptive Selection
When natural selection moves a
character to both its extremes.
Occurs much less frequently in
nature.
Results in two separate species
26. Finches in West Africa
When food is scarce,
large-billed 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.
