Biology
Chapter 16
Evolution of
Populations
Part II
•
MICROEVOLUTION: Evolution is any
change in the relative frequency of
alleles in a population.
Microevolution
refers to
small scale
change in allele
frequency over
time.
III. Sources of Genetic Variation
A. Two sources of genetic variation
1. Mutation
a. Ultimate source of variation.
b. Any change in a sequence of DNA
c. Most mutations
are bad.
Example: UV,
radiation, toxins
d. Mutations that
produce changes
in an organism’s
phenotype and
increase an
organism’s fitness,
or its ability to
reproduce in its
environment, will
be passed on.
2. Genetic shuffling
that results from
sexual reproduction.
a. Independent
assortment during
meiosis produces
8.4 million possible
combinations.
b. Crossing-over.
16-2 Evolution as Genetic Change
I. Natural Selection on Single-Gene Traits
A. Reminder: Evolution is any change over
time in the relative frequencies of alleles
in a population. Populations, not
individual organisms, evolve over time.
B. Natural selection on single-gene traits
can lead to changes in allele
frequencies and thus to evolution.
Effect of Color Mutations on Lizard Survival
(Figure 16-5):
1. Organisms of one color may produce
fewer offspring than organisms of
other colors.
Example: Red lizards are more visible to
predators and therefore, may be more
likely to be eaten and not pass on that
red gene.
II. Natural Selection on Polygenic Traits
Natural selection can affect the distribution
of phenotypes in any of three ways:
(1) directional selection
(2) stabilizing selection
(3) disruptive selection.
A. Directional Selection
1. One of the two
possible
extremes is
favored.
Example: Darkcolored peppered
moths in regions
of England with
industrial
pollution.
Section 16-2
Directional Selection
Figure 16–6
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
B. Stabilizing Selection
1. Intermediate characteristics are favored.
Examples: Human babies with very high or
very low birth weights have lower survival
than babies with intermediate weights.
Stabilizing Selection
Figure 16–7
Section 16-2
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
C. Disruptive Selection
1. Natural selection moves characteristics
toward both extremes, and intermediate
phenotypes become rarest.
Example: Populations of West African
birds with either large or small, but not
intermediate size beaks.
Section 16-2
Disruptive Selection
Figure 16–8
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Bottleneck
2. Founder effect: A population can
become limited in genetic variability if
it’s founded by a small number of
individuals.
Example: Polydactyly in Amish.
Figure 16-9: Founder Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
IV. Hardy-Weinberg and Genetic Equilibrium
A. What would be necessary for no change
to take place?
1. Hardy-Weinberg principle states that
allele frequencies in a population
will remain constant unless one or
more factors cause those
frequencies to change.
2. If allele frequencies remained constant
then it there would be genetic
equilibrium.
5. Five conditions necessary for HardyWeinberg Equilibrium
NOTE: Hardy-Weinberg equilibrium
rarely exists in natural populations but
understanding the assumptions behind it
gives us a basis for understanding how
populations evolve.
Conditions necessary for
Hardy-Weinberg Equilibrium
a. The population is very large.
b. The population is isolated (no migration of
individuals, or alleles, into or out of the
population).
c. Mutations do not alter the gene pool.
d. Mating is random.
e. All individuals are equal in reproductive
success (no natural selection).
Large Ground Finch
Small Tree Finch
Woodpecker
Finch
16-3 The Process of Speciation
I. How do we get new species?
A. What is a Species?
1. Species:
a group of interbreeding
organisms that breed with one another
and produce fertile offspring.
This means that the individuals of the same
species share a common gene pool.
Diversity in Humans
2. If a beneficial genetic change
occurs in one individual, then that
gene can be spread through the
population as that individual and its
offspring reproduce.
B. Isolating Mechanisms (Leads to a new
species!)
Reproductive Isolation – members of
two populations cannot interbreed and
produce fertile offspring.
PRE-Mating Reproductive Isolation –
involves mechanisms which do not allow
mating to occur in the first place.
1. Behavioral Isolation: Members of two
populations are capable of interbreeding
but have differences in mating displays
or courtship rituals.
a. specific scents (pheromones of insects).
b. color patterns/strutting.
c. specific sounds or calls.
Courtship Dance
Different Mating Songs
2. Geographic/Ecological Isolation: Two
populations are separated by geographic
barriers such as rivers, mountains, or
bodies of water.
When has speciation occurred?
3. Temporal Isolation: Two or more
species live in the same habitat but have
different mating/reproductive seasons.
a. Brown trout and Rainbow trout are
found in the same streams but
Rainbow trout spawn in the Spring and
Brown trout spawn in the Fall.
b. Three similar species of orchid living in
the same tropical habitat each release
pollen on different days; therefore, they
cannot pollinate one another.
NOTE: Several isolating mechanisms can
compound one another to insure mating
doesn’t occur. This permits two species to
occupy the same valuable habitat and
prevents wastage of valuable gametes.
POST-Mating Reproductive Isolation –
(fertilization occurred and zygote formed)
1. Hybrid inviability: hybrid zygotes fail
to develop or fail to reach sexual
maturity.
2. Hybrid sterility: Hybrids fail to
produce functional gametes.
Example: horse x donkey => mule (sterile).
Hybrid Sterility
+
Donkey
Horse
=
Mule (sterile)