Chapter 16
POPULATION
GENETICS
In order to understand the
genetics behind populations
we must revisit Darwin
Charles Darwin
 Darwin’s Theory:
 All organisms compete for limited space

Organisms produce more offspring than can survive

Natural selection states that organisms best suited to the
environment survive while those not suited may eventually
die
Charles Darwin
 Darwin’s Evolution:
 Variation exists within a species
 Some variations are favorable
 Survival of the fittest
The strongest will survive and reproduce
 The weak will die out
 Organisms better adapted to the environment will survive


Adaptations will happen gradually

Gradualism
Charles Darwin
Gene Pools
 Biologists today study a particular population
 Gene pool- combined genetic information of all the members
of that population
 Relative frequency- the number of times an allele appears in a
population as compared with the other alleles
 Sources of Genetic Variation
 Mutations
 Genetic reshuffling during sexual reproduction
Single vs Polygenetic Variation
 Inheritable variation can be expressed in a number
of ways

Single trait- controlled by a single gene


Example widows peak
 Since single gene controls the trait usually there are only two
phenotypes
Polygenic trait- controlled by a multiple genes

Example height in humans
 Due to the multiple number of genes controlling this allele
there are multiple phenotypes that result
 End up with a bell shaped curve (most people fall around the
average, you have some that are well above and some well below
average
Evolution as Genetic Change
 Natural Selection on Single Gene Traits
 Lizard example, peppered moths
Natural Selection on Polygenic Traits
 Can effect the distribution of phenotypes in any number
of three ways:

Disruptive Selection


Directional Selection


Selection can act against the middle of a normal distribution after an
environmental change, this is selection against the most common
variation (ex. African Swallowtale Butterfly
After several generations, the normal distribution shifts in the
direction of change (ex. DDT and insects)
Stabilizing Selection

Environments may go through long periods of stability, when
conditions remain about the same. Organisms that are best adapted
to the existing environment will be favored, and there is selection
against the extremes
Disruptive Selection
 Disruptive Selection When individuals at the
upper and lower ends of the curve have
higher fitness than individuals near the
middle, disruptive selection takes place. In
this example, average-sized seeds become less
common, and larger and smaller seeds become more
common. As a result, the bird population splits into
two subgroups specializing in eating different-sized
seeds.
Disruptive Selection
Directional Selection
 Directional Selection Directional selection
occurs when individuals at one end of the
curve have higher fitness than individuals in
the middle or at the other end. In this example,
a population of seed-eating birds experiences
directional selection when a food shortage causes the
supply of small seeds to run low. The dotted line
shows the original distribution of beak sizes. The
solid line shows how the distribution of beak sizes
would change as a result of selection.
Directional Selection
Stabilizing Selection
 Stabilizing Selection Stabilizing selection
takes place when individuals near the center
of a curve have higher fitness than
individuals at either end. This example shows
that human babies born at an average mass are more
likely to survive than babies born either much
smaller or much larger than average
Stabilizing Selection
Natural Selection on polygenic Traits
 Natural Selection
Other Sources of Genetic Variation
 Genetic Drift- In small populations, individuals
that carry a particular allele may leave more
descendants than other individuals do, just
by chance. Over time, a series of chance
occurrences of this type can cause an allele to
become common in a population.



May occur when small group colonizes new habitat
Not caused by natural selection but by chance
situation in which allele frequencies change as a
result of the migration of a small subgroup of a
population is known as the founder effect.
Founders effect
 One example of the founder
effect is the evolution of
several hundred species of
fruit flies found on different
Hawaiian Islands. All of
those species descended
from the same original
mainland population. Those
species in different habitats
on different islands now
have allele frequencies that
are different from those of
the original species.
Understand?
 Genetic drift is
 A. colonization of a new habitat by small groups of
individuals.
 B. random change in allele frequencies.
 C. migration of a small subgroup of a population.
Understand?
 Genetic drift is
 A. colonization of a new habitat by small groups of
individuals.
 B. random change in allele frequencies.
 C. migration of a small subgroup of a population.
Population Genetics
 Population
 Group of organisms that live in the same are & interbreed
 Evolution can only occur when there is a change in
the kinds or % of genes in the gene pool of a
population (allele frequencies)
Hardy-Weinberg Principle
 States that allele frequency will stay constant unless
one or more factors cause those frequencies to
change


Describes the conditions that must be met in order for the
allele frequencies to remain constant
It describes genetic equilibrium

Five conditions
Hardy-Weinberg Principle
 1. No Mutations
 2. Random Mating
 3. No Genetic Drift
 4. No Natural Selection
 5. No Gene Flow
THESE CONDITIONS
CAN BE MET FOR LONG
PERIODS OF TIME. IF
HOWEVER THESE
CONDITIONS ARE NOT
MET THEN THE
GENETIC EQUILIBRIUM
WILL BE DISRUPTED
AND THE POPULATION
WILL EVOLVE
Key Concepts
 Can you answer the following:
 Describe three patterns of natural selection on polygenic traits.
Which one leads to two distinct phenotypes?
 How does genetic drift lead to a change in a population's gene
pool?
 What is the Hardy-Weinberg principle?
 Describe how natural selection can affect traits controlled by
single genes.
The Process of Speciation
 Isolating Mechanisms- As new species evolve,
populations become reproductively isolated from
each other.

Reproductive isolation through:
Behavioral isolation
 Geographic isolation
 Temporal isolation

Behavioral Isolation
 Occurs when two populations are capable of
interbreeding but have differences in courtship
rituals or other reproductive strategies that involve
behavior.
Behavioral Isolation
The eastern
meadowlark (left)
and western
meadowlark
(right) have
overlapping
ranges. They do
not interbreed,
however, because
they have
different mating
songs.
Geographic Isolation
 Populations
are separated
by geographic
barriers such
as rivers,
mountains, or
bodies of
water.
Temporal Isolation
 Two or more species reproduce at different times.
 three similar species of orchid all live in the same rain forest.
Each species releases pollen only on a single day. Because the
three species release pollen on different days, they cannot
pollinate one another.
Testing Natural Selection
 Peter and Rosemary Grant continued Darwin’s
observations on the finches of Galapagos Islands

When food for the finches was scarce, individuals with the
largest beaks were more likely to survive, as shown in the
graph below. Beak size also plays a role in mating behavior,
because big-beaked birds tend to mate with other big-beaked
birds. The Grants observed that average beak size in that
finch population increased dramatically over time.
Understand?
 This graph shows
that



A. the larger a bird's beak,
the smaller are its chances
of survival.
B. the smaller a bird's
beak, the greater are its
chances of survival.
C. the larger a bird's beak
the greater are its chances
of survival.
Understand?
 This graph shows
that



A. the larger a bird's beak,
the smaller are its chances
of survival.
B. the smaller a bird's
beak, the greater are its
chances of survival.
C. the larger a bird's beak
the greater are its chances
of survival.
Understand?
 What type of natural selection did the Grants
observe in the Galápagos?



A. disruptive selection
B. directional selection
C. stabilizing selection
Understand?
 What type of natural selection did the Grants
observe in the Galápagos?



A. disruptive selection
B. directional selection
C. stabilizing selection
Speciation in Darwin’s Finches
 Speciation- When one or more new organisms
evolve from a single ancestral species






Founders Effect- few finches arrive from mainland
Separation of populations- some birds cross to other islands
Changes in gene pool- over time populations become adapted
to their environment
Reproductive isolation- no longer will mat e with one another
Ecological competition- compete for available
resources…best suited to environment wins
Continued Evolution- repeats process time and time again.
Over many generations it produced 13 different species of
finches (see page 410 in your book)
Understand?
 When two species do not reproduce because of
differences in mating rituals, the situation is referred
to as



A. temporal isolation.
B. geographic isolation.
C. behavioral isolation.
Understand?
 When two species do not reproduce because of
differences in mating rituals, the situation is referred
to as



A. temporal isolation.
B. geographic isolation.
C. behavioral isolation.
Understand?
 One finding of the Grants' research on generations of
Galápagos finches was that



A. natural selection did not occur in the finches.
B. natural selection can take place often and very rapidly.
C. beak size had no effect on survival rate of the finches.
Understand?
 One finding of the Grants' research on generations of
Galápagos finches was that



A. natural selection did not occur in the finches.
B. natural selection can take place often and very rapidly.
C. beak size had no effect on survival rate of the finches.
Understand?
 All of the following played a role in speciation of
Galápagos finches EXCEPT



A. no changes in the gene pool.
B. separation of populations.
C. reproductive isolation.
Understand?
 All of the following played a role in speciation of
Galápagos finches EXCEPT



A. no changes in the gene pool.
B. separation of populations.
C. reproductive isolation.