Population Genetics and
Patterns of Evolution
Chapter 11
Are these organisms the same
species?
Are these organisms the same
species?
How do we study evolution?
• A species is defined as a group of similar
organisms that are capable of producing
fertile offspring.
KEY CONCEPT
A population shares a common gene pool.
How do we study evolution?
• Evolution is change over time, which
means it occurs within a group whose
individuals are actually breeding with
each other;
• and therefore, we study evolution by
examining genetic change within a
population.
• INDIVIDUALS do NOT EVOLVE, a
population evolves.
Genes and Variation
1. Inheritable traits are coded for by genes,
and the different forms of a gene are
called alleles. There exists variation
within a population for many of these
alleles.
11.1
Genetic Variation Within Populations
TEKS 7C, 7E, 7F
Genetic variation in a population increases the chance
that some individuals will survive.
• Genetic variation leads to phenotypic variation.
• Phenotypic variation is necessary for natural selection.
• Genetic variation is stored in a population’s gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have offspring
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
1. The gene pool consists of all the genes
present in a population.
2. The relative frequency of an allele in a
population is often expressed in a
percentage.
Calculate the frequency of “A” and
“a”
A = 24/72 = .33 or 33%
a = 48/72 = .67 or 67%
1. How many genes are in
our gene pool?
2. What is the frequency of
the black allele?
–
–
20 out of 50
0.4
3. What is the frequency of
the brown allele?
–
–
30 out of 50
0.6
In this sample
population, is the
most common allele
the dominant one?
The most common
allele does not
have to be dominant!!
When a change in the relative frequency of an
allele occurs in a population, “change over time”
has occurred, and this is evolution on a small
scale.
This is called MICROEVOLUTION
– observable change in the allele frequencies
– can result from natural selection
i.
•
•
Example: Consider alleles for polydactyly in the gene pool,
the allele coding for extra digits, the polydactyly allele (P), is
only 1% of the population, the frequency is 0.01.
The allele for 5 fingers and toes (p) is 99% of the population,
or a frequency of 0.99.
If over time, extra fingers was an advantage, and natural
selection selected FOR individuals with extra digits, a shift in
that allele frequency might happen, and evolution on a small
scale would have occurred!
11.1
Genetic Variation Within Populations
TEKS 7C, 7E, 7F
Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
• Recombination forms new combinations of alleles.
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
Section 2
Selection on a Single-Gene trait
•
A single-gene trait with two alleles will
show two phenotypes. A change in
frequency is easy to see in a population.
Selection on a Polygenic Trait
• A polygenic trait is one that is controlled
by more than one gene.
• Things such as height in humans are
polygenic traits.
• If you were to graph out the frequencies of
the phenotypes, you would get a bell
shaped curve.
Range of Phenotype
Selection on a Polygenic Trait
1. Directional Selection- occurs when individuals
at one end of the curve (with phenotypes at
one end of the spectrum) are advantaged, and
selection against the other end occurs. The
individuals with the higher fitness, or ability to
survive and reproduce, will succeed. Over
time the population will shift in its phenotypes
to one direction.
–
Example: Food becomes scarce and one type of
beak is most efficient
Selection on a Polygenic Trait
Number vs. Running speed of Rabbits
Selection on a Polygenic Trait
2. Stabilizing selection- occurs when
individuals in the middle of the curve are
more advantaged, or have a higher
fitness, than individuals at the ends. This
causes the frequency of the midphenotypes to increase, and the ends to
decrease
• Example: Birth weight in humans
Number of spiders vs. body size
Increasing body size
Selection on a Polygenic Trait
3. Disruptive Selection- occurs when
individuals at the ends of the curve are
more advantaged, or have a higher
fitness, than the individuals at the middle
of the curve. This is less common. A
single curve will appear to split in two.
– Example: Larger and smaller seeds
become more common
Selection on a Polygenic Trait
Directional, Stabilizing or Disruptive
selection?
•
The Siberian Husky is a dog bred for working in the
snow. The Siberian Husky is a medium dog with the
males weighing 16-27kg (35-60lbs). These dogs have
strong pectoral and leg muscles, allowing it to move
through dense snow. The Siberian Husky is well
designed for working in the snow. If the Siberian Husky
had heavier muscles, it would sink deeper into the
snow, so they would move slower or would sink and
get stuck in the snow. Yet if the Siberian Husky had
lighter muscles, it would not be strong enough to pull
sleds and equipment, so the dog would have little
value as a working dog. What type of selection does
this illustrate?
Stabilizing Selection
Directional, Stabilizing or Disruptive
selection?
•
In a species of African butterfly Pseudacraea eurytus,
the colorations range from a reddish yellow to blue. In
both cases, these extremes of color, from different
ends of the spectrum, look like (mimic) other species
of butterflies that are not normally the prey of other the
local predator group of birds and other insects.
Accordingly, selection favors the extremes in
coloration within the Pseudacraea eurytus population.
Those butterflies that are moderate in coloration are
eaten in far greater numbers that those at the
extremes of the color spectrum. As a consequence,
those butterflies with extremes of coloration survive as
a greater percentage of the population available to
pass on those genes for coloration to the next
generation. What type of selection does this illustrate?
Disruptive Selection
Directional, Stabilizing or Disruptive
selection?
•
The greyhound breed of dog was originally used to
hunt the fastest of game, fox and deer. Their breed
dates to Egypt in 3BC. Early breeders were interested
in dog with the greatest speed. They carefully selected
from a group of hounds those who ran the fastest.
From their offspring, the greyhound breeders again
selected those dogs who ran the fastest. By continuing
this selection for those dogs who ran faster than most
of the hound dog population, they gradually produced
a dog who could run up to 64km/h (40mph). What type
of selection does this illustrate?
Directional Selection
Section 3
11.3 Other Mechanisms of Evolution
Gene flow is the movement of alleles between
populations.
• Gene flow occurs when
individuals join new
populations and
reproduce.
• Gene flow keeps
neighboring populations
similar.
• Low gene flow increases
the chance that two
populations will evolve
into different species.
bald eagle migration
TEKS 7D, 7F
Genetic Drift
• Genetic drift is the change in a
population’s allele frequencies due to
chance.
• There are 2 situations in which a
population is shrunk and genetic drift can
take place.
Genetic Drift
• The Bottleneck Effect
– Disasters such as
earthquakes, flood,
droughts and fires can
greatly reduce the size of a
population. Those that
survive may not be
representative of the
original gene pool.
– This can greatly reduce
genetic variability.
Genetic Drift
• The Founder Effect
– Takes place when a few individuals from a
larger population colonize an isolated habitat.
– There is very little genetic variety in the gene
pool because not all genes from the original
population are represented.
11.3 Other Mechanisms of Evolution
TEKS 7D, 7F
• The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– The founder effect is genetic drift that occurs after start
of new population.
Genetic Drift
• How might the bottleneck effect and the
founder effect negatively affect a forming
population?
Bottleneck or Founder?
•
Cheetahs were once widespread in
Africa and Asia. Their numbers fell
drastically during the last ice age about
10,000 years ago. Those few that survive
reproduced and made up the population
that exists today.
Bottleneck or Founder?
•
Northern elephant seals have reduced
genetic variation probably hunting
reduced their population size to as few
as 20 individuals at the end of the 19th
century. Their population has since
rebounded to over 30,000. Because of
they over-hunting, the Northern Elephant
seals have much less genetic variation
than a population of southern elephant
seals that was not so intensely hunted.
Bottleneck or Founder?
•
The Afrikaner population of Dutch
settlers in South Africa is descended
mainly from a few colonists. Today, the
Afrikaner population has an unusually
high frequency of the gene that causes
Huntington’s disease, because those
original Dutch colonists just happened to
carry that gene with unusually high
frequency.
Bottleneck or Founder?
•
European bison, faced extinction in the
early 20th century due to overhunting,
nearly leading to extinction. The animals
living today are all descended from 12
individuals and they have extremely low
genetic variation, which may be
beginning to affect the reproductive
ability of bulls
Bottleneck or Founder?
•
The Amish populations in the United
States, which have grown from a very
few founders, have not recruited
newcomers, and tend to marry within the
community. Though still rare,
phenomena such as polydactyly (extra
fingers and toes) are more common in
Amish communities than in the American
population at large.
11.3 Other Mechanisms of Evolution
TEKS 7D, 7F
Sexual selection occurs when certain traits increase
mating success.
• Sexual selection occurs
due to higher cost of
reproduction for females.
– males produce many
sperm continuously
– females are more
limited in potential
offspring each cycle
11.3 Other Mechanisms of Evolution
TEKS 7D, 7F
• There are two types of sexual selection.
– intrasexual selection: competition among males
– intersexual selection: males display certain traits to
females
Section 4
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
KEY CONCEPT
Hardy-Weinberg equilibrium provides a framework for
understanding how populations evolve.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Biologists use models to study populations.
• Hardy-Weinberg equilibrium is a type of model.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Genotype frequencies stay the same if five conditions are
met.
– very large population: no genetic drift
– no emigration or immigration: no gene flow
– no mutations: no new alleles added to gene pool
– random mating:
no sexual selection
– no natural selection:
all traits aid equally
in survival
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Real populations rarely meet all five conditions.
– Real population data is
compared to a model.
– Models are used to
studying how populations
evolve.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
The Hardy-Weinberg equation is used to predict genotype
frequencies in a population.
• Predicted genotype frequencies are compared with actual
frequencies.
– used for traits in simple dominant-recessive systems
– must know frequency of recessive homozygotes
– p2 + 2pq + q2 = 1
"The Hardy-Weinberg equation
is based on Mendelian genetics.
It is derived from a simple
Punnett square in which p is the
frequency of the dominant allele
and q is the frequency of the
recessive allele."
Hardy-Weinberg Formulas:
p is the frequency of the dominant allele
q is the frequency of the recessive allele
p+q=1
p2 + 2pq + q2 = 1
Homozygous dominant
Heterozygous
Homozygous recessive
In these pigs, the allele for pink coat is
dominant and the allele for black coat is
recessive.
p+q=1
p2 + 2pq + q2 = 1
P = .5
Q = .5
P2 = .25
Q2 = 4/16 = .25
2PQ = .5
What is p? What is q?
Determine the percent of the pig
population that is heterozygous for pink
coat.
Hardy Weinberg Derivation
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
• Genetic drift changes allele frequencies due to chance
alone.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
• Gene flow moves alleles from one population to another.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
• Mutations produce the genetic variation needed for
evolution.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
• Sexual selection selects for traits that improve mating
success.
11.4 Hardy-Weinberg Equilibrium
TEKS 7C, 7D, 7F
• Natural selection selects for traits advantageous for
survival.
11.4 Hardy-Weinberg Equilibrium
• In nature, populations evolve.
– expected in all populations
most of the time
– respond to changing
environments
TEKS 7C, 7D, 7F
Speciation
• First of all what is a species?
– As defined by Ernst Mayr- the Biological
Species Concept states: “Species are
groups of actually or potentially
interbreeding natural populations which
are reproductively isolated from other
such groups.”
– When natural selection acts on a population,
certain characteristics are favored and others
are not.
Section 5
Speciation
• What causes new species to arise?
– They must be separated and no longer be
able to produce fertile offspring, or become
reproductively isolated, in order to become
officially a different species.
– This is called speciation.
• Two types: phyletic speciation and divergent
speciation
Types of Reproductive Isolating
Mechanisms (RIMs):
• Prezygotic (prevent
mating or fertilization)
– Geographical
isolation- different
habitats or rarely
encounter each other
Types of Reproductive Isolating
Mechanisms (RIMs):
• Prezygotic (prevent mating or fertilization)
– Temporal isolation- breed/flower at different
times of the year or day
Types of Reproductive Isolating
Mechanisms (RIMs):
• Prezygotic (prevent mating or fertilization)
– Behavioral isolation- differences in
mating/courting; usually a result of sexual
selection
Ex. The eastern meadowlark (left)
and western meadowlark (right) have
overlapping ranges. They do not
interbreed because they have different
mating songs.
Types of Reproductive Isolating
Mechanisms (RIMs):
• Prezygotic (prevent mating or fertilization)
– Mechanical isolation- anatomically
incompatible sex organs on plants or animals
Types of Reproductive Isolating
Mechanisms (RIMs):
• Postzygotic (reduce viability of hybrids)
– Hybrid inviability- hybrids do not develop or
are less likely to survive
• Hypotheses for why this occurs include
– differences in gene regulation interfering with
development OR
– immunological differences between mother and the
hybrid fetus
– Either way- the fetus does not develop properly
Types of Reproductive Isolating
Mechanisms (RIMs):
• Postzygotic (reduce viability of hybrids)
– Hybrid sterility- F1 hybrids develop, but
cannot reproduce
Types of Reproductive Isolating
Mechanisms (RIMs):
• Postzygotic (reduce viability of hybrids)
– Hybrid breakdown- F1 hybrids are viable, but
F2s are not
• Ex: Certain species of cotton plants can make
fertile offspring, but the offspring of those are
sterile
How did speciation occur in the
Galapagos?
1. Because the Galapagos are a group of
islands, there are separate ecosystems
on each.
2. Founder populations arrived on an island
from the mainland of South America.
How did speciation occur in the
Galapagos?
3. Reproductive isolation occurred (geographic).
4. Frequencies of different traits changed in that
population over time because of natural
selection, based on food source, soil types,
predators, etc.
5. Eventually, over a long period of time, the
original population and the founder population
on the second island are very different and are
considered different species.
What type of beak would each bird have?
• Notice the beaks’ structure fits their functions
Salamander Speciation
• On their ways to diverging into separate
species
• What RIMs are in effect here?
Origin and Complexity of Life
• How did life begin?
• We don’t know for sure, but we have a
hypothesis of how life began.
– Biogenesis - the idea that macromolecules
were created without oxygen and that bacteria
and other cells evolved from these
macromolecules.
• The early atmosphere contained hydrogen
gas, methane, ammonia, and water
– What is missing that we rely on?
• An experiment has been replicated that
unites these compounds into organic
macromolecules
Miller and Urey’s experiment
• With the right
conditions
simulating lightning
striking the “soup”
of small molecules,
we can recreate
perhaps the origins
of organic
molecules.
Contains amino acids and
nuclein acids
Origin and Complexity of Life
• These proteins and nucleic acids are self
replicating, and could be the precursors to
life
– There are findings and articles about the first
“life” all the time in science magazines- check
out their arguments sometime!
Origin and Complexity of Life
• Once life did exist, what was likely the first
life?
– Bacteria! (Prokaryotes)
• Cyanobacteria can harvest light energy and make
food by photosynthesis
– What waste product do they release?
Origin and Complexity of Life
• Later evolved Eukaryotes
• How did eukaryotes evolve? We talked about this
last semester!
• By ENDOSYMBIOSIS
Origin and Complexity of Life
• The simplest eukaryotes alive today are
Protists. Early eukaryotes were like
protists of today.
• Later still we get simple multicellular
organisms
• Life began in water, and only later was it
adapted to life on land
• From “soup” to “cells”
Patterns of Evolution
• 1. Increasing complexity of cellular life
– the rise of oxygen in the atmosphere
drove some life forms to extinction, while
other life forms evolved new, more
efficient metabolic pathways that use
oxygen for respiation.
Patterns of Evolution
2.
•
•
Extinction- 99% of all species
that have ever lived are now
extinct. In the struggle for
existence, species compete
for resources, and some
lose, and die.
Sudden changes in the
environment or natural
disasters can cause mass
extinctions.
A mass extinction allows for
a new radiation of species to
fill all the empty niches.
• The dodo bird has been extinct
for several hundred years after
humans introduced predators
to their habitat
11.6 Patterns in Evolution
TEKS 7B, 7D, 7E, 7F
Species can become extinct.
• Extinction is the elimination of a species from Earth.
• Background extinctions occur continuously at a very low
rate.
– occurs at roughly the same
rate as speciation
– usually affects a few species
in a small area
– caused by local changes in
environment
11.6 Patterns in Evolution
TEKS 7B, 7D, 7E, 7F
• Mass extinctions are rare but much more intense.
– destroy many species at global level
– thought to be caused by catastrophic events
– at least five mass extinctions in last 600 million years
Mass Extinction
Patterns of Evolution
3. Adaptive Radiation- several vastly
different species arise from a single
species to fill available niches.
Patterns of Evolution
4. Convergent Evolutionunrelated organisms come
to resemble each other
because their adaptations
resemble each other.
•
Penguins are birds,
dolphins are mammals,
and they have modified
structures that are like that
of a fish for swimming!
•
They are structures with
the same functions, but
are not on related animals
are called analogous
structures.
Patterns of Evolution
5. Coevolution- two species evolve along
with each other based on a close
relationship with each other. Plants and
their pollinators, parasites with their
hosts, etc.
11.6 Patterns in Evolution
TEKS 7B, 7D, 7E, 7F
• Divergent evolution describes evolution toward different
traits in closely related species.
kit fox
red fox
ancestor
How do convergent and divergent
evolution illustrate the directional
nature of natural selection?
Patterns of Evolution
6. Punctuated equilibrium- long periods of
time with stable species broken with
rapid period of change.