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POPULATION EVOLUTION
INDIVIDUALS DON’T EVOLVE, POPULATIONS DO.

As Charles Darwin and Aflred
Wallace perceived long ago,
individuals don’t evolve,
populations do.

A population is a group of
individuals of the same species in
a specified area.

To under stand how it evolves, start
with variations in the traits that
characterizes it.
VARIATION IN POPULATIONS
•
Individuals in a population have
traits in common.
–
Morphological traits
•
–
Physiological traits
•
–
Features like two feathered wings,
or five forward-facing toes.
Metabolic activities
Behavioral traits
•
Respond to stimuli
– Individuals also have variation in
the details of their shared
traits.
VARIATIONS IN POPULATIONS

For sexually reproducing species,
a population is a group of
individuals that are interbreeding,
reproductively isolated from
other species, and produce fertile
offspring.

The offspring typically have two
parents, and they have mixes of
parental forms of traits.
VARIATION IN POPULATIONS
•
Dimorphism
The persistence of two forms of a trait in a population.
– For example: humans have sexual dimorphism in which males and females have
specific characteristics, and can (usually) be told apart easily.
–
•
Polymorphism
The persistence of three or more forms of a trait.
– For example: humans have polymorphism in skin color.
–
THE GENE POOL

Genes encode information about
heritable traits.

The individuals of a population
inherit the same number and kind
of genes (except for their
gametes).

Together, they and their offspring
represent a gene pool—a source of
genetic resources.
THE GENE POOL—SOURCES OF VARIATION

For sexual reproducers, nearly all
genes available in the shared pool
have two or more slightly
different molecular forms, or
alleles.

Any individual might or might not
inherit alleles for any trait.
This is the source of variations in
phenotype.
 Mutation also leads to new alleles.
 New alleles may come as a result of
genetic recombination during sexual
reproduction.

MUTATION REVISITED

Mutations are a major source of
variation in populations.
Most mutations are neutral and
have no effect on the individual.
 Some mutations lead to death.



These lethal mutations will not be
passed onto future generations.
Every so often, a beneficial
mutation will occur, and if it gives
an organism an advantage, may
increase their fitness.

This is an adaptation.
STABILITY AND CHANGE IN ALLELE FREQUENCIES

Researchers typically track allele
frequencies, or the relative
abundance of alleles of a given gene
among all individuals of a population.

For example: Count the number of
students with red hair, blond hair,
brown hair, and black hair in class.

If there are 40 students in class, and 2 of
them have red hair, 10 have blond hair, 25
have brown hair, and 3 have black hair,
then the allele frequencies for each hair
color will be:
 5% red hair
 25% blond hair
 62.5% brown hair
 7.5 % black hair
STABILITY AND CHANGE IN ALLELE FREQUENCIES

Microevolution refers to small-scale changes in allele
frequencies that arise as an outcome of mutation, natural
selection, genetic drift or gene flow, or some combination of
these.

An example of microevolution would be…

Zombies are taking over the world! To zombies, the brains of brunettes
and blonds are the tastiest, so they eat them first. They will eat the
brains of people with black hair occasionally, but only eat the brains of
red-heads when they absolutely have to. (Environmental pressure).
 As a result, the allele frequencies in this population will change.
 Having red hair is now an adaptation! Those with red hair will be the
most fit in the changed environment, and will have the most
opportunities to have children.
 If you were to take a survey of a 40-person classroom, the allele
frequencies would be…
 85% red hair
 2% blond hair
 3% brown hair
 10% black hair
Mmmm….blonds….
WHEN IS A POPULATION NOT EVOLVING?

When a population stops evolving, they are under
genetic equilibrium.

Genetic equilibrium is exceedingly rare in nature, and
can lead to the extinction of a species.

Genetic Equilibrium can only be achieved if five conditions
are met.
 1. There is no mutation.
 2. The population is infinitely large.
 3. The population is isolated from all other populations
of the species (no gene flow).
 4. Mating is random.
 5. All individuals survive and produce the same number
of offspring.
 This scenario can never truly happen in nature, and
is called the Hardy-Weinburg Formula.
THE HARDY-WEINBURG FORMULA

When scientists are studying a population undergoing
change, they may utilize the Hardy-Weinburg formula to
attempt to discern the reason behind that change.

Remember, the 5 conditions of equilibrium are:
 1. There is no mutation.
 2. The population is infinitely large.
 3. The population is isolated from all other populations of the
species (no gene flow).
 4. Mating is random.
 5. All individuals survive and produce the same number of
offspring.
 If a scientist notices a change in allele frequencies, she
can ask…are there new mutations? Is the population
getting smaller? Is the population interbreeding with a
new population? Are some mates more desirable than
others? Are offspring dying at different rates or are
more offspring being produced by some individuals than
others?
 The answers to these questions may give the scientist
an idea about why allele frequencies are changing.
NATURAL SELECTION REVISITED

Natural selection is the most influential process in microevolution.

Natural selection can cause populations to undergo…
Directional selection
 Stabilizing selection
 Disruptive selection

DIRECTIONAL SELECTION

In directional selection, allele
frequencies shift in a consistent
direction, so forms at one end of a
phenotypic range become more
common than mid-range forms.
Suddenly, the environment runs out of the dragons
favorite food, hot dogs, and the smaller dragons, who
need to eat less, become more common than the large
dragons, who need more.
For example: Dragons come in a variety of
sizes, as evidenced by this bell curve.
Suddenly, people develop a taste for dragon and begin to
hunt them. Small dragons are easier to catch and eat, and
so the larger dragons become more common.
Both of these scenarios are examples of directional selection
THE PEPPERED MOTH

Populations of peppered moths are a
classic example of directional selection.

The moths feed and mate at night and rest
motionless on trees during the day. Their
behavior and coloration (mottled gray to
nearly black) camouflage them from dayflying, moth-eating birds.
In the 1850’s, the industrial revolution started
in England, and factory smoke altered
conditions in much of the countryside.
 Before then, light moths were the most
common form, and a dark form was rare.
 Also, light-gray speckled lichens had grown
thickly on tree trunks. Light moths but not dark
moths that rested on the lichens were
camouflaged.

THE PEPPERED MOTH

Lichens are sensitive to air
pollution. Between 1848-1898,
soot an other pollutants started
to kill the lichens and darken the
tree trunks.

The dark moth form was better
camouflaged.

The dark moths became the most
common form of moth until 1952
when pollution controls allowed
lichens to make a comeback, and the
light colored moths, once more,
became more common.
 Researchers hypothesized: If
the original conditions favored
light moths, then the changed
conditions favored dark ones.
POCKET MICE

In the Sonoran Desert (that’s here!) there
are two main colors of pocket mice: tawny
and black.
Rock pocket mice are small mammals that
spend the day in underground burrows and
forage for seeds at night. Those who live in
tawny-colored outcroppings of granite,
are…well…tawny colored!
 Those who live in the dark basalt of ancient
lava flows (but the same area and same
species) tend to be black.


We can expect that night-flying predatory birds are
selective agents that affect fur color. This placed
selective pressure on the Rock Pocket Mice living
in the two separate environments causing the
allele frequencies to change.
RESISTANCE TO PESTICIDES AND ANTIBIOTICS

Pesticides can cause directional selection.

Typically a heritable aspect of body form,
physiology, or behavior helps a few individuals
to survive the first pesticide doses.


As the most resistant ones are favored,
resistance become more common.
 There are 450 species of pests that are now
resistant to one or more types of pesticides—
including bed bugs! Ewww.
Antibiotics can also cause directional
selection.

Antibiotics are used to fight pathogenic
bacteria, and have been notoriously
overprescribed, overused, and not used
correctly.

This has lead to the evolution of “super bugs”! We
will talk more on those later!
Bed bugs were virtually wiped out in
America in the 1930’s thanks to the
pesticide DDT.
Research into recent infestations in large
cities show that bed bugs are now
resistant to many of the traditional
pesticides used to the control them.
STABILIZING SELECTION

With stabilizing selection,
intermediate forms of a trait in a
population are favored, and
extreme forms are not.



Small baby dragons aren’t strong enough to
compete with their siblings to get food and
they die.
Large baby dragons need too much food to
survive, and they die.
This mode of selection can
counter mutation, genetic drift,
and gene flow.

For example: Baby dragons come in
all sizes, as shown in this bell
curve.

This is stabilizing selection.
SOCIABLE WEAVERS

Between 1993-2000, scientists captured,
measured, tagged, released, and
recaptured 70 to 100 percent of the birds
living in communal nests during the
breeding season. (Can we say alien
abduction?!)
Their field studies supported a prediction
that body mass is a trade-off between risks
of starvation and predation.
 Intermediate-mass birds have the selective
advantage.

Foraging is not easy in this habitat, and lean
birds do not store enough fat to avoid starvation.
 The largest birds are more attractive to
predators and not as good at escaping.

Social weaver nest (above) and birds (below).
DISRUPTIVE SELECTION

With disruptive selection, forms
at both ends of the range of
variation are favored, and
intermediate forms are selected
against.



Small dragons are able to find food in the
smallest rock crags, and large dragons are
able to hunt big game for food.
Medium sized dragons compete
aggressively with other predators to get
food, and die more frequently.
For example: Dragons come in a
variety of sizes as evidenced by
this bell curve.

This is disruptive selection.
BLACK-BELLIED SEED CRACKER

The black-bellied seed crackers of Cameroon
come in two sizes and two sizes only—large
billed or small billed, with nothing in between.

Factors that affect feeding performance are the
key. Cameroon’s swamp forests flood in the wet
season; lightning-sparked fires burn in the hot,
dry season. Most plants are fire-resistant,
grasslike sedges. One species produces hard
seeds and the other, soft seeds.

All Camaroon seedcrackers prefer soft seeds, but
birds with large bills are better at cracking hard
ones.
 In the dry season, the birds compete fiercely for
scare seeds.
 Birds with intermediate sizes are being selected
against, and now all bills are either 12 or 15 mm
wide.
12 mm beak size
15 mm beak size
SEXUAL SELECTION

The individuals of many sexually
reproducing species show a distinct male
or female phenotype, or sexual
dimorphism.
Often the males are larger and flashier than
females.
 Courtship rituals and male aggression are
common.


These adaptations and behaviors seem puzzling.
All take energy and time away from an
individual’s survival activities. Why do they
persist if they do not contribute directly to
survival? The answer is sexual selection.
 By this mode of natural selection, winners
are the ones that are better at attracting
mates and successfully reproducing
compared to others of the population.
Sexy and I know it!
SEXUAL SELECTION

By choosing mates, a male or
female is a selective agent acting
on its own species.

The selected males and females
pass on their alleles to the next
generation.

Flashy body parts and behaviors are
often observable signs of health and
vigor. Such traits may improve the
odds of producing more healthy,
vigorous offspring.
GENETIC DRIFT—THE CHANCE CHANGES

Genetic drift is a random change
in allele frequencies over time,
brought about by chance alone.

There are two types of genetic
drift, the bottleneck effect and
the founder effect.
The Black Plague would have caused genetic drift in the
European countries that it greatly affected.
BOTTLENECKS

Bottlenecks occur when there is a drastic
reduction in population size is brought on by
extreme pressure.

Suppose that contagious disease, habitat loss, or
hunting nearly wipes out a population.


Even if moderate numbers of individuals survive a
bottleneck, allele frequencies will have been altered at
random.
In the 1890’s, hunters killed all but twenty of a
large population of northern elephant seals.

Government restrictions allowed the population to
recover to about 130,000 individuals.
 Each is homozygous for all the genes analyzed so
far! (No genetic variation).
THE FOUNDER EFFECT

The founder effect is when
unpredictable genetic shifts
occur after a few individuals
establish a new population.

Genetic diversity might be greatly
reduced relative to the original
gene pool,

For example: If the people on Lost
were stuck on a real island (rather
than a metaphorical one), and
decided to continue a population,
that population would be incredibly
good-looking as compared with the
population in general.
INBRED POPULATIONS

Inbreeding is nonrandom mating among
very close relatives, which share many
identical alleles.

It leads to the homozygous condition, and can
lower fitness if harmful recessive alleles are
increasing in frequency.

For example: The Old Order Amish in
Pennsylvania are moderately inbred, and they
have a high frequency of a recessive allele that
causes Ellis-Van Creveld syndrome, in which
individuals have extra fingers, toes, or both.
 The allele might have been rare when a few
founders entered Pennsylvania, but now 1 in
8 individuals are heterozygous for the allele,
and 1 in 200 are homozygous for it.
GENE FLOW

Individuals of the same species don’t always stay put.
A population loses alleles when an individual leaves it for
good—emigration.
 A popualtion gains alleles when an individual permanently
moves in—immigration.


In both cases, gene flow—the physical movement of alleles
into and out of a population—occurs.
 For example: According to Y-chromosome data, there may
be as many as 16 million men in the world today who can
claim to be descendants of Genghis Khan.
 Genghis Khan slaughtered the populations of the cities and
nations he took over, and kept the prettiest girls for
himself. His son, Tushi, got the next pick.
 His grandson, Kubilai Khan, established the Yuan Dynasty,
and supposedly added 30 virgins to his harem every year
of his reign.
 Now that’s gene flow!
Genghis Khan founded the Mongol
Empire, the largest contiguous empire
in history.
He united the tribes of northeast Asia
and his empire extended through
Central Asia and China.