Transcript Populations PP

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.
VARIATION IN POPULATIONS
•
Individuals in a
population have traits in
common.
– Morphological traits
– Physiological traits
– Behavioral 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.
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
 Genetic variation makes
up a population’s gene
pool—the combined
alleles (traits) of all the
individuals in a
population.
THE GENE POOL—SOURCES OF VARIATION
 Any individual might or
might not inherit alleles
for any trait.
Mutation also leads to new
alleles.
 New alleles may come as a
result of genetic
recombination during sexual
reproduction.

MUTATION

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..
 Every so often, a beneficial
mutation will occur, and if it
gives an organism an
advantage, may increase
their fitness (adaptation).
STABILITY AND CHANGE IN ALLELE FREQUENCIES
 Researchers typically track
allele frequencies.
 An allele frequency is a measure
of how common a certain allele
is in the population.
 For example: Count the
number of students with red
hair, blond hair, brown hair,
and black hair in class.
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.
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.

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.

THE HARDY-WEINBURG FORMULA

When scientists are studying a population
undergoing change, They may utilize the HardyWeinburg 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.
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.
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 day-flying,
moth-eating birds.
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.
POCKET MICE
 In the Sonoran
Desert 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.
RESISTANCE TO PESTICIDES AND ANTIBIOTICS
 Pesticides can cause directional
selection.

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.
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.
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.
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.
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.

Cameroon’s swamp forests flood in the
wet season; lightning-sparked fires burn in
the hot, dry season. Most plants are fireresistant, grasslike sedges. One species
produces hard seeds and the other, soft
seeds.
12 mm beak size
15 mm beak size
SEXUAL SELECTION

The individuals of many sexually
reproducing species show a distinct
sexual dimorphism.
Often the males are larger and flashier than
females.
 Courtship rituals and male aggression are
common.


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.
http://www.youtube.com/watch?v=nS
1tEnfkk6M
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.
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.

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,
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.
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 population 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.