Download Evolution of Populations

Evolution of Populations:
Georgia Performance Standards:
SB5b: Explain the history of life in terms of
biodiversity, ancestry, and the rates of evolution.
SB5d: Relate natural selection to changes in
organisms.
Essential Questions:
1.
2.
3.
Why is important to understand evolutionary theory?
What is the role of natural selection in speciation?
Why are there species alive now that were not found in the past fossil
record?
4. How does fossil and biochemical evidence support the evolutionary
theory?
Process of Speciation:
• Factors such as natural selection and
chance events can change the relative
frequencies of alleles in a population.
• But how do these changes lead to the
formation of new species, or
speciation?
Process of Speciation:
• A species as a group of organisms that
breed with one another and produce fertile
offspring.
– They share a common gene pool.
– A genetic change that occurs in one individual can
spread through the population as that individual and
its offspring reproduce.
– If a genetic change increases fitness, that allele will
eventually be found in many individuals of that
population.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Mechanisms of Evolution
 Population genetics
 Hardy-Weinberg principle states that when allelic frequencies
remain constant, a population is in genetic equilibrium.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 This equation allows us to determine the
equilibrium frequency of each genotype in
the population.
 Homozygous dominant (p2)
 Heterozygous (2pq)
 Homozygous recessive (q2)
Hardy-Weinberg Equation:
• Used to calculate the frequency of alleles
p2 + 2pq + q2 = 1
• Frequency of WW + Frequency of Ww + Frequency of ww = 1
• The combined frequencies of all alleles
must be 100%
Five conditions are required for HardyWeinberg equilibrium
Evolution v/s Equilibrium
• Five conditions
are required to
maintain genetic
equilibrium from
generation to
generation
1. The population is very
large
2. The population is
isolated
3. Mutations do not alter
the gene pool
4. Mating is random
5. All individuals are
equal in reproductive
success
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Hardy-Weinberg principle
• The Hardy-Weinberg principle states that
allele frequencies in a population will remain
constant unless one or more factors cause those
frequencies to change.
• The situation in which allele frequencies remain
constant is called genetic equilibrium (juh-netik ee-kwih-lib-ree-um).
• If the allele frequencies do not change, the
population will not evolve.
Checkpoint Questions:
1. Describe how natural selection can affect traits
controlled by single genes.
2. Describe three patterns of natural selection on
polygenic traits. Which one leads to two distinct
phenotypes?
3. How does genetic drift lead to a change in a
population’s gene pool?
4. What is the Hardy-Weinberg principle?
5. How are directional selection and disruptive
selection similar? How are they different?
Microevolution
• When the relative frequencies of alleles
in a population change over a number
of generations, evolution is occurring
on its smallest scale (microevolution)
Genes and Variation:
• Genetics, molecular biology, and
evolutionary theory work together to
explain how inheritable variation
appears and how natural selection
operates on that variation
There are several potential
causes of microevolution
• Genetic drift is a change in a gene pool
due to chance
– Genetic drift can cause the bottleneck effect
• Gene flow can change a gene pool due to
the movement of genes into or out of a
population
• Mutation changes alleles
• Nonrandom mating
• Natural selection leads to differential
reproductive success
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Genetic Drift
 A change in the allelic frequencies in a
population that is due to chance
 In smaller populations, the effects of genetic
drift become more pronounced, and the
chance of losing an allele becomes greater.
Genetic Drift
• Natural selection is not the only source of
evolutionary change.
• The smaller a population is, the farther the
results may be from what the laws of
probability predict. This kind of random change
in allele frequency is called genetic drift.
• How does genetic drift take place?
– 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.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Founder Effect
 Occurs when a small sample of a
population settles in a location separated
from the rest of the population
 Alleles that were uncommon in the original
population might be common in the new
population.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Bottleneck
 Occurs when a population declines to a
very low number and then rebounds
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Gene Flow
 Increases genetic variation within a
population and reduces differences between
populations
Nonrandom Mating
 Promotes inbreeding and could lead
to a change in allelic proportions
favoring individuals that are
homozygous for particular traits
Gene Pools:
• All members of a population can interbreed, they
share a common group of genes, called a gene
pool.
– A gene pool is the combined genetic information
of all the members of a particular population.
• Typically contains two or more alleles—or forms of a
certain gene—for each inheritable trait.
– The relative frequency of an allele is the
number of times that allele occurs in a gene pool
compared with the number of times other alleles
occur.
Relative Frequencies of Alleles
Section 16-1
Sample Population
Frequency of Alleles
allele for brown fur
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for black fur
Sources of Genetic Variation
• The two main sources of genetic variation
are mutations and the genetic shuffling
that results from sexual reproduction.
– Sexual reproduction can thus produce many
different phenotypes, but this does not
change the relative frequency of alleles in a
population. (Card deck analogy)
Evolution as Genetic Change
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– Ex: Color Mutations
Single-Gene and Polygenic Traits
• Inheritable variation can be expressed in a
variety of ways.
• The number of phenotypes produced
for a given trait depends on how many
genes control the trait
Single-gene trait
• Trait controlled by a
single gene
• Variation in this gene
leads to only two distinct
phenotypes
• The number of
phenotypes a given trait
has is determined by how
many genes control the
trait.
In humans, having a widow’s
peak or not having a widow’s
peak is controlled by a single
gene with two alleles. As a result,
only two phenotypes are
possible.
Polygenic Traits:
• Most traits are controlled by
two or more genes and are,
therefore, called polygenic
traits.
• Each gene of a polygenic
trait often has two or more
alleles.
• As a result, one polygenic
trait can have many possible
genotypes and even more
possible phenotypes.
EX: height (A bell-shaped curve is
also called a normal distribution)
Checkpoint Questions:
1. What two processes can lead to inherited
variation in populations?
2. How does the range of phenotypes differ
between single-gene traits and polygenic
traits?
3. What is a gene pool? How are allele
frequencies related to gene pools?
4. How could you distinguish between a species
in which there is a lot of variation and two
separate species?
Evolution as Genetic Change
• Natural selection
– does not act
directly on genes,
but on phenotypes.
– affects which
individuals having
different
phenotypes survive
and reproduce and
which do not.
– determines which
alleles are passed
from one
generation to the
next.
– can change the
relative
frequencies of
alleles in a
population over
time.
Exactly what factors change the relative
frequencies of alleles in a population?
• In genetic terms, any factor that causes alleles
to be added to or removed from a population
will change the relative frequencies of alleles.
• Evolution is any change in the relative
frequencies of alleles in a population’s gene
pool.
• Evolution acts on populations, not on
individuals.
Natural Selection on Single-Gene Traits
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– EX: Color Mutations (organisms of one color
may produce fewer offspring than organisms
of another color.
Evolution as Genetic Change
Natural Selection on
Polygenic Traits
• Fitness varies in
polygenic traits.
• Where fitness varies,
natural selection can
act.
• Natural selection
can affect the
distributions of
phenotypes in any
of three ways:
– directional selection
– stabilizing selection
– disruptive selection.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Natural Selection
 Acts to select
the individuals
that are best
adapted for
survival and
reproduction
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Directional selection makes an organism
more fit.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Disruptive selection is a process that splits
a population into two groups.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Sexual selection operates in populations
where males and females differ significantly
in appearance.
 Qualities of sexual attractiveness
appear to be the opposite of qualities
that might enhance survival.
Natural
Selection
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Prezygotic isolation
prevents reproduction
by making fertilization
unlikely.
 Prevents genotypes
from entering a
population’s gene pool
through geographic,
ecological, behavioral,
or other differences
Eastern meadowlark and Western meadowlark
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Postzygotic isolation occurs when fertilization
has occurred but
a hybrid offspring
cannot develop
or reproduce.
 Prevents
offspring survival
or reproduction
Liger
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Allopatric Speciation
 A physical barrier divides one population
into two or more populations.
Abert squirrel
Kaibab
squirrel
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Sympatric Speciation
 A species evolves into a new species
without a physical barrier.
 The ancestor species and the new species
live side by side during the speciation
process.
Isolating Mechanisms
• What happens to a gene pool as one species
evolves into one or more species?
• As new species evolve, populations become
reproductively isolated from each other.
• When the members of two populations
cannot interbreed and produce fertile
offspring, reproductive isolation has
occurred.
– At that point, the populations have separate gene
pools.
Reproductive Isolation
• Develops in a variety of ways:
– behavioral isolation = occurs when two
populations are capable of interbreeding but have
differences in courtship rituals or other types of
behavior.
– geographic isolation = two populations are
separated by geographic barriers such as rivers,
mountains, or bodies of water.
• do not guarantee the formation of new species
– temporal isolation = two or more species
reproduce at different times.
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Section 17-4
Species
that are
Unrelated
form
Related
in
under
under
in
in
Interrelationshiops
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Coevolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation
Go to
Section:
Checkpoint Questions:
1. How is reproductive isolation related to the
formation of new species?
2. What type of isolating mechanism was
important in the formation of different
Galápagos finch species?
3. Explain how behavior can play a role in the
evolution of species.
4. Leopard frogs and tree frogs share the same
habitat. Leopard frogs mate in April; tree frogs
mate in June. How are these species isolated
from each other?