Download Chapter 23 EVOLUTION AND GENETIC VARIATION

EVOLUTION AND
VARIATION
Dr. Uche Amaefuna
16-1 Genes and Variation
• As Darwin developed his theory
of evolution, he worked under a
serious handicap
• He didn’t know how heredity
worked
• This lack of knowledge left two
big gaps in Darwin’s thinking
1. He had no idea how heritable traits pass
from one generation to the next
2. He had no idea how variation appeared,
even though variation in heritable traits
was central to Darwin’s theory
• During the 1930’s Evolutionary biologists
connected Mendel’s work to Darwin’s
• By then biologists understood that genes
control heritable traits
How Common Is Genetic
Variation?
• Many genes have at least 2 forms
or alleles
• Animals such as horses, dogs,
mice, and humans often have
several alleles for traits such as
body size or coat color
Variation and Gene Pools
• Genetic variation is studied in
populations
Population
• A group of individuals of the same
species that interbreed
• Because members of a
population interbreed, they share
a common group of genes called
a gene pool
Gene pool
• All the genes including all the
different alleles, that are present
in a population
Relative Frequency
• The number of times that the
allele occurs in a gene pool,
compared with the number of
times other alleles for the same
gene occur
• In genetic terms, evolution is any
change in the relative frequency
of alleles in a population
Sources of Genetic
Variation
• The 2 main sources of genetic
variation are mutations and the
genetic shuffling that results from
sexual reproduction
Mutations
•
•
•
•
Any change in a sequence of DNA
Mutations can occur because of
Mistakes in DNA replication
Radiation or chemicals in the
environment
• Some mutations don’t affect the
phenotype but some do
Gene shuffling during
sexual reproduction
• Mutations are not the only source of variation
• Most heritable differences are due to gene
shuffling that occurs during the production of
gametes
• The 23 pairs of chromosomes can produce 8.4
milliondifferent combinations of genes
• Crossing over further increases the number of
different genotypes that can also appear in
offspring
Single – Gene and
Polygenic Traits
• The number of phenotypes
produced for a given trait
depends on how many genes
control the trait
Single – gene trait
• Controlled by a single gene that
has two alleles
• Variation in these genes leads to
only 2 distinct phenotypes
Polygenic traits
• Traits controlled by two or more
genes
• Each gene of a polygenic trait has
two or more alleles
• As a result one polygenic trait can
have many possible genotypes and
phenotypes
Ex.) height
Polygenic traits
16-2 Evolution as Genetic
Change
• A genetic view of evolution offers a new way to
look at key evolutionary concepts
• If each time an organism reproduces, it passes
copies of its genes to its offspring…
• We can therefore view evolutionary fitness as
an organism’s success in passing genes to the
next generation
• We can also view an evolutionary adaptation as
any genetically controlled physiological,
anatomical, or behavioral trait that increases an
individuals ability to pass along its genes
Evolution as Genetic
Change
• Remember that evolution is any
change over time in the relative
frequency of alleles in a population.
• This reminds us that it is
populations, not individual
organisms that can evolve overtime
Natural Selection on
Single – Gene Traits
• Natural selection on single gene
traits can lead to changes in allele
frequencies and thus to evolution
Natural Selection on
Polygenic Traits
• Natural selection can affect the
distributions of phenotypes in any
of three ways
1. Directional Selection
• When individuals at one end of
the curve have higher fitness than
individuals in the middle or at the
other end
2. Stabilizing Selection
• When individuals near the center
of the curve have higher fitness
than individuals at either end of
the curve
3. Disruptive Selection
• When individuals at the upper and
lower ends of the curve have
higher fitness than individuals
near the middle
• Can create 2 distinct phenotypes
Genetic Drift
• Natural Selection is not the only
source of evolutionary change
• In small populations, an allele can
become more or less common by
chance
Genetic Drift
• A random change in allele frequency
Genetic Drift
• These individuals may carry alleles in
different relative frequencies than did
the larger population from which they
came
• If so, the population that they found
will be genetically different from the
parent population
• This cause is not natural selection,
but chance
Founder effect
• A situation in which allele
frequency changes as a result of
the migration of a small subgroup
of a population
Evolution vs. Genetic
Equilibrium
• To clarify how evolutionary
change operates, scientists often
find it helpful to determine what
happens when no change takes
place
Hardy – Weinberg principle
• States that allele frequency in a
population will remain constant
unless one or more factors cause
these frequencies to change
Genetic equilibrium
• The situation in which allele
frequencies remain constant
5 conditions are required to
maintain genetic equilibrium
1. There must be random mating
2. The population must be very
large
3. There can be no movement into
or out of the population
4. No mutations
5. No natural selections
16-3 The 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
speciation?
Speciation
• The formation of new species
Isolating Mechanisms
• Since members of the same species
share a common gene pool, in order
for a species to evolve into 2 new
species, the gene pools must be
separated into 2
• As new species evolve, populations
become reproductively isolated from
each other
Reproductive isolation
• When the members of 2
populations cannot interbreed and
produce fertile offspring
Behavioral Isolation
• When two populations are
capable of interbreeding but have
differences in courtship rituals or
other reproductive strategies
Eastern & Western Meadowlark
Geographical Isolation
• When two populations are
separated by geographic barriers
such as rivers, mountains, or
bodies of water
Albert & Kaibab Squirrels
Temporal Isolation
• When 2 or more species reproduce
at different times
Rana aurora - breeds January March
Rana boylii - breeds late March May
Testing Natural Selection in Nature
Q: Can evolution be observed in nature?
A: YES
The Grants
Testing Natural Selection in Nature
• Darwin hypothesized that finches had
descended from a common ancestor
and overtime, natural selection
shaped the beaks of different bird
populations as they adapted to eat
different foods
• The Grants, realized that Darwin’s
hypothesis relied on two testable
assumptions
1. There must be enough heritable
variation in these traits to
provide raw materials for natural
selection
2. Differences in beak size and
shape must produce differences
in fitness that cause natural
selection to occur
Variation
• The Grants identified and
measured every variable
characteristic of the birds on the
island
• Their data indicated that there is a
great variation of heritable traits
among the Galapagos finches
Natural Selection
• During the…
• Rainy season – enough food for
everyone, no competition
• Dry season – some foods become
scarce
• At that time, differences in beak sizes
can mean the difference between life
and death
• Birds become feeding specialists
Natural Selection
• The Grants discovered that individual
birds with different size beaks had
different chances of survival during a
drought
Speciation in Darwin’s
Finches
• Speciation in the Galapagos finches
occurred by founding of a new
population, geographical isolation,
changes in the new population’s gene
pool, reproductive isolation and
ecological competition
Founders Arrive
• Many years ago, a
few finches from
South American
mainland
• Species A, flew or
were blown to one
of the Galapagos
Islands
Geographic Isolation
• Later on, some birds from
species A crossed to
another island in the
Galapagos group
• The finches then became
unable to fly from island
to island and become
isolated from each other
and no longer share a
common gene pool
Changes in the Gene Pool
• Overtime,
populations on each
island became
adapted to their
local environments
Reproductive Isolation
• Now imagine that a
few birds from the
second island cross
back to the first island
• Q: Will the population
A birds, breed with the
population B birds?
• A: Probably not
Ecological Competition
• As these two new
species live together
in the same
environment, they
compete with each
other for available
seeds
• The more different
birds are, the higher
fitness they have, due
to less compitition
Continued Evolution
• This process of isolation
on different islands,
genetic change, and
reproductive isolation
probably repeated itself
time and time again
across the entire
Galapagos island chain
• Over many generations, it
produced the 13 different
finch species found there
today
Studying Evolution Since
Darwin
• It is useful to review and critique the strength
and weakness of evolutionary theory
• Darwin made bold assumptions about heritable
variation, the age of the Earth, and the
relationships among organisms
• New data from genetics, physics, and
biochemistry could have proved him wrong on
many counts, and they did not
• Scientific evidence supports the theory that living
species descended with modification from
common ancestors that lived in the past
Limitations of Research
• The Grants data shows how
competition and climate change
affects natural selection
• However, they did not observe
the formation of a new species
Unanswered Questions
• Many new discoveries have led to
new hypotheses that refine and
expand Darwin’s original ideas
• No scientist suggests that all
evolutionary processes are fully
understood. Many unanswered
questions remain
Why Understanding Evolution is
Important?
• Evolution continues today
Example:
• Drug resistance in bacteria and viruses
• Pesticide resistance in insects
• Evolutionary theory helps us understand
and respond to these changes in ways
that improve human life