Chapter 16 Evolution of Populations
I. Genes and Variation
A. 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. Although variation in heritable traits was central to
Darwin’s theory, he had no idea how that variation
appeared.
B. How Common Is Genetic Variation?
1. We now know that many genes have at least two
forms, or alleles.
2. All organisms have additional genetic variation that is
“invisible” because it involves small differences in
biochemical processes.
3. Individual fishes, reptiles, and mammals are typically
heterozygous for between 4 and 8 percent of their genes.
C. Variation and Gene Pools
1. Genetic variation is studied in populations.
A population is a group of individuals of the same
species that interbreed.
a)
Because members of a population interbreed, they
share a common group of genes called a gene pool. A
gene pool consists of all genes, including all the
different alleles, which are present in a population.
b)
2. The relative frequency of an allele is 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.
Relative frequency is often expressed as a
percentage.
a)
Example: in a mouse population the relative
frequency of the dominant B allele (black fur) is
40%, and the relative frequency of the recessive b
allele (brown fur) is 60%.
(1)
The relative frequency of an allele has nothing to
do with whether the allele is dominant or recessive.
b)
3. Gene pools are important to evolutionary theory,
because evolution involves changes in populations over
time.
4. In genetic terms, evolution is any change in the relative
frequency of alleles in a population.
D. Sources of Genetic Variation
1. The two main sources of genetic variation are
mutations and the genetic shuffling that results from
sexual reproduction.
a)
Mutations
(1)
Any change in a sequence of DNA.
Mutations can occur because of mistakes in the
replication of DNA or as a result of radiation or
chemicals in the environment.
(2)
Mutations do not always affect an organism’s
phenotype.
(3)
b)
Gene Shuffling
Most heritable differences are due to gene
shuffling that occurs during the production of
gametes, which is known as crossing-over.
(1)
When alleles are recombined during sexual
reproduction, they can produce dramatically different
phenotypes. Thus, sexual reproduction is a major
source of variation within many populations.
(2)
Sexual reproduction can produce many different
phenotypes, but it does not change the relative
frequency of alleles in a population.
(3)
(a) Example: The probability of drawing an ace off
the top of the deck will always be 4 in 52, or 1/13. No
matter how many times you shuffle the deck, this
probability will remain the same.
E. Single-Gene and Polygenic Traits
1. The number of phenotypes produced for a given trait
depends on how many genes control the trait.
2. Among humans, a widow’s peak is a single-gene trait.
a)
It is controlled by a single gene that has two alleles.
The allele for a widow’s peak is dominant over the
allele for a hairline with no peak.
b)
3. In real populations, phenotypic ratios are determined
by the frequency of alleles in the population as well as by
whether the alleles are in the dominant or recessive form.
4. Allele frequencies may not match Mendelian ratios.
5. Many traits are controlled by two or more genes and
are, therefore, called polygenetic traits.
One polygenetic trait can have many possible
genotypes and phenotypes.
a)
Height in humans is one example of a polygenetic
trait.
b)
II. Evolution as Genetic Change
A. Natural selection never acts directly on genes. Why?
1. Because it is an entire organism—not a single gene—
that either survives and reproduces or dies without
reproducing.
2. Natural selection can only affect which individuals
survive and reproduce and which do not.
3. If an individual dies without reproducing, the individual
does not contribute its alleles to the population’s gene
pool. If an individual produces many offspring, its alleles
stay in the gene pool and may increase in frequency.
B. Natural Selection on Single-Gene Traits
1. Natural selection on single-gene traits can lead to
changes in allele frequencies and thus to evolution.
Imagine that a hypothetical population of lizards is
normally brown, but experiences mutations that
produce red and black forms.
a)
If red lizards are more visible to predators, they
might be less likely to survive and reproduce, and the
allele for red coloring might not become common.
b)
Black lizards, on the other hand, might absorb
more sunlight and warm up faster on cold days. If
high body temperature allows them to move faster to
feed and to avoid predators, they might produce more
offspring than brown forms.
c)
The allele for black color might then increase in
relative frequency.
d)
If a color change has no effect on fitness, the allele
that produces it would not be under pressure from
natural selection.
e)
C. Natural Selection on Polygenic Traits
1. Natural selection can affect the distributions of
phenotypes in any of three ways.
a)
Directional selection
When individuals at one end of the curve have
higher fitness than individuals in the middle or at the
other end, directional selection takes place.
(1)
The range of phenotypes shifts as some
individuals fail to survive and reproduce while others
succeed.
(2)
Example: Beak size increasing because seeds
production changes.
(3)
b)
Stabilizing selection
When individuals near the center of the curve
have higher fitness than individuals at either end of
the curve, stabilizing selection takes place.
(1)
This situation keeps the center of the curve at its
current position but it narrows the overall graph.
(2)
(3)
c)
Example: Human birth weight
Disruptive selection
When individuals at the upper and lower ends of
the curve have higher fitness than individuals near
the middle, disruptive selection takes place.
(1)
In such situations, selection acts most strongly
against individuals of an intermediate type.
(2)
If the pressure of natural selection is strong
enough and lasts long enough, this situation can
cause the single curve to split into two.
(3)
In other words, selection creates two distinct
phenotypes.
(4)
(5)
Example: birds with medium-sized beaks
D. Genetic Drift
1. Natural selection is not the only source of evolutionary
change.
2. In small populations, an allele can become more or
less common simply by chance.
3. 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.
4. In small populations, individuals that carry a particular
allele may leave more descendants than other individuals,
just by chance. Over time, a series of change
occurrences of this type can cause an allele to become
common in a population.
5. Genetic drift may occur when a small group of
individuals colonizes a new habitat. These individuals
may carry alleles in different relative frequencies than did
the larger population that they came from.
6. A situation in which allele frequencies changes as a
result of the migration of a small subgroup of a
population is known as the fonder effect.
E. Evolution Versus Genetic Equilibrium
1. Scientists often find it helpful to determine what
happens when no change takes place.
2. The Hardy-Weinberg principle states that allele
frequencies in a population will remain constant unless
one ore more factors causes those frequencies to
change.
3. The situation in which allele frequencies remain
constant is called genetic equilibrium.
4. If the allele frequencies do not change, the population
will not evolve.
5. Under what conditions does the Hardy-Weinberg
principle hold?
Five conditions are required to maintain genetic
equilibrium from generation to generation.
a)
There must be random mating – All members of
the population must have an equal opportunity to
produce offspring. In natural populations, however,
mating is rarely completely random.
(1)
(2)
The population must be very large
There can be no movement into or out of the
population
(3)
(4)
No mutations
(5)
No natural selection
III. The Process of Speciation
A. 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?
B. Isolating Mechanisms
1. As new species evolve, populations become
reproductively isolated from each other.
2. 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.
3. Behavioral Isolation
One type of isolation mechanism, behavioral
isolation, occurs when two populations are capable of
interbreeding but have differences in courtship rituals
or other reproductive strategies that involve behavior.
a)
b)
Example: Birds of paradise
4. Geographic Isolation
With geographic isolation, two populations are
separated by geographic barriers such as rivers,
mountains, or bodies of water.
a)
b)
Example: Abert and Kaibab Squirrel
Geographic barriers do not guarantee the
formation of new species. If two formerly separated
populations can still interbreed, they remain a single
species.
c)
5. Temporal Isolation
The third isolating mechanism is temporal
isolation, in which two or more species reproduce at
different times.
a)
Example: 3 similar species of orchid all live in the
same rain forest, each only release pollen one day, but
they all release on different days, which means they
cannot pollinate each other.
b)
C. Testing Natural Selection in Nature
1. All three basic mechanisms of evolutionary change
can be seen in nature.
2. Darwin had a reasonable hypothesis about his varied
finches but had no way of testing it.
3. The work of Peter and Rosemary Grant form Princeton
University realized that Darwin’s hypothesis relied on two
testable assumptions.
First, in order for beak size and shape to evolve,
there must be enough heritable variation to provoke
natural selection.
a)
Second, differences in beak size and shape must
produce differences in fitness that cause natural
selection to occur.
b)
The Grants tested these hypotheses on the medium
ground finch on Daphne Major.
c)
4. Variation
The Grants first identified and measured as many
individual birds as possible on the island, noting all
their different characteristics.
a)
5. Natural Selection
During dry-season drought made some foods
become scarce, and other disappear altogether. To
survive, birds become feeding specialists.
a)
The Grants’ most interesting discovery was that
individual birds with different-sized beaks had
different changes of survival during a drought.
b)
Changes in the food supply on the Galapagos
caused measurable fluctuations in the finch
populations over a period of only decades.
c)
D. Speciation in Darwin’s Finches
1. Speciation in the Galapagos finches occurred by
founding of a new population, geographic isolation,
changes in the new population’s gene pool, reproductive
isolation, and ecological competition.
2. Founders Arrive
Many years ago a few finches from the South
American mainland flew or were blown to one of the
Galapagos Islands and managed to survive and
reproduce.
a)
3. Geographic Isolation
Later some of these finches crossed to another
island in the Galapagos group and essentially became
isolated from the original group.
a)
4. Changes in the Gene Pool
Over time populations on each island became
adapted to their local environments. Natural
selection acted on the two groups of birds differently
creating a change in the gene pool for each
population.
a)
5. Reproductive Isolation
Finches prefer to mate with finches that have a
similar beak to theirs. With the two populations
having different sized beaks they will likely not
interbreed.
a)
6. Ecological Competition
If the birds begin to live in the same habitat again
they will have to compete for seeds. The more
specialized birds will often have less competition.
a)
7. Continued Evolution
All of these factors combined continue to lead to
more changes between the two populations which
continue evolution to the point of speciation.
a)
E. Studying Evolution Since Darwin
1. It is useful to review and critique the strengths and
weaknesses of evolutionary theory.
2. Limitations of Research
The Grants’ research clearly shows the effects of
directional selection in nature. Their data also show
how competition and climate changes affect natural
selection.
a)
The work does have limitations though; they did
not observe the formation of a new species.
b)
3. Unanswered Questions
No scientist suggests that all evolutionary processes
are fully understood. Many unanswered questions
remain.
a)
Because evolution continues today, driving changes
in the living world such as drug resistance in bacteria
and viruses, and pesticide resistance in insects.
Evolutionary theory helps us understand and respond
to these changes in ways that improve human life.
b)