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Biology 1 Notes- Chapter 16 (pages 393-410)
Evolution of Populations
16–1 Genes and Variation
A. How Common is Genetic Variation? 16–3
A.
B. Variation and Gene Pools
C. Sources of Genetic Variation
1. Mutations
2. Gene Shuffling
B.
Single-Gene and Polygenic Traits
16–2 Evolution as Genetic Change
A. Natural Selection on Single-Gene Traits
B. Natural Selection on Polygenic Traits
C.
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
C. Genetic Drift
D. Evolution Versus Genetic Equilibrium
1. Random Mating
2. Large Population
D.
3. No Movement Into or Out of the
Population
4. No Mutations
5. No Natural Selection
The Process of Speciation
Isolating Mechanisms
1. Behavioral Isolation
2. Geographic Isolation
3. Temporal Isolation
Testing Natural Selection in Nature
1. Variation
2. Natural Selection
3. Rapid Evolution
Speciation in Darwin’s Finches
1. Founders Arrive
2. Separation of Populations
3. Changes in the Gene Pool
4. Reproductive Isolation
5. Ecological Competition
6. Continued Evolution
Studying Evolution Since Darwin
How Common is Genetic Variation?
• Genetic variation is studied within populations.
• Because members within populations interbreed, they share a
common group of genes called a gene pool.
• A gene pool is made up of all of the genes (including all of the
alleles) that are present in a population.
• The relative frequency of an allele is the number of times that
allele shows up, compared to the number of times other alleles for
that same gene occur.
• In genetic terms, evolution is any change in the relative frequency
of alleles in a population.
• (change from 10-20% allele frequency= evolution)
Sources of Genetic Variations
• Gene pool for a fur color in mice
50 alleles total
20 are B black
48%
heterozygous
black
Frequency of
Alleles
30 b brown
allele for
brown fur
16%
homozygous
black
36%
homozygous
brown
• Two main sources of genetic variation
– Mutations
– Genetic shuffling from sexual reproduction
allele for
black fur
Phenotypes for Single-Gene Trait
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
No widow’s peak
Phenotype
• A single gene trait is one that is controlled by one gene with 2
alleles for that gene. Widows peak is an example of a single gene
trait and the frequency of phenotype is shown in the graph.
Generic Bell Curve for Polygenic Trait
Frequency of Phenotype
• Polygenic traits are traits that are controlled by
2 or more genes.
Phenotype (height)
• The graph shows
there are many
possibilities for
phenotypes.
• Also, most people
have the average
or medium
version of the
phenotype, with a
small number of
individuals at
either extreme.
Evolution as Genetic Change
Natural Selection on Single Gene
Traits
• Natural selection can lead to
changes in allele frequency,
and thus evolution.
Natural Selection on Polygenic Traits
• Some variations increase or decrease an organism’s chance of survival
in an environment.
• Fitness is the ability of an organism to survive and reproduce in its
environment.
• The range of phenotypes of a polygenic trait normally fit a bell curve.
• Natural Selection can affect the distribution of phenotypes.
• There are three different types of natural selection that act on
variations: stabilizing, directional, and disruptive.
Directional Selection
• Directional Selection takes
place when individuals at one
end of the curve have a higher
fitness than those in the middle
or at the other end.
• Natural selection favors one of
the extreme variations of a
trait.
Selection for
longer beaks
Normal
variation
Key
Directional Selection
Food becomes scarce.
High mortality,
Low mortality,
low fitness
high fitness
High mortality,
low fitness
Selection for
average size
spiders
• Stabilizing selection is a
natural selection that favors
average individuals in a
population.
• This keeps the curve in the
same place, but makes it
more narrow.
Normal
variation
Stabilizing
Selection
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Disruptive Selection
Selection for
light limpets
Normal
variation
Selection for
dark limpets
• In disruptive selection, individuals
with either extreme of a trait’s
variation are selected for
(individuals at the ends of the
curve have higher fitness than
individuals near the middle)
• Over time, it is less likely that
species with extreme variations
will mate, therefore giving rise to
new species.
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Drift
• Genetic drift is the change of allelic frequencies by chance events.
• In small populations, individuals that carry a particular allele
may leave more descendants than other individuals, just by
chance.
• Over time, a series of chance occurrences of this type can cause an
allele to become common in a population.
• A situation in which allele frequencies change as a result of the
migration of a small subgroup of a population is known as the
founder effect.
Evolution Versus Genetic Equilibrium
• Genetic Equilibrium is when allele frequencies remain constant
(there is no evolution or change in the population)
• The Hardy –Weinberg Principle states that allele frequencies in a
population will remain constant as long as the following five
conditions are met:
•
•
•
•
•
Random mating- no preference in mate selection
Large population size- so that small changes will not be
significant
No migration- no gene flow: no new alleles brought into
the population
No mutations- no new alleles added to the population
No Natural selection- all organisms are reproductively
successful therefore no genes are favored
• If any of the above are not met, then the population will evolve
The Process of Speciation
• Recall that a species is defined as a group of
organisms that look alike and can interbreed to
produce fertile offspring in nature.
• The evolution of new species is called speciation.
• Reproductive isolation occurs when formerly
interbreeding organisms can no longer mate and
produce fertile offspring.
• There are three types of reproductive isolation:
behavioral, temporal, and geographic.
Types of Reproductive
Isolation
Behavioral Isolation
• Two organisms are capable of interbreeding,
but they have different courtship rituals.
Ex: different mating songs of birds eastern
and western meadowlarks
Temporal Isolation
• Two or more species reproduce at different
times. Ex: Pollen released at different times
from orchids.
Geographic Isolation
• Geographic isolation occurs whenever a
physical barrier divides a population . Ex:
tree frogs
Geographic Isolation
• When geographic isolation divides a population of tree frogs, the individuals no
longer mate across populations.
• Tree frogs are a single population.
• The formation of a river may divide the frogs into two populations.
• Over time, the divided populations may become two species that may no longer
interbreed, even if reunited.
Reproductive Isolation Flowchart
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
Testing Natural Selection in Nature
•
•
Peter and Rosemary Grant have worked for the past twenty
years to show that Darwin’s hypothesis is correct.
They realized that Darwin relied on two assumptions:
1. In order for beak size and shape to evolve in finches, there
must have been many varieties in those traits for natural
selection to work on.
2. Differences in beak size and shape must produce
differences in fitness that cause natural selection to occur.
Variation
• The Grants found that the finches of the Galapagos
Islands had a great variety of heritable traits.
• Many of the characteristics appeared in a bell-shaped
distributions typical of polygenic traits.
Natural Selection
• Individual birds with different sized
beaks had different chances of survival
during a drought. (food, mating)
• The average beak size in that finch
population increased dramatically over
time. Natural Selection
• This change in beak size is an example
of directional selection operating on an
anatomical trait
Rapid Evolution
• The Grants found that
natural selection takes
place frequently—and
sometimes very rapidly.
• Changes in the food
supply caused
measurable fluctuations
in the finch populations
over a period of only
decades.
• This is very different
from the slow, gradual
evolution that Darwin
envisioned.
Speciation in Darwin's Finches
• The Grants' work demonstrates that finch beak size can
be changed by natural selection.
• 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.
• Small groups of finches moved from one island to
another, became reproductively isolated, and evolved into
new species.
Founders Arrive
• A few finches travel from South
America to one of the islands, there they
survive and reproduce.
Separation of populations
• Some birds from species A cross to a
second island. The two populations no
longer share a gene pool.
Changes in the gene pool
• Seed sizes on the second island favor
birds with larger beaks. The population
on the second island evolves into a
population, B, with larger beaks.
Eventually, population A and B will
evolve into a separate species.
Reproductive Isolation
• Even if a few birds from population B move back to the
first island, they will not breed with the birds from
population A.
• The differences in beak size and mating behavior will
lead to reproductive selection.
Ecological Competition
• The two species will compete with each other for seeds
when they live together.
Continued Evolution
• The process of isolation on different islands,
genetic change, and reproductive isolation
will continue to repeat itself.
• Thirteen different species of finches live on
the Galapagos islands today.
Studying Evolution since Darwin
Why is understanding evolution so important?
• Evolution continues today.
• For example, bacteria and viruses are evolving
resistance to drugs.
• Insects are evolving resistance to pesticides.
• Evolutionary theory can help us understand and
respond to these changes in ways that can improve
human life.
There are still many unanswered questions that need to
be addressed.