Download Polygenic trait

Evolution of Populations
Ch 16
Where does Variation come from?
•
Mutations
•
•
•
errors in mitosis & meiosis
environmental damage
Meiosis
• mixing of genes through crossing over
and independent assortment.
Natural Selection and Populations
-Single gene trait: controlled by single gene with two alleles
♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling
-Polygenic trait: controlled by 2 or more
genes with 2 or more alleles
♦ Examples: height, hair color, skin color, eye
color.
Natural Selection Affecting Polygenic
Traits
A. Directional Selection: Individuals at one
end of the curve have higher fitness so evolution
causes increase in individuals with that trait
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
♦ Example: Galapagos finches – beak size
Key
Natural Selection Affecting Polygenic
Traits
B. Stabilizing Selection: individuals at the
center of the curve have highest fitness; evolution
keeps center in the same position but narrows the
curve
♦ Example: human
birth weight
Natural Selection Affecting Polygenic
Traits
C. Disruptive Selection: individuals at both
ends of the curve have highest fitness.
♦ Example: birds where seeds are either large or small
Evolution
and
Speciation
How can natural
selection lead to the
formation of new
species???
Speciation
Natural selection and chance events can change the
relative frequencies of alleles (# times an allele occurs
in the gene pool) in a population and lead to
speciation.
•Speciation
•A species
is the formation of new species.
is a group of organisms that breed with one
another and produce fertile offspring.
What Factors are involved in
the formation of New
species??
The gene pool of two populations must
become separated for them to become
new species.
Reproductive Isolation
As new species evolve, populations
become reproductively isolated from each
other.
When members of the two
populations can’t breed with each
other and produce fertile offspring,
reproductive isolation has occured
Isolating Mechanisms
How can reproductive isolation occur?
1)Behavioral Isolation: when two populations are
capable of interbreeding, but have differences in
courtship rituals or other reproductive behaviors.
Ex: Eastern and western meadow larks have
overlapping ranges but don’t breed because they
have a different matting call.
Isolating Mechanisms
How can reproductive isolation occur?
2. Geographic Isolation: two populations are
separated by geographic barriers, like rivers,
mountains, or bodies of water.
Ex: grand canyon squirrels: Colorado river
separated 2 populations 10,000 years ago.
Isolating Mechanisms
How can reproductive isolation occur?
3. Temporal isolation: when two or more species
reproduce at different times (different months or
seasons)
Ex: Northern Leopard Frog, North American Bullfrog
Mates in April
Mates in July
Genetic Drift
•
Genetic Drift: random change in allele
frequency that occurs in small populations
•
Genetic drift can be caused by:
1. Founder Effect
2. Bottleneck Effect
1. Founder Effect
•
When a new population is started by only a small group of
individuals. example: colonization of New World
•
just by chance some rare alleles may be at high
frequency; others may be missing
•
skew the gene pool of new population
2. Bottleneck Effect
•
When a large population is drastically reduced by a
disaster.
•
Famine, natural disaster, loss of habitat.
•
Loss of variation by chance event.
•
Alleles lost NOT
due to fitness!
Testing Natural Selection
& Speciation in Nature
Can natural selection actually be observed in
nature?? YES!
Important studies of natural selection in action
have been done in the Galapagos Islands, with the
descendants of the finches that Darwin first studied
Darwin’s Hypothesis: The finches he saw were
different from each other, but he hypothesized that
they all had a common ancestor
Testing Darwin’s
Hypothesis
Two assumptions of Darwin’s finch hypothesis:
1. For beak size and shape to evolve, there must
be enough heritable variation in those traits
for natural selection to act on them
2. Difference in beak size and shape must
produce differences in fitness, allowing
natural selection to occur
Testing Darwin’s
Hypothesis
The Galapagos
Islands
Researchers from Princeton University--Peter and
Rosemary Grant--tested Darwin’s finch hypothesis
The Grants focused on the Medium Ground Finch on
Daphne Major, one of the Galapagos islands
The Grants’ Observations
During the rainy season plenty of food
During droughts food becomes scarce, & birds
with bigger beaks have better chance of survival
This shows that beak size can be changed by
natural selection
1. A few finches, founders
(species A), travel from S.
America to the Galapagos
Survive and reproduce
2. A group of birds from Species A
travels to another island in the
Galapagos- Geographic Isolation.
The two groups no longer share a
gene pool
Speciation in Galapagos
Finches
3. Seed sizes on the second
island favor birds with bigger
beaks. Species B evolves over
time.
4. If birds from population B
migrate back to the original
island, they will not mate with
birds from population A
Populations A and B are
separate species
Speciation in Galapagos
Finches
5. Ecological Competition: If species A and B
compete for seeds on the first island, they continue to
evolve in ways that increase the difference between
them.
A new species--C-- might evolve
Continued Evolution
The process of isolation,
genetic change, and
reproductive isolation probably
repeated itself often across the
entire Galapagos Island chain.
The End. Answer your review
questions.
1) Name and describe 3
types of selection that
affect polygenic traits.
2) What is speciation?
3) Name and describe 3
types of reproductive
isolation.