Transcript Mechanisms of Evolution

Warm-up
• What is evolution?
• What is heredity?
Mechanisms of
Evolution
What do we know?
• Evolution is:
A change in species over a very long
period of time
• Heredity is:
Passing of alleles through generations of
organisms
How are Evolution and Alleles Related?
Changes in genes (mutations) produce
variations that can be passed on to offspring.
Natural selection works on these variations.
•Genetic variation is studied in populations.
•Because members of populations reproduce
together (interbreed), they share a common
group of genes called a gene pool.
Relative frequency of alleles:
The number of times a particular allele
occurs in a gene pool
• In a population of 100 slugs, there are 50
yellow slugs (YY), 35 yellow-brown (YB),
and 15 brown slugs (BB).
Remember: in a population of 100 slugs, there are:
50 yellows (YY), 35 yellowish-browns (YB), and 15 browns (BB).
• To figure out relative frequencies, we must see
how many times the alleles for Y and B occur in
our population.
• There are 100 slugs. Each has two alleles for
color. That means there are 200 alleles total.
• For Y, there are 50+50+35= 135 Y alleles.
• Divide 135/200=.675 x100 = 67.5%!
• Do the same thing for B. You should get…
35+15+15 = 65 B alleles
65/200=.325 x 100 = 32.5%!
• So, we have a relative frequency of 67.5% for the
Y allele and 32.5% for the B allele.
• Let’s say that a species of frog started to eat these
slugs. The yellow ones were much easier for the
frogs to see! What might happen?
• If the relative frequencies change, EVOLUTION
has occurred!
What are some sources of genetic variation?
• A mutation is any change in the sequence of DNA.
Mutations may or may not have an effect on the
organism’s phenotype. If there is a change in
phenotype, it could have a negative or positive effect
on that organism’s fitness.
• Independent Assortment describes how the 23
chromosomes from your mother and 23 chromosomes
from your father came together in a unique way
during meiosis.
– The biological selection of a particular allele for one trait
has nothing to do with the selection of an allele for another
trait
Three types of natural selection:
• Stabilizing selection
• Disruptive selection
• Directional selection
Stabilizing Selection
Average traits are favored; Genetic Diversity
decreases as population stabilizes
Example: Human Birth Weight
Babies that are too small
tend to lose too much heat
and have more illnesses.
Average weight babies
(around 8 lbs.) tend to be
the healthiest
Babies that are too
large have a hard
time fitting through
the birth canal.
Disruptive Selection
Favors traits at both extremes
Ex: Darwin’s finches in the Galapagos
Because there were different seed sizes
available on the different islands, birds
with bigger beaks ate the big seeds and
birds with smaller beaks ate the small
seeds.
Medium beaks had difficulty retrieving
small seeds and were also not tough
enough for the bigger seeds, and were
hence maladaptive.
Directional Selection
Allele frequency shifts in one direction
Example: Peppered moths
Peppered moths, found in
England, were originally
light colored and hid on
light colored trees.
Then came the
industrial revolution,
covering England
with soot…
So, the darker colored
moths were selected for,
since now they blended in
with the soot-covered
trees!
Once more…
Genetic Drift
Random changes in allele frequency
What could cause genetic drift?
• Population Bottleneck: When a population’s
size decreases dramatically in a short
period of time (due to environmental onetime disaster—hard frost, forest fire, flood,
etc.)
What could cause genetic drift?
• Founder Effect: when a small group in
a population splinters off from the
original population and forms a new
one.
Founder effect in Amish
populations...
Ellis-van Creveld Syndrome
Warm-up
• Get out your guided notes from Friday.
• On the very last line change pq to 2pq
• EVOLUTION TEST THIS FRIDAY 3/8
What did G. H. Hardy say?
Allele frequencies will stay the same from
generation to generation IF:
• There are no mutations
• Mating is random
• Populations are infinitely large
• There is no selection
*This is the Hardy-Weinberg Principle!
Hardy-Weinberg Equation
• A = dominant allele; a = recessive allele
p = (freq)A
q = (freq)a
So p + q = 1
• If the population is in EQUILIBRIUM:
– p2=(freq)AA (homozygous dominant)
– q2=(freq)aa (homozygous recessive)
– 2pq=(freq) Aa (heterozygous)
So p2 + 2pq + q2 = 1
p + q = 1 AND p2 + 2pq + q2 = 1
• You have sampled a population in which
you know that the percentage of the
homozygous recessive genotype (aa) is
36%. Using that knowledge, calculate the
following:
a.
b.
c.
d.
Frequency of “AA” genotype
Frequency of “a” allele
Frequency of “A” allele
Frequencies of genotype “Aa”
p + q = 1 AND p2 + 2pq + q2 = 1
• You have sampled a population in which
you know that the percentage of the
homozygous recessive genotype (aa) is
36%. Using that knowledge, calculate the
following:
a.
b.
c.
d.
Frequency of “AA” genotype = 0.16 (16%)
Frequency of “a” allele = 0.6 (60%)
Frequency of “A” allele = 0.4 (40%)
Frequencies of genotype “Aa” = 0.48 (48%)
Mechanisms of
Evolution
part deux
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Lots of things could cause allele
frequencies to change! These are
things that Hardy-Weinberg does not
take into account.
Mutations
Individuals moving into the population
Individuals moving out of the population
Disease
Predation
What else???
Speciation:
The formation of new species
How does it happen?
As species experience genetic changes over
time (evolution), populations become
reproductively isolated from each other.
(Populations’ gene pools don’t mix!)
What are some types of
reproductive isolation?
Behavioral isolation (Species have different
reproductive behaviors)
Courting individual (usually male) does not
display to members of other species
Signal and response are species-specific
What are some types of
reproductive isolation?
Geographic isolation (Populations are
physically separated by geographical barriers
like water, canyons, or mountains; continental
drift)
Differences in populations are the consequences
of geographic isolation; Separate populations
evolve independently (from adaptation and
genetic drift)
What are some types of
reproductive isolation?
Temporal isolation (Species reproduce at
different times)
Can be because of timing of breeding cycles
(seasons) or the time of day/night that species
search for mates
Once populations are
separated…
Adaptive Radiation
Diversification of a single ancestral species into
several forms that are each specialized to a
particular environmental niche
What does
adaptive
mean?
What does
radiation
mean?
Convergent Evolution
Occurs when organisms that are
not closely related
independently evolve
(acquire)similar traits
Wings evolved separately in:
insects (arthropods)
pterosaurs (extinct flying reptiles)
birds (birds)
bats (mammals)
So, flight has evolved at least four separate times!
How was the adaptation for flight useful?
What is an analogous structure, and how is it different from
a homologous structure?
Analogous structures have a SIMILAR function but
DIFFERENT evolutionary origin.
Homologous structures are similar in structure AND show
common ancestry!
Sometimes, two or more species can
evolve in response to each other. This
is called coevolution.
This often happens between
predators and prey. Predators
that are good at catching prey are
naturally selected. This means
they survive because they can
catch food. Prey animals might
evolve to have better camouflage
or better escaping or fighting
skills. Predator and prey evolve
in response to each other.
Punctuated equilibrium vs. gradualism
• 2 ways in which
evolution of a species
can occur
• Punctuated Equilibrium=
speciation is rapid; long
periods of stasis
• Gradualism= constant,
gradual evolution
(smooth and
continuous)
Gradualism
Punctuated
Equilibrium