Transcript Chapter 16 Evolution of Populations

Chapter 16
Evolution of Populations
16.1 Genes and Variation
A. Populations and Gene Pools
a. population: individuals of the same species in a given
area.
b. gene pool is the combined genetic material of all the
alleles in the population
c. allele frequency = % of an allele in the pool
* 20 bugs =the gene pool has 40 alleles (some B, b)
30 B alleles = .75
10 b alleles = .25
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Genes are responsible for the phenotypes
in organisms
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if the frequency of allele changes, then
the population will start to look different
as nature selects for these alleles. –
breeding bunnies for example

In genetic terms = evolution is any
change in the relative frequency of alleles
in a population.
B. Sources of Genetic Variation
Evolution is fueled by variations in individuals
that result from sexual reproduction:
a. Mutations
b.Gene Shuffling
During meiosis independent assortment and
crossing over increase the number of
genotypes that can occur.
crossing over in meiosis
random homologous pairing in meiosis
C. Expression of Variation
a. Some traits are single-allele traits
example: widow’s peak.
2 alleles and 2 phenotypes
b. Most traits are polygenic trait.
Controlled by 2 or more genes
There is a lot of variation in the phenotype
Ex: height, hair color
Two forms of the trait in
single allele traits
A lot of variation in
Polygenic traits
16.2 Evolution as Genetic Change
A. What causes allele frequencies in a gene
pool to change?
1. Natural Selection on Single Allele Traits
If an organism survives and produces many
offspring, its alleles stay in the gene pool and
may increase in frequency. The adaptations
that helped it to survive will be magnified.
Example: Breeding Bunnies fur or no fur
2. Natural Selection on Polygenic Traits
With many phenotypes, distribution of traits can
be affected in may ways.
a. Directional Selection
When individuals at one end of the of the curve have
higher fitness than the middle or the other end.
Low mortality,
high fitness
Section 16-2
Key
Food becomes scarce.
High mortality,
low fitness
b. Stabilizing Selection
When individuals near the center of the curve
have higher fitness than individuals at either end
of the curve
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
c. Disruptive Selection
When individuals at the upper and lower ends of
the curve have higher fitness than individuals
near the middle.
Key
High mortality,
low fitness
Beak Size
Population splits
into two subgroups
specializing in
different seeds.
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
3. Genetic Drift – Random change in the frequency
of a gene. (not natural selection) Occurs more in small
population.
a. Can happen by natural disasters – certain
phenotypes in a population die and it then changes
the gene pool
b. Founders effect – a group leaves a population and
settles in new location after generations, new looks
different than original population. Chance – not
natural selection.
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Founding Population B
Notice the New populations look different than the original, because of the
Random members that left and breed new populations
B. Evolution vs. Genetic Equilibrium
a. Evolution is a change in allele frequencies
b. Genetic Equilibrium – no genetic change.
If allele freq do not change then pop will not
evolve (look the same)
Galapagos Islands show an example of SLOW
change – more equilibrium
c. Hardy-Weinberg Principle says……
allele frequencies will remain constant unless one or
more factors causes those frequencies to change.
For genetic equilibrium to occur(no evolution
1. random mating
2. population must be large
3. can be no migration or emigration
4. no mutation
5. no natural selection

What would happen if one of these things
occurred/changed?
The allele frequencies for a trait would change and
Evolution !!!!!!
Equation:
p2 + 2pq + q2 = 1
Used to determine if the frequencies of alleles change in
a population.
P2= homozygous dominant in pop
2pq= heterozygous dominant in pop
q2 = recessive phenotype in pop
p= frequency of dom allele in a pop
q= frequency of rec allele in a pop
p+q = 1 total alleles
Example:
If there are 50 people = 100 alleles (T, t)
40 T alleles = 40 % = .4 = p
60 t alleles = 60% = .6 = q

p2 + 2pq + q2 = 1
p+q=1
p = dom allele
PTC Tasting is dominant
q = rec allele
Example: 17 student: 12 tasters, 5 nontasters
tasters (TT, Tt)= 12/17 = .7 = p2 + 2pq
non-tasters (tt) = 5/17= .3 = q2 so q =.54
p+ q=1
p + .54= 1
p = .46 (T)
p2 = .46 x .46 = .2 = ………. 20 % TT tasters
2pq = 2 x .54 x .46 = .5 …….50% Tt tasters
q2 = .54 x .54 = .3 = ………..30 % tt non-tasters

The point is, then…..if p & q change over
time then the phenotype ratios will change
too = EVOLUTION

Use HW at 2 points and compare the p &
q values to see if there is a shift
16.3 Speciation: Evolution to the MAX
Species : a group of similar organisms that breed
produce fertile offspring in a natural environment.
Speciation: new species evolving from old species.
When the gene pools change so much from the
original, a new species develops.
Reproductive Isolation  When members of a population can’t or don’t
reproduce….the gene pools become isolated
 Specific mating instead of random mating
 3 kinds of Rep Isolation
a. Behavioral isolation: different courtship rituals or other
types of mating behavior that prevent mating
lacewing
Geographic isolation: A pop is
separated by geographic barriers – rivers, mountains,
other water.
b.
no random mating
now, but only within those
members that are together.
Galapagos Islands = example
california salamanders
Temporal isolation: 2 or more
species reproduce at different times.
Seasonal differences in mating
c.
Example of speciation though a combination of factors:
geographical, behavioral, niche changes
Chimps and bonobos
diverge into 2 species.
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Chimps & Bonobos
Example of how there more than
geographical isolation affects speciation.
environmental factors mold evolution too.

PBS Hummingbird
Reproductive isolation leads to new species
Section 16-3
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
Speciation in Darwin's Finches
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Speciation in Darwin's Finches
– Speciation in the Galápagos finches occurred by:
 founding of a new population
 geographic isolation
 changes in new population's gene pool
 reproductive isolation
 ecological competition
 Continued evolution
Speciation in Darwin's Finches
Founders Arrive
A few finches—
species A—travel
from South
America to one of
the Galápagos
Islands.
There, they survive
and reproduce.
Speciation in Darwin's Finches
– Geographic Isolation
Some birds from
species A cross to
a second island.
The two
populations no
longer share a
gene pool.
Speciation in Darwin's Finches

Changes in the Gene Pool
Seed sizes on the
second island
favor birds with
large beaks.
The population on
the second island
evolves into
population B, with
larger beaks.
Speciation in Darwin's Finches
– Reproductive Isolation
 If population B birds cross back to the first
island, they will not mate with birds from
population A.
 Populations A and B have become separate
species.
Testing Natural Selection
in Nature
Speciation in Darwin's Finches
– Ecological Competition
 As species A and B compete for available
seeds on the first island, they continue to
evolve in a way that increases the
differences between them.
 A new species—C—may evolve.
Testing Natural Selection
in Nature
Speciation in Darwin's Finches
– Continued Evolution
 This process of isolation, genetic change,
and reproductive isolation probably
repeated itself often across the entire
Galápagos island chain.
Chapter 17
History of Life
17.1 Fossil Record
Section 17-1
a. Shows
change in organisms over time
b. Sedimentary rock forms layers, encasing any dead organisms
that have fallen into that layer. Older fossils are on bottom
rock layers
Dead organisms are buried
The preserved remains
Water carries small rock
particles to lakes and seas.
by layers of sediment, which
forms new rock.
may later be discovered
and studied.
Pressure turns sediment into rock and many bones are preserved
by mineral saturation.
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Fossilization
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PBS Fossilization
Show how fossils are used to piece
together the ancestry of whales from that
of land mammals to aquatic mammals.
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Lucy fossilization
Show how fossils are formed
Compare/Contrast Table
2 Ways of Determing the Age of Fossils
Section 17-1
Comparing Relative and Absolute Dating of Fossils
Can determine
Is performed by
Drawbacks
Relative Dating
Absolute Dating
Age of fossil with respect to
another rock or fossil (that is,
older or younger)
Age of a fossil in years
Comparing depth of a fossil’s
source stratum to the position
of a reference fossil or rock
Determining the relative
amounts of a radioactive
isotope and nonradioactive
isotope in a specimen
Imprecision and limitations of
age data
Difficulty of radioassay
laboratory methods
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Carbon 14 is a radioactive isotope. Its half-life is 5730 years.
The amount of C14 left in a fossil sample can determine the
age of the fossil.
Radiometric dating PBS
17.2 Earth’s Early History
Hypotheses about the beginning of earth and life are based on a
small amount of scientific data. As new evidence is found,
scientists ideas might change. Part of the process of science is
Collecting data, evaluating, and revising!
A. 1950’s Urey & Miller designed experiments to examine
how inorganic compounds
gases
spark
could from into organic
molecules (proteins, DNA).
Water vapor
Condensed amino acids
* When a spark was added to their “soup” of chemicals,
simple amino acids were formed.
* There were problems with the experiment, but more
recent experiments have been able to produce
cytosine and uracil which are in RNA.
B. Which came first…DNA or RNA?
Need DNA to make RNA and RNA
is easier to make…….
Thought that pieces of RNA
were produced first and helped in
the formation of DNA then proteins….
C. Simple prokaryotes formed first. Did not use oxygen. They
became photosynthetic and oxygen levels began to rise in the
atmosphere.
D. How did Eukaryotes form? One thought is the Endosymbiotic
Theory. Was not recognized as a viable theory until the
1960’s by Lynn Margulis. (Boston!)
Eukaryotic Cells developed from a symbiotic relationship
between several kinds of prokaryotes (bacteria) – each had
its own “specialty” and together – formed a great “unit”
Aerobic bacteria
Nuclear envelope
Ancient prokaryotic
Photosynthetic bacteria
Plant and
Plantlike cells
mitochondrion
Primitive eukaryote
Animals, fungus
cells
Concept Map
Section 17-2
4 billion
3.8 billion
Evolution of Life
Early Earth was hot; atmosphere contained poisonous gases.
Earth cooled and oceans condensed.
Simple organic molecules may have formed in the oceans..
Small sequences of RNA may have formed and replicated.
3.5 billion
First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.
2 billion
Later prokaryotes were photosynthetic and produced oxygen.
An oxygenated atmosphere capped by the ozone layer protected Earth.
First eukaryotes may have been communities of prokaryotes.
500 mil
Multicellular eukaryotes evolved.
Sexual reproduction increased genetic variability, hastening evolution.
17.4 Patterns of Evolution
Macroevolution – Long term change (whole new orgnisms)
5 Patterns of Macroevolution
1.
Extinction: large numbers of species disappear.
The result is the remaining species now have new niches (job)
to fill, and may then thrive and evolve.
Permian Extinction – PBS
extinction
invasive species - extinction
2. . Adaptive Radiation: single species evolved into
several different species with different living habits.
3. Convergent Evolution: is when adaptive radiation
produces two unrelated species that actually appear
similar.
Started out with completely different genetic material,
niches, and habitats. Over time, natural selection
molds similar body forms – analogous structures
Not related but have
similar body structure
due to the long period
of selection for these
traits in separate
environments
Convergent Evolution
 PBS Anteater

4. Coevolution
The evolution of two organisms together.
The two organisms benefit each other and
therefore they change together
Toxic Newt
Leafcutter
red queen
allergies and asthma
5. Punctuated Equilibrium
There is variation in the rate of evolution.
* There is evidence of a slow and gradual evolution (Darwin’s
big thing). Tortoises – state of slow equilibrium
gradualism.
* There is also evidence of bursts of rapid evolution in
which several new species have formed. (rapid is still
thousands or millions of years. Punctuated equilibrium
Affects small populations
more dramatically.
* isolation
* migration
* extinction
Gradualism