Transcript lecture 13, part 2, how populations evolve, 051209c

How Populations Evolve: The Modern
Synthesis
Lecture 13, Part 2
http://www.tpwd.state.tx.us
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Much of the text material in the lecture notes is from our textbook,
“Essential Biology with Physiology” by Neil A. Campbell, Jane B.
Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship.
Other sources were sometimes used, and are noted.
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Outline
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Darwin, Mendel, and the modern synthesis
Evolution of populations
Population genetics
Phenylketonuria
Sources of genetic variation
Mechanism of microevolution
Retinitis pigmentosa
A closer look at natural selection
Species extinctions
Words and terms to know
Possible test items
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Our Planet at Night
Composite image
http://www.theodoregray.com
Many human population centers are apparent.
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Darwin and Mendel
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Natural selection involves hereditary processes that Darwin was unable
to explain.
Although Darwin and Mendel worked at about the same time, Mendel’s
discoveries went unnoticed for many years.
Mendel defined the hereditary processes for natural selection in his
experiments.
The works of Darwin and Mendel were consolidated in the mid-1900s,
many years after their discoveries.
This fusion of evolutionary biology with genetics is known as the ‘modern
synthesis.’
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Populations
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A population is defined as a group of individuals from the same species
living in the same area at the same time.
Natural barriers can form over time to separate populations of animals,
plants, or other organisms.
A population, for example, may be isolated from another by mountain
range or river.
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Some Natural Barriers
River
http://www.100thpenn.com
Mountain range
http://proprphotos.com
http://www.michigandnr.com
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Gene Exchange
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The genes in isolated populations are infrequently exchanged unless
there are opportunities to interbreed.
Populations, however, are usually not completely isolated and rarely
have sharp boundaries.
One population center may blur into another where members of both
populations are present.
Individuals are more concentrated in population centers, and are most
likely to breed with other local members.
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Evolution of Populations
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To a biologist, a population is the smallest biological unit that can evolve.
A common misconception is that individual organisms evolve during their
lifetimes.
Natural selection does act on individuals—inherited characteristics affect
their survival and reproductive success.
The evolutionary impact of the natural selection process is only apparent
when population changes are tracked over generations.
For DDT resistance in mosquitoes, evolution is measured by the change
in the percentage of resistant individuals over the span of several generations.
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Unnatural Selection
http://caspar.bgsu.edu
Far Side cartoon by Gary Larson
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Population Genetics
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The focus on populations as the basic evolutionary unit is known as population genetics.
Biologists examine genetic variation within populations and track genetic
changes over time.
http://images.perasoned-ema.com
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Faces in Groups
http://mdhsimage.mdhs.org
http://www.worth100.com
http://debate.uvm.edu
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Genetic Variation in Populations
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We generally have no trouble in recognizing the faces of our friends and
acquaintances in a crowd.
Each person has a unique genome as reflected in individual variations in
her or his physical appearance (phenotype) and temperament.
Individual variation exists in the populations of all species that reproduce
sexually.
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Factors in Genetic Variation
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Most populations have substantial variation that can only be detected
through biochemical means (such as DNA analysis).
Not all of the variation in a population is heritable—a phenotype can
result from a combination of genotype and environmental influences.
Strength-training, for example, can build-up muscle mass beyond what
would normally be expected for a person’s genotype—this would not be
passed to offspring.
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Polygenic Inheritance
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Many of the variable traits in a population result from combined effects of
several genes.
Polygenic inheritance produces traits that can vary continuously, such as
skin color and human height.
http://bp2.blogger.com
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Polymorphism
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Other features such as ABO blood groups result from a single gene with
different alleles that produce distinct phenotypes.
In-between blood types do not exist.
The contrasting phenotypes (blood types A, B, AB, and O) are known as
‘morphs.’
A population is said to be polymorphic if two or more morphs are present
in substantial numbers (such as what usually happens with blood types).
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Sources of Genetic Variation
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Genetic variation is produced by mutations or sexual recombination of
alleles—both are random processes.
A mutation in the nucleotide sequence of a gene can lead to harmless,
harmful, or beneficial effects.
On rare occasions, a mutant allele may enhance reproductive success
especially if the environment has changed—for example, the mosquito
populations sprayed with DDT.
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Generations and Life Spans
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In bacteria, which have short life spans and can evolve rapidly, the only
form of genetic variation is mutation since they reproduce asexually by
a process known as budding.
Natural selection can increase the frequency of a beneficial mutation in
a few days or even a few hours because bacteria can multiply quickly.
The outcome of natural selection for most animals and plants is not as
apparent because of the longer length of time between generations.
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Gene Pool
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The gene pool consists of all alleles—alternative forms of genes—in all
individuals making-up a population.
The gene pool is the reservoir from which the next generation draws its
genes.
Genes, as we have discussed, consist of DNA nucleotide sequences (A,
T, G, and C).
http://www.ruf.rice.edu
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Gene Pool Analysis
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Consider a wildflower population of only two varieties (morphs) for flower
color.
The allele for red flowers (R) is dominant to the allele (r) for white flowers
(so far, just like Mendel’s pea plant experiments).
Let’s say the R allele represents 80 percent of genes in the gene pool, or
p = 0.8.
Since there are only two possibilities, we can infer the r allele represents
the other 20 percent, or q = 0.2.
Using just this information, the frequencies of genotypes in the gene pool
can be calculated.
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Calculation of Genotype Frequencies
Allele frequencies: p = 0.8 (R) and q = 0.2 (r)
Sperm
p = 0.8
R p = 0.8
R
RR
q = 0.2
p2 = 0.64
r
r
rR
Rr
p * q = 0.16
p * q = 0.16
Eggs
q = 0.2
rr
q2 = 0.04
Hardy-Weinberg formula: p2 + 2pq + q2 = 1.00
Genotype frequencies: p2 = 0.64 (RR), 2pq = 0.32 (Rr), and q2 = 0.04 (rr)
RR = 64 percent, Rr = 32 percent, and rr = 4 percent
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http://www.sciencecartoonplus.com
Sometimes It May Seem Like This…
Cartoon by Sid Harris
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Phenylketonuria
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The Hardy-Weinberg formula can be used to calculate the percentage of
a population that carries the allele for an inherited disorder.
Phenylketonuria (PKU) is an inherited disorder of the inability of the body
to breakdown the amino acid, phenylalanine.
PKU, the result of a recessive allele from both parents, affects about one
in 10,000 newborn in the United States.
The disorder can cause severe mental retardation if left untreated early in
life.
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Genetic and Biochemical Bases
http://upload.wikimedia.org
http://newbornscreening.org
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Frequency of PKU Carriers
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Newborn are routinely tested for the disorder—the symptoms can be prevented by following a strict diet.
The frequency of U.S. newborn with PKU corresponds to q2 in the HardyWeinberg formula.
If q2 = 0.0001, q = .01, the frequency of homozygous recessive carriers in
the general population.
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Microevolution
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Microevolution is evolution at its smallest scale, such in found in Darwin’s
finches or a population of wildflowers.
Four mechanisms can change the gene pool of a population: genetic drift,
genetic flow, mutations, and natural selection.
Darwin’s finches
http://www.d113.lake.k12.il.us
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Small Sample Sizes
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If we toss a coin 1000 times, and it comes up heads 700 times, we might
conclude something is wrong.
If we tossed the coin ten times, and it comes up heads seven times, we
may think nothing of it.
The smaller the sample size, the greater potential deviation from the
expected result (50/50) for a random event with only two possibilities.
http://www.moderncoinmart.com
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Sampling Error
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The deviation from the expected result is known as sampling error—it is
often a problem with small sample sizes.
Sampling error applies in population biology in what is known as genetic
drift.
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A Wildflower Population
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The coin toss logic can be applied to a population’s gene pool where we
substitute the population size for the sample size of the number of coin
tosses.
For instance, say we have a population of wildflowers that had drawn its
alleles at random from the previous generation.
The larger the population of the previous generation, the more likely the
new generation (offspring) will represent the previous generation’s gene
pool.
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Genetic Drift
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While a large gene pool tends to maintain its status quo, a small gene
pool may not be accurately represented in the next generation due to
sampling error.
A change in the gene pool of a small population due to chance is known
as genetic drift.
The bottleneck effect and founder effect can shrink a population to a size
where genetic drift can occur.
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Wildfires
Portugal
Wildfires are not limited
just to California and
other western states.
Both images from http://upload.wikimedia.org
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Bottleneck Effect
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Large-scale disasters—including wildfires, floods,
earthquakes, and droughts—can drastically reduce
the size of a population.
The surviving members of the now much smaller
population may not represent the original population’s gene pool.
By chance, some alleles might be overrepresented
among the survivors while others may be underrepresented.
Other alleles may have been eliminated altogether
from the surviving population.
This bottleneck effect reduces genetic variability in
the population since some alleles are lost from the
gene pool.
http://www.fao.org
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Cheetahs
Cheetahs, the fastest of all running animals, were once widespread in
Africa and Asia.
• Their populations fell drastically during the last ice age about 100,000
years ago.
• The species suffered a severe bottleneck—it then experienced a second
bottleneck in the 19th century when they were hunted to near-extinction.
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Painting
http://www.onlineartdemos.co.uk
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Perils of Low Genetic Variability
Genetic variability is low in the few small cheetah populations existing in
the wild.
• With so little genetic variability, the population has a reduced capacity to
adapt to environment changes including loss of habitat and the spread of
disease.
• The reduced genetic variability makes its future precarious in its natural
state.
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Founder Effect
Genetic drift can also occur when a few individuals—including animals,
plants, and bacteria—colonize a new habitat such as an island or lake.
• The smaller the colony, the less its genetic makeup will represent the
gene pool of the larger population from where the colonists originally
came.
• Genetic drift in a newly-established colony is known as the founder
effect.
• The effect explains the relatively high frequency of certain inherited
disorders among human populations established by a few colonists.
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Tristan da Cunha
http://www.pmel.noaa.gov
http://www.visitandlearn.co.uk
Tristan da Cunha
http://volcano.und.edu
Tristan da Cunha is one of the most remote
places on Earth. The volcanic island is
astride the mid-Atlantic Ridge between two
tectonic plates. The surrounding ocean is
rich in crawfish, a major source of revenue
for the ~300 islanders. A second major
revenue source is the production and sale of
postage stamps to collectors.
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Retinitis Pigmentosa
Fifteen people established a colony on the island of Tristan de Cunha in
1814.
• One of the colonists happened to carry the recessive allele for a rare and
progressive form of blindness known as retinitis pigmentosa.
• Among the 240 residents on the island in the 1960s, four had the genetic
disorder and nine were known to be heterozygous carriers.
• The incidence on Tristan de Cunha is much higher than in Great Britain,
the homeland of the original colonists.
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Gene Flow
Most populations are not completed isolated—a population may gain or
lose alleles by gene flow, that is, the exchange of genetic material with
other populations.
• Gene flow occurs when fertile individuals or gametes migrate between
populations.
• Consider two similar species of wildflowers in adjacent woodland areas.
• Wind may carry pollen from one flower population to the other enabling
the populations to interbreed.
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Reduction in Genetic Differences
Gene flow tends to reduce the genetic differences among populations.
• If extensive, gene flow can result in the combination of populations into
a single population with a common gene pool.
• Gene flow is not always possible due to reproductive barriers between
species as we will discuss next week.
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Human Migration and Gene Flow
The migration of people
throughout the world is
transferring alleles between
populations that were once
isolated.
http://courses.washington.edu
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Genetic Mutations
A mutation results from a change in the DNA nucleotide sequence, the
genetic language of life.
• A new mutation can immediately change the gene pool of a population
by substituting one allele for another.
• Although a mutation on any one gene is rare, the cumulative impact of
mutations can be substantial since each individual in a population has
thousands of genes.
• Over many generations, mutations can be important to the evolutionary
process.
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http://berkeley.evolution.edu
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A Closer Look at Natural Selection
We return to natural selection for a closer look at its basic mechanisms.
• Genetic drift, gene flow, and mutation can result in microevolution, but
they usually do not lead to adaptation since they are largely based on
chance.
• Of all of the forms of microevolution, only natural selection is considered
adaptive.
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Adaptation
On the average, those individuals best suited to the environment leave
the most offspring and have a disproportionate impact on the gene pool.
• When fields were sprayed with DDT, the resistant mosquitoes survived
and increased the frequency of the pesticide-resistant allele in the gene
pool.
• Natural selection is adaptive because it enables some members of a
species to survive and reproduce in a changed or new physical environment.
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Darwinian Fitness
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The ‘struggle for existence’ and ‘survival of the fittest’ are sometimes
interpreted by the ‘Social Darwinists’ as competitive contests between
individuals.
While the male members in some animal species do battle for mating
privilege, reproductive success is generally more subtle and passive.
A frog, for example, may produce more eggs than others because she is
more efficient at catching insects for food.
Individuals in a wildflower population may differ in reproductive success
because some are better able to attract pollinators due to subtle differences in their color, shape, or fragrance.
Darwinian fitness is the contribution a individual makes to the gene pool
of the next generation relative to the contributions of other individuals.
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Attracting Pollinators
Ruby-throated hummingbird and honeybee
http://www.fs.fed.us
Darwinian fitness of flowering plants depends in part on a competitive
advantage in attracting pollinators.
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Directional Selection
Three modes of natural selection are directional selection, diversifying
selection, and stabilizing selection.
• Directional selection shifts a population to an extreme phenotype—it is
most common when the local environment changes or when organisms
migrate to a new environment.
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Diversifying and Stabilizing Selection
Diversifying selection can lead to a balance between two or more morphs,
or phenotype forms.
• Stabilizing selection maintains the variation of a phenotype within a narrow
range—it typically occurs in a stable environment in which populations are
already well adapted.
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Survival of a Species
When challenged with a new set of environmental problems, a population
either adapts through natural selection or can become extinct in that locale.
• Some human activities can be so sudden or overwhelming that a species is
unable to survive.
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http://upload.wikimedia.org
Dodo Bird
The dodo bird lived on Mauritius Island in the Indian Ocean. Related
to the pigeon and dove, it was about three-feet tall. The dodo was
flightless, unafraid, and lived on fruit. In the 1700s, it was one of the
first species to be extinguished due to indiscriminate hunting.
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http://www.stanford.edu
Passenger Pigeon
Female (top branch) feeding a squab
http://vrcoll.fa.pitt.edu
Over five billion passenger pigeons were thought
to have resided in the Eastern United States in the
1800s. Migratory flocks were reported as long as
100 miles, a mile wide, and several birds deep.
Within 20-to-30 years the passenger pigeon was
largely extinct due to indiscriminate hunting and
other human activities.
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The Last Survivor
“In 1857, a bill was brought forth to the Ohio State
Legislature seeking protection for the passenger
pigeon. A Select Committee of the Senate filed a
report stating, ‘The passenger pigeon needs no
protection. Wonderfully prolific, having the vast
forests of the North as its breeding grounds,
traveling hundreds of miles in search of food, it is
here today and elsewhere tomorrow, and no ordinary destruction can lessen them, or be missed
from the myriads that are yearly produced.’ ”
Source:
http://en.wikipedia.org/wiki/passenger_pigeon
Martha
https://www.geology.ucdavis.edu
‘Martha’ died in the Cincinnati, Ohio
Zoo on September 1, 1914. There
are no more passenger pigeons.
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Words and Terms to Know
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Bottleneck effect
Darwinian fitness
Extinction
Founder effect
Gene flow
Gene pool
Genetic drift
Genetic variation
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Hardy-Weinberg formula
Microevolution
Mutation
Natural barrier
Phenylketonuria
Polymorphism
Population
Retinitis pigmentosa
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Possible Test Items
1. What is a population from a biological perspective? Describe how
populations can evolve.
2. Describe the mechanisms of microevolution. How are they adaptive
or a matter of chance?
3. Describe how the bottleneck effect can reduce genetic variability in an
animal population.
4. How does Darwinian fitness differ from the social term, survival of the
fittest?
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