Download 01 Microevolution Unique Gene Pools and Genetic Variation NMSI

UNIT 9 Evolution Part 1
Microevolution: Unique Gene Pools and
Changing Allele Frequencies
Adaptations and Fitness
• An adaptation is a genetically
controlled trait that is favored by
natural selection and gives the
organism a reproductive advantage
ensuring the trait is passed on to its
descendants.
• This trait may also allow the individual
to survive longer thus increasing the
reproductive rate of that individual.
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Adaptations and Fitness
• The antelope hare lives in the desert,
and the snowshoe hare lives in the
mountains.
• Explain how the differences in their
traits enhance their ability to survive in
their respective environments.
• Evolutionary success or fitness refers to
the contribution of genes to the gene
pool and NOT how long an organism
lives.
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The Effect of Environmental Change
• Earth’s environment is NOT STATIC, but rather ever
changing.
• As a consequence, traits or adaptations that were favorable
may become unfavorable.
• The peppered moth, Biston betularia is native to England
and exists in two forms, one is dark and the other light
with a “peppered” appearance. Birds are its main predator.
• Prior to the industrial revolution, only 2% of the moths
were dark.
• The industrial revolution produced vast amounts of sulfur
dioxide and soot from the burning of coal which altered
the environment.
• Fifty years later 95% of the moths were dark.
• Propose an explanation!
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Industrial Melanism
England has since
regulated the burning of
coal and as a result, the
trees are returning to their
original state (A).
Consequently, the coloring
among the population of
moths in Britain has
shifted back so that the
peppered moths are once
again favored.
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Evolution Defined
• Evolution is defined as a change in the
inherited characteristics of biological
populations over successive
generations.
• Evolutionary processes give rise to
diversity at every level of biological
organization, from the molecular to the
macroscopic.
• As a result diversity is prevalent among
molecules such as DNA as well as
individual organisms and species of
organisms.
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Microevolution
Microevolution is simply a change in gene frequency
within a population.
• Evolution at this scale can be observed over short periods of time
such as from one generation to the next.
• Example: The frequency of a gene for pesticide resistance in a
population of crop pests increases.
• Such a change might come about because
– natural selection favored the gene
– the population received new immigrants carrying the gene (gene flow)
– nonresistant genes mutated into a resistant version of the gene
– of random genetic drift from one generation to the next
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Microevolution
• A gene is a sequence of DNA nucleotides
that specify a particular polypeptide
chain.
• Genes code for proteins.
• An allele is a particular form of a gene.
For example: B represents the allele for
black coat color and b for white coat color.
• Selection acts on phenotype because differential reproduction and
survivorship depend on phenotype not genotype.
• Natural selection acts on individuals, but only populations
evolve.
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Macroevolution
Macroevolution is evolution on a scale of separated
gene pools (not individuals).
• Think of it as an accumulation of changes which result in
speciation (forming a new species).
• Macroevolutionary studies focus on change that occurs at or
above the level of species, in contrast with microevolution,
which refers to smaller evolutionary changes (typically
described as changes in allele frequencies) within a species or
population.
• The process of speciation may fall within the purview of either,
depending on the forces thought to drive it.
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More Evolution Terms
• Species-a group of interbreeding organisms that produce viable and fertile
offspring in nature
• Gene pool-sum total of all the genes in a given species
• Allelic frequency-is the percent occurrence for a given allele
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Sources of Genetic Variation
How does variation in a population or gene pool arise?
1.
Mutations, gene duplication and chromosome fusion provide the raw
material for evolution.
2.
Meiosis and sexual reproduction produce new recombinants of phenotypes
upon which natural selection operates.
The wisteria pictured
on the right has a
mutation causing it
to produce white
flowers instead of
purple flowers.
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Types of Mutations
• MOST mutations are deleterious as
well as recessive.
• Obviously, mutations occurring in
somatic cells do not affect future
generations.
• Only mutations occurring in
gametes affect future generations.
• Mutations can occur at either the
gene or chromosomal level.
Mutations may cause a
sheep to have a 5th leg.
But this is not evolution!
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Neutral Mutations
Naturally evolving proteins gradually accumulate
mutations while continuing to fold into stable structures.
This process of neutral evolution is an important mode of
genetic change and forms the basis for the molecular
clock.
• Cytochrome c is a small protein found on the
mitochondrial membrane.
• Between mammals and reptiles there are 15 different
amino acids or mutations.
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Neutral Mutations
•
Mammals and reptiles diverged 265 million years ago.
•
That means on average cytochrome c mutated every 17 million years.
•
In comparing the evolution of other organisms and their cytochrome c
one mutation every 17 million years holds true.
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Cytochrome c Comparison
Molecular homology of cytochrome c (see three-letter code of amino acids)
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Human
Gly Asp Val Glu Lys Gly Lys Lys
Pig
Chicken
- Ile Dogfish
Drosophila <<< Wheat <<< - Asn Pro Asp Ala - Ala Yeast <<< - Ser Ala Lys - Ala Thr
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Ile Phe
Val Leu
Leu -
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17 18
20
Ile Met Lys Cys Ser Gln Cys His Thr Val Glu Lys
Val Gln - Ala Val Gln Val Gln - Ala - Asn
Val Gln Arg
Ala - Ala
Lys Thr - Ala - Asp Ala
Lys Thr Arg - Glu Leu -
• A dash indicates that the amino acid is the same one found at that position in the
human molecule.
• All the vertebrate cytochromes (the first four) start with glycine (Gly).
• The Drosophila, wheat, and yeast cytochromes have several amino acids that
precede the sequence shown here (indicated by <<<).
• In every case, the heme group of the cytochrome is attached to Cys-14 and
Cys-17 (human numbering).
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Hemoglobin Comparison
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Human Impact on Gene Pools
It is well documented that humans have had an impact on certain gene pools.
For example, humans have selected for certain desirable traits within the mustard
family and cultivated different agricultural products for human consumption.
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Artificial Selection
When humans manipulate a gene pool it is called artificial selection. There are
often consequences involved in such manipulations. For example in agriculture,
farmers try to increase crop production, which may lead to many farmers growing
only one variety of a particular crop such as corn. This leads to a loss of genetic
diversity. If a disease attacks that particular variety of corn, the farmers growing
that variety lose their entire crop.
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Antibiotics and Artificial Selection
• When antibiotics are applied to a population of microorganisms to
treat an infection, some of the microorganisms may be naturally
immune to the drug.
• Why? A random mutation occurred in the genetic code of the
microorganism conferring its resistance.
• These resistant microorganisms continue to flourish and cause
disease.
• The only remaining option a physician has is to treat the infection
with a different antibiotic and hope that none of the surviving
microorganisms possess a different random mutation that makes
them resistant to the second antibiotic as well.
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Antibiotics and Artificial Selection
• The increase in antibioticresistant bacteria has caused
doctors to reduce the number
of prescriptions written for
antibiotics in general.
• About 70% of pathogenic
bacteria are resistant to at
least one antibiotic and are
called “super bugs” or MDR
bacteria. (multidrug resistant)
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MRSA or Methicillin-resistant
Staphylococcus aureus
• MDR bacteria do not
respond to “first line of
defense” antibiotics.
• These types of bacteria
are most commonly
found in hospitals.
• Skin boils or similar
lesions that do not heal
often result.
• MDR bacteria can attack
internal organs upon
gaining entry into the
body.
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Effect of Sexual Reproduction
Sexual reproduction recombines genes in new ways. This results in unique
offspring that differ from either parent or sibling. Humans make 223 different
kinds of gametes. Fertilization means that the uniqueness of an individual is
223  223. Or the probability that two siblings will be genetically identical
(excluding identical twins) is 446.
Sexual reproduction is like shuffling a deck of cards and every
time getting a new and unique hand dealt. It is the major driving
force of evolution.
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Genetic Drift
Small populations can experience changes in allele frequencies
more dramatically than large populations. In very large populations
the effect can be insignificant. Also in small populations genes can
be lost more easily. When there is only one allele left for a
particular gene in a gene pool, that gene is said to be fixed , thus
there is no genetic diversity.
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Genetic Drift
• Genetic drift can be most
profound in populations that
are dramatically reduced
(bottle neck populations)
usually due to some
environmental catastrophe.
• Also genetic drift occurs when
a small population arrives at a
new habitat such as an island.
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Bottleneck Example
In 1900, the population of prairie
chickens in Illinois was 100
million but by 1995, the
population was reduced to
around 50 in Jasper County due
to over hunting and habitat
destruction which caused the
bottleneck to occur.
A comparison of the DNA from
the 1995 bird population
indicated the birds had lost most
of their genetic diversity.
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Bottleneck Example
• Additionally, less than 50%
of the eggs laid actually
hatched in 1993.
• Populations outside IL do
not experience the egg
hatching problem.
• Bottleneck populations
generally experience a
severe reduction in genetic
diversity within the
population.
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Founder Effect
• The founder effect is the loss of genetic
variation that occurs when a new
population is established by a very small
number of individuals from a larger
population and is a special case of
genetic drift.
• Founder effects are very hard to study!
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Founder Effect
• Biologist got their chance after a hurricane wiped out all the
lizard species on certain islands in the Bahamas, scientists repopulated the small islands with two lizard pairs, one having
long limbs and one having short limbs.
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Founder Effect
• Before the hurricane, these
islands supported populations
of a Caribbean lizard, the
brown anole, Anolis sagrei.
• After the hurricane, seven of
the islands were thoroughly
searched. No lizards were
found.
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Founder Effect
• In May 2005, the researchers
randomly selected one male
and one female brown anole
from lizards collected on a
nearby larger island to found
new anole populations on
seven small islands.
• They then sat back and
watched how those lizards
evolved to get an up-close
look at the Founder Effect.
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Founder Effect
• During the next four years, the researchers repeatedly
sampled lizards from the source island, from the seven
experimental founder islands, and from 12 nearby
islands that served as a control.
• The team found that all lizard populations adapted to
their environment, yet retained characteristics from their
founders.
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A Human Founder Effect Example
• The Amish community was founded
by a small number of colonist.
• The founding group possessed the
gene for polydactyly (extra toes or
fingers).
• The Amish population has increased
in size but has remained genetically
isolated as few outsiders become a
part of the population.
• As a result polydactyly is much
more frequent in the Amish
community than it is in other
communities.
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Impact of Nonrandom Mating
• Nonrandom mating also changes allele frequency.
• Nonrandom mating implies a choice of mates which is
more prevalent in animals.
• Random mating is less common.
• Time and space also factors into non-random mating.
For example, pollen from Ohio is more likely to crosspollinate a nearby tree in Ohio, rather than Oregon.
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Sexual Selection
• Sexual selection of mates also
affects allele frequency.
• The peacock provides a
particularly well known example
of intersexual selection, where
ornate males compete to be
chosen by females.
• The result is a stunning feathered
display, which is large and
unwieldy enough to pose a
significant survival disadvantage.
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Sexual Selection
• Female birds of many
species choose the male.
• Males that are “showier”
will better attract
females.
• These males have a
selective advantage even
though they are more
susceptible to predators.
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Sexual Selection
• Females that are drab, blend in to their
surroundings and as a result, avoid
predators which giving females a
survival advantage.
• This illustrates that the importance of
mating with the correct male
outweighs the importance of being
preyed upon.
• Sexual selection can lead to sexual
dimorphism where there is a distinct
difference between males and females.
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Natural Selection
Natural Selection is the only mechanism that consistently causes
adaptive evolution.
• Evolution by natural selection is a blend of chance and “sorting”.
– Chance in the context of mutations causing new genetic
variations
– Sorting in the context of natural selection favoring some alleles
over others
• This favoring process causes the outcome of natural selection to be
anything but random!
• Natural Selection consistently increases the frequencies of alleles
that provide reproductive advantage and thus leads to adaptive
evolution.
Three Modes of Natural Selection
• Natural selection can alter the frequency distribution of
heritable traits in three ways depending on which
phenotype is favored:
– Directional Selection
– Disruptive Selection
– Stabilizing Selection
Directional Selection
• Directional selection occurs when conditions favor individuals
exhibiting one extreme of a phenotypic range.
• Commonly occurs when the population’s environment changes or
when members of a population migrate to a new (and different)
habitat.
Possible Effect of
Continual Directional Selection
If continued, the variance may decrease.
after
Phenotype (trait)
before
after
Frequency
before
Frequency
after
Frequency
before
Phenotype (trait)
Phenotype (trait)
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
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Stabilizing Selection
• Stabilizing selection removes extreme variants from
the population and preserves intermediate types.
• This reduces variation and tends to maintain the
status quo for a particular phenotypic character.
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Diploidy
• In diploid eukaryotes each organism has two copies
of every gene and a considerable amount of genetic
variation is hidden from selection in the form of
recessive alleles.
• Often alleles are recessive and less favorable than
their dominant counterparts.
• By contrast, haploid organisms express every gene
that is in their genome. What you see is what you
get. It reduces genetic variability.
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Diploidy
• Recessive alleles persist by propagation in
heterozygous individuals.
• This latent variation is exposed to natural selection
only when both parents carry the same recessive
allele and two copies end up in the same zygote.
• As you might expect, this happens rarely if the
allelic frequency of the recessive allele is very low.
• Why is heterozygote protection of potentially
negative recessive alleles important to species
survival?
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Balancing Selection
• Balancing selection occurs when natural selection
maintains two or more forms in a population.
• This type of selection includes heterozygote advantage
and frequency-dependent selection.
• Heterozygote advantage involves an individual who is
heterozygous at a particular gene locus thus has a
greater fitness than a homozygous individual.
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Heterozygote Advantage
• A well-studied case is that of sickle
cell anemia in humans, a hereditary
disease that damages red blood cells.
• Sickle cell anemia is caused by the
inheritance of a variant hemoglobin
gene (HgbS) from both parents.
• In these individuals, hemoglobin in
red blood cells is extremely sensitive
to oxygen deprivation, and this
causes shorter life expectancy.
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Heterozygote Advantage
• A person who inherits the
sickle cell gene from one
parent, and a normal
hemoglobin gene (HgbA)
from the other, has a normal
life expectancy.
• However, these heterozygote
individuals, known as carriers
of the sickle cell trait, may
suffer problems from time to
time.
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Heterozygote Advantage
• The heterozygote is resistant to
the malarial parasite which
kills a large number of people
each year in Africa.
• There exists a balancing
selection between fierce
selection against homozygous
sickle-cell sufferers, and
selection against the standard
HgbA homozygotes by malaria.
• The heterozygote has a
permanent advantage (a higher
fitness) wherever malaria
exists.
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Heterozygote Advantage
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Frequency-Dependent Selection
• The fitness of a phenotype depends on how common it
is in the population.
• In positive frequency-dependent selection the fitness
of a phenotype increases as it becomes more common.
• In negative frequency-dependent selection the fitness
of a phenotype increases as it becomes less common.
• For example in prey switching, rare morphs of prey are
actually fitter due to predators concentrating on the
more frequent morphs.
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