Transcript Microevolution: Unique Gene Pools

Microevolution: Unique Gene Pools
Charles Darwin
• Charles Darwin (1809-1882) is credited
with proposing that the mechanism for the
process of evolution is natural selection.
• English naturalist and geologist
• Darwin spent five years on a voyage (HMS
Beagle) that took him around the world
with the majority of his time spent in South
America and its neighboring islands.
• Darwin published his theory (20 years later)
with compelling evidence for evolution in
his 1859 book On the Origin of Species,
overcoming scientific rejection of earlier
concepts of transmutation of species.
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Charles Darwin
He established that all species of life have descended over time from
common ancestors (descent with modification), and proposed the
scientific theory that this branching pattern of evolution resulted from
a process that he called natural selection, in which the struggle for
existence has a similar effect to the artificial selection involved in
selective breeding.
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Charles Darwin
• By the 1870s the scientific community and much of
the general public had accepted evolution as a fact
(change over time/descent with modification).
• However, many favored competing explanations (e.g.
Theists evolution, Neo-Lamarckism, Saltationism). It
was not until the emergence of the modern
evolutionary synthesis from the 1930s to the 1950s
that a broad consensus developed in which natural
selection was the basic mechanism of evolution.
• In modified form, Darwin's scientific discovery is the
unifying theory of the life sciences, explaining the
diversity and unity of life.
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Darwin’s Influences & Observations
• Thomas Malthus, Charles Leyell, George
Cuvier
• Populations change over time as evidenced by
the fossil record.
• There are always more offspring produced
than the preceding generation.
• Populations, if left unchecked, grow at a
geometric rate (exponential rate) rather than
an arithmetic rate.
• Darwin used an example involving elephants
to illustrate the points above. He estimated
that if elephants underwent unrestricted
reproduction, that in 740-750 years there
would be 15 million elephants produced from
just one original pair.
Exponential vs. Logistic growth
Darwin’s Elephant Problem Cont’d
“There is no exception to the rule that every
organic being naturally increases at so high a
rate that if not destroyed, the earth would soon
be covered by the progeny of a single pair ....
(Darwin, 1859 p.64)
(Note: Darwin’s calculation flaws)
* What prevents/”destroys” this innate ability
of living creatures to reproduce so prolifically?
• Darwin’s answer: There is always a struggle
for existence. What did he mean?
• What is involved in the struggle and who
“wins”?
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Darwin’s Observations Cont’d
•
There is variation within a given species and the majority of this variation is inherited.
This litter of kittens vary with respect to coat pattern and color.
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Any variation may, to some degree, affect the ability of an organism to reproduce and
contribute genes to the gene pool, thus affecting evolutionary success.
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Species change over time. These changes are related to traits that are inherited or arise
from an alteration of the genetic code.
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Some inherited traits are beneficial and contribute to survival.
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Whether a trait is beneficial or not is a function of the environment in which it lives.
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Take-away points:
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Overproduction leads to:
A struggle for existence but also creates:
Tremendous variation (inherited)
Natural selection “selects” those variants
that are best suited (have the
adaptations that lead to differential
reproductive success in local
environments)
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A closer look: 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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Cont’d Adaptations and Fitness
• The antelope hare lives in the desert, and the
snowshoe hare lives in the mountains.
• What adaptations do you see in these two
hares that might provide them with differential
reproductive success in their local
environments?
• Evolutionary success or fitness refers to the
contribution of genes to the gene pool and
NOT necessarily how long an organism lives.
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Classic example of adaptations to a “local
environment”
• 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 (morphs), 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. How so?
• Fifty years later 95% of the moths were dark.
• Propose an explanation.
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Industrial Melanism (I.M.)
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.
What role does inherent
variation play in I.M.?
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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 below the
species level (at the level of populations).
• 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.
• Potential causes include:
– natural selection favored the gene (SC gene heterozygous advantage)
– the population received new immigrants carrying the gene (gene flow: humans
past and present)
– nonresistant genes mutated into a resistant version of the gene
– coevolution
– of random genetic drift from one generation to the next (sudden environmental
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changes)
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What are genes?
• A gene is a sequence of DNA nucleotides
that specify a particular polypeptide
chain.
• Many genes code for proteins (polypeptides).
• 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
success depends on phenotype not genotype.
• Natural selection “selects”/”favors” individuals, but only
populations evolve.
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Macroevolution (in brief)
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 but not necessarily).
• What determines if a single species has become 2 different
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 occurrence for a given allele in a population
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N.S. needs Variation to work.
Sources?
How does variation in a population or gene pool arise?
1.
Mutations, (gene duplication and chromosome fusion) provide the raw
material for evolution.
2.
Most variation that N.S. acts on comes from 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. Which means?
• 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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Point Mutations:
Synonymous vs. Nonsynonymous
Point mutations occur when one
nucleotide is substituted for another.
The genetic code contains “synonyms” for
the coding of amino acids. For example the
DNA codons GGA, GGG, GGT, GGC all
code for the amino acid proline.
Therefore, as long as the codon has GG in
positions 1 & 2, a mutation in position three
has no consequence, proline will be coded
for regardless.
This sort of mutation is called a
synonymous or silent mutation.
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Point Mutations:
Synonymous vs. Nonsynonymous
Point mutations that do result in a different amino acid are called a nonsynonymous or
missense mutations (if polypeptide is produced) vs nonsense.
Missense mutations can affect the protein in one of THREE ways: (Remember the new
amino acid will have a different R group on the protein)
1.
It can result in a protein that does not function as well as the original protein. (This
happens most often.)
2.
It can result in a protein that functions better than the original protein.
3.
It can result in a protein that functions like the original protein. This is usually
because the R groups are similar. (both polar or both nonpolar, etc.)
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Mutations cont’d Gene Duplication
Genes can be duplicated and occasionally the duplication moves a gene from
one chromosome to another. Each gene will accumulate different mutations
altering the protein that is subsequently synthesized.
Globins are proteins. A class of the group binds with oxygen (in muscles,
erythrocytes, plant root cells). This gene has been duplicated and modified
many times. It has given rise to the hemoglobin genes (alpha, beta, fetal,
embryonic) as well as myoglobin, leghemoglobin and psuedogenes.
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Evolution of Hemoglobin Gene
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Neutral Mutations
Naturally evolving proteins gradually accumulate mutations while
continuing to fold into stable structures.
Ex. Hb4 and its forms (slightly diff seq = slightly diff polypeptide =
no functionality lost)
This process of neutral evolution is an important mode of genetic
change and forms the basis for the molecular clock.
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Cytochrome c is a small protein found on the mitochondrial
membrane. (a highly conserved protein)
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Between mammals and reptiles there are 15 different amino acids
or mutations. (neutral Cyt c mutations have been passed on for
millions of years)
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Neutral Mutations
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Mammals and reptiles diverged 265 million years ago.
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That means on average cytochrome c mutated every 17 million years.
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In comparing the evolution of other organisms and their cytochrome c
one (NEUTRAL) mutation every 17 million years holds true.
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Changes in Cytochrome C
Above is a comparison ancestral cytochrome c and human cytochrome c. This
gene has been highly conserved as it is a protein used in the electron transport
chain of the mitochondria. Missense mutations occur more frequently in
pseudogenes (genes that have been duplicated, then mutated and are no longer
functional) than in functional genes. Why?
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Hemoglobin Comparison
Human beta chain
0
Gorilla
1
Gibbon
2
Rhesus monkey
8
Dog
15
Horse, cow
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Mouse
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Gray kangaroo
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Chicken
45
Frog
67
Lamprey
125
Sea slug (a mollusk)
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Soybean (leghemoglobin) 124
• This is a comparison between the
differences in the amino acid
sequence of human hemoglobin
and different species.
• The last three species do not have a
distinction between a and b chains.
• There is an direct relationship
between the amino acid sequence
and how closely related the
organisms are to humans.
• The b chain of hemoglobin has
146 amino acids.
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Hemoglobin Comparison
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Frameshift Mutation
• A frameshift mutation (nonsense) occurs as a result of either an
insertion or deletion of a nucleotide. Polypeptide produced is
massively dysfunctional or completely non-functional
• This changes the amino acid sequence of the protein from that point
forward.
• THECATATETHEFATFATRATBUTNOTHISHAT
• TECATATETHEFATFATRATBUTNOTHISHAT
• THECATATETHEFATFATRATBUTNOOTHISHAT
• Insertion mutations and deletions commonly produce these frameshifts
• Almost all frame shift mutations are deleterious.
• Tay Sachs & Cystic fibrosis are examples
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Chromosomal Rearrangement
There have also been major changes
in chromosome structure that result in
changes within populations which
can, in turn, result in the emergence
of new species.
These include:
a. inversions
b. deletions
c. duplication
d. translocations
e. fusions
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Chromosomal Rearrangement
Compare the karyotype of a
human (H) and a chimpanzee (C).
Notice the great apes have 24
pairs of chromosomes compared
to 23 pairs of chromosomes in a
human.
Why the difference?
Chromosome #2 in the human is
the result of a fusion of two
chimpanzee chromosomes.
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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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Reducing or Eliminating Gene Pools
• Human activities often augment genetic drift
and diminish gene flow for many species.
• This reduces genetic variation thereby
disrupting adaptive processes both locally
and globally within a species.
• This impact is illustrated within populations
of collared lizards (Crotaphytus collaris)
living in the Missouri Ozarks.
• Forest fire suppression has reduced habitat
and disrupted gene flow in this lizard, thereby
altering the balance toward drift and away
from gene flow. This balance can be restored
by managed landscape burns.
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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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Created by:
Carol Leibl
Science Content Director
National Math and Science