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.
•
Darwin spent five years on a voyage that
took him around the world with the
majority of his time spent in South America
and its neighboring islands.
• Darwin published his theory 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, 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.
• However, many favored competing explanations
and 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 of life.
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Darwin’s Observations
• 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 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 19 million elephants
produced from just one original pair.
Darwin’s Observations
Darwin’s Elephant Problem
“There is no exception to the rule that every
organic being naturally increase at so high a rate
that if not destroyed, the earth would soon be
covered by the progeny of a single pair ....
The Elephant is reckoned to be the slowest
breeder of all known animals, and I have taken
some pains to estimate its probable minimum
rate of natural increase: it will be under the
mark to assume that it breeds when thirty years
old, and goes on breeding till ninety years old,
bringing forth three pairs of young in this
interval; if this be so, at the end of the fifth
century there would be alive fifteen million
elephants, descended from the first pair.”
(Darwin, 1859 p.64)
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Darwin’s Observations
•
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.
•
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.
•
Species change over time. These changes are related to traits that are inherited or arise
from an alteration of the genetic code.
•
Some inherited traits are beneficial and contribute to survival.
•
Whether a trait is beneficial or not is a function of the environment in which it lives.
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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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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.
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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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.
Myoglobin is a protein that binds with oxygen in the muscles. This gene has
been duplicated and modified many times. It has given rise to the hemoglobin
gene.
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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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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.
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Cytochrome c Comparison
Molecular homology of cytochrome c (see three-letter code of amino acids)
1
6
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 -
14
17 18
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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
Human beta chain
0
Gorilla
1
Gibbon
2
Rhesus monkey
8
Dog
15
Horse, cow
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Mouse
27
Gray kangaroo
38
Chicken
45
Frog
67
Lamprey
125
Sea slug (a mollusk)
127
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 inverse relationship
between the difference in 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 occurs as a result of either an insertion or
deletion of a nucleotide.
• This changes the amino acid sequence of the protein from that point
forward.
• Almost all frame shift mutations are deleterious.
• Recently, bacteria were found growing in a pool of nylon wastes.
(Flavobacterium)
• These bacteria were actually digesting the nylon waste.
• Upon examining the genome of these bacteria, it was found there
was a frameshift mutation in their DNA that caused the production
of three different enzymes that could digest the nylon.
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Evolution of Hemoglobin Gene
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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