Download Evidence of Evolution

Evidence of
Evolution
Genetic Variation and Patterns of Evolution
Elements of Natural Selection
1. All species have genetic variation.
2. Since organisms generally produce more offspring than can
be supported by the environment, individuals within a
species frequently complete with each other for survival.
3. The environment itself presents many challenges for an
organism's survival.
4. Survival of the fittest occurs
5. The traits of the organisms best suited to a certain habitat
tend to become more frequent in a population over time.
A species is a group of similar organisms that can interbreed and produce
fertile offspring.
A population is a group of individuals of the same species that
interbreed.
A gene pool consists of all genes, including all the different alleles, that
are present in a population.
Populations, not individual organisms, can evolve over time.
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Variation and Gene Pools
Natural selection can affect the distributions of
phenotypes in any of three ways:
• directional selection
• stabilizing selection
• disruptive selection
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Natural selection on single-gene traits can lead to
changes in allele frequencies and thus to evolution.
Directional Selection
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When individuals at one end of the curve have higher fitness than
individuals in the middle or at the other end, directional selection
takes place.
Stabilizing Selection
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When individuals near the center of the curve have higher fitness than
individuals at either end of the curve, stabilizing selection takes place.
Disruptive Selection
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When individuals at the upper and lower ends of the curve have higher
fitness than individuals near the middle, disruptive selection takes
place.
Consider that a flying beetle has been introduced to a new
tropical island. Identify the type of natural selection at work in
each example. – stabilizing, directional, or disruptive
1. Only the beetles with the darkest bodies thrive in the new
environment.
2. The smallest beetles cannot compete for food. The largest
beetles are easy prey for birds.
3. The beetles with the smallest wings a thrive in rotten tree
trunks. The beetles with the largest wings thrive along the
coastline.
Variation and Gene Pools
A gene pool consists of all genes, including all the different alleles, that are present in a
population.
The relative frequency of an allele is the number of times the allele occurs
in a gene pool, compared with the number of times other alleles for the
same gene occur.
Relative frequency is often expressed as a percentage, and it is not related
to whether an allele is dominant or recessive.
Variation and Gene Pools
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Gene Pool for Fur Color in Mice
In genetic terms, evolution is any change in the
relative frequency of alleles in a population.
The two main sources of genetic variation are
mutations and the genetic shuffling that results
from sexual reproduction.
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Variation and Gene Pools
What is genetic drift?
• A random change in allele frequency
• Genetic drift may occur when a small group of
individuals colonizes a new habitat.
• Individuals may carry alleles in different relative
frequencies than did the larger population from which
they came.
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Genetic Drift
Genetic Drift
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Genetic Drift
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Genetic Drift
Population A
Population B
When allele frequencies change due to migration of a small subgroup of a population it is
known as the founder effect.
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Descendants
Evolution Versus Genetic Equilibrium
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The Hardy-Weinberg principle states that allele frequencies in a
population will remain constant unless one or more factors cause
those frequencies to change.
When allele frequencies remain constant it is called genetic
equilibrium.
Five conditions are required to maintain genetic
equilibrium from generation to generation:
1. there must be random mating,
2. the population must be very large,
3. there can be no movement into or out of the
population,
4. there can be no mutations, and
5. there can be no natural selection.
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Evolution Versus Genetic Equilibrium
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The Process of Speciation
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Natural selection and chance events can change the relative
frequencies of alleles in a population and lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that breed with one another
and produce fertile offspring.
Isolating Mechanisms
What factors are involved in the formation of new species?
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The gene pools of two populations must become separated for
them to become new species.
Isolating Mechanisms
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As new species evolve, populations become reproductively
isolated from each other.
When the members of two populations cannot interbreed and
produce fertile offspring, reproductive isolation has occurred.
Isolating Mechanisms
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Behavioral isolation occurs when two populations are capable of
interbreeding but have differences in courtship rituals or other
reproductive strategies that involve behavior.
Isolating Mechanisms
Abert
Kaibab
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Geographic isolation occurs when two populations are separated by
geographic barriers such as rivers or mountains.
Isolating Mechanisms
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Temporal isolation occurs when two or more species reproduce at
different times.
Speciation in Darwin's Finches
A few finches—
species A—travel
from South
America to one of
the Galápagos
Islands.
There, they survive
and reproduce.
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Founders Arrive
Speciation in Darwin's Finches
Some birds from
species A cross to
a second island.
The two
populations no
longer share a
gene pool.
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Geographic Isolation
Speciation in Darwin's Finches
Seed sizes on the
second island
favor birds with
large beaks.
The population on
the second island
evolves into
population B, with
larger beaks.
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Changes in the Gene Pool
Speciation in Darwin's Finches
Reproductive Isolation
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If population B birds cross back to the first island, they will not mate
with birds from population A.
Populations A and B are separate species.
Patterns of Evolution
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Macroevolution refers to large-scale evolutionary patterns and
processes that occur over long periods of time.
Six important topics in macroevolution are:
• extinction
• adaptive radiation
• convergent evolution
• coevolution
• punctuated equilibrium
• changes in developmental genes
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Macroevolution
Extinction
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More than 99% of all species that have ever lived are now extinct.
Adaptive radiation is the process by which a single species or a small
group of species evolves into several different forms that live in
different ways.
For example, in the adaptive radiation of Darwin's finches, more than a
dozen species evolved from a single species.
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Adaptive Radiation
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Different organisms undergo adaptive radiation in different places or at
different times but in similar environments.
The process by which unrelated organisms come to resemble one
another is called convergent evolution. Results in analogous
structures.
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Convergent Evolution
Coevolution
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Sometimes organisms that are closely connected to one another by ecological
interactions evolve together.
The process by which two species evolve in response to changes in each other
over time is called coevolution.
Punctuated Equilibrium
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Darwin felt that biological
change was slow and
steady, an idea known as
gradualism.
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Punctuated equilibrium is a pattern of
evolution in which long stable periods are
interrupted by brief periods of more rapid
change.
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Developmental Genes and Body Plans
Hox Genes are the master control genes of body layout.
Evolution of Wings in Insects