Download Natural Selection

Charles Darwin 1809-1882
Natural Selection
Natural Selection “Survival of
the fittest”
*Natural Selection
1. There is genetic variation in populations
2. There is competition/a struggle for existence
in nature—for food, resources, mates, etc.
Invidiuals which are more fit in a specific
environment have a better chance of surviving
and reproducing than less fit organisms.
3. Over time, generations will consist of
organisms whose genetic variations aid
survival and reproduction in that
environment.
Natural Selection
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Important note: The fittest does not
always mean the largest, strongest, or
smartest.
Fitness depends upon specific
environmental conditions.
Sometimes fittest means smallest, or best
camouflaged, or most adaptable, etc.
Evolution
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Natural Selection leads to evolution,
because natural selection causes change
Evolution—change in a
population/species over time
Artificial Selection
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Sometimes evolution can be “man-made”
Selective breeding— When humans breed
exceptional plants and animals in order to
produce desired offspring.
Ex: racing horses, purebred dogs
Genetic engineering – Evolution by design
Mechanisms of Evolution—how
it happens
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Populations (species) evolve, not
individuals.
Evolution occurs when a population’s
gene frequencies change over time.
Small populations that interbreed will
have many mutations and therefore will
see greatest potential for evolutionary
changes.
The genes
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Gene pool - a population’s collective genes
Allelic frequency—The % of an allele in a
population (ex: 25% “t” in a pea plant
population)
Genetic Equilibrium—When the allelic
frequency remains the same over generations
(no evolution) This can happen when there is
no movement into or out of a population.
Mechanisms of Evolution
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So, when a population is NOT in genetic
equilibrium, when the gene pool changes,
populations evolve.
When mutations occur, creating new
alleles, and individuals with that mutation
survive and reproduce, the gene pool is
altered, leading to evolution.
Natural Selection Revisited
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There are 3 types of natural selection:
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Stabilizing Selection—the average
individual is selected for
Disruptive Selection—Both extreme
variations of a trait are selected for
Directional Selection—One extreme
variation of a trait is selected for
3 types of selection graphs
Genetic Drift “Evolution by
chance”
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Genetic drift—Changes in the allelic
frequency of a population due to chance events.
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Founder effect—evolution as the result of a
migration of a small subgroup of a population to a
new environment.
Population Bottleneck—When an environmental
catastrophe drastically reduces the numbers of a
population, eliminating huge amounts of genetic
diversity. Ex: humans 70,000 years ago
Speciation
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Speciation—Evolution of a new species
Reproductive Isolation
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Reproductive isolation—When subgroups
of a population can no longer interbreed
and produce fertile offspring
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Behavioral isolation—isolation due to
different courtship rituals or reproductive
strategies
Ex: Eastern & Western Meadowlark songs
Reproductive Isolation
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Temporal Isolation—Isolation due to
population groups reproducing at different
times.
Ex: flowers being pollinated at different times
of the year
Reproductive Isolation
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Geographic isolation—Isolation of
populations due to geographic barriers
Ex: South American and African monkeys
Adaptive Radiation
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Adaptive Radiation—When one ancestral
species spreads out geographically, and many
new species form over time.
One species radiates outward and each subgroup
adapts to its own unique environment
Speciation of multiple species from one ancestral
species—a type of divergent evolution
Ex: “Darwin’s Finches” of the Galapagos Islands
Darwin’s Finches—Adaptive
Radiation
Mutations lead to Evolution
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DNA mutations can lead to new genes/new
traits in individuals within a population.
If these new traits are favorable in the
organisms’ environment, and help it
survive and reproduce, then the genes
will be passed on to offspring.
The greatest potential for evolutionary
change would involve a small population,
with many mutations
Mutations and bacteria
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Antibiotic Resistance—DNA mutations in
bacteria can prevent them from being killed
by medicine. They survive, then continue
to grow and divide as bacteria do. Soon,
all of that type of bacteria has evolved
resistance.
Gradualism vs. Punctuated
Equilibrium
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Gradualism—
Speciation occurs at a
slow, steady pace
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Punctuated
Equilibrium—
Speciation occurs
quickly, with long
periods of genetic
equilibrium in
between
Most likely both are true, both are supported
by fossil evidence