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Observing Variation
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(1) Variation exists in living populations.
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(2) Some variations are helpful and increase
life span.
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(3) Some variations are harmful and
decrease life span.
Observing Variation
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(4) A population may become physically
separated, so two groups form.
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(5) As mutations and meiosis occur, new
variations will appear.
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(6) Some offspring will survive better than
others.
Observing Variation
(7) Offspring that survive in one area may
not survive in another area.
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(8) Over time, more variations will
accumulate in the two populations.
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(9) The two populations will become
different because different variations occur
in the two groups.
Observing Variation
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(10) Individuals that die out do not get to
reproduce.
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(11) In some cases, the two groups become
so different from each other that they can no
longer interbreed.
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(12) Sometimes, a new species has formed.
What is this Process?
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Evolution of species
A set of natural processes that causes
change in a population of living things over
time.
Biological evolution, simply put, is descent
with modification.
Why was this little sticker so controversial?
Source: http://www.swarthmore.edu/NatSci/cpurrin1/textbookdisclaimers/CobbDisclaimer.jpg
Understanding Evolution
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Scientists study natural processes and look
for natural explanations.
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I will introduce you to some of the evidence
that leads biologists to view evolution as the
great unifying theory of the field.
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Science cannot answer every question.
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What if you disagree with photosynthesis?
Evolution by Natural Selection
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Variation exists in every population.
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Sources of inherited variation: meiosis,
crossing over, mutation.
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Natural selection (predators, disease,
drought, cold, famine, flood, heat…..)
causes some individuals to survive while
others do not.
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Allele frequency change is called evolution.
Sometimes it causes speciation.
Charles Darwin
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Age 25: set sail on 5 year voyage on HMS
Beagle as “naturalist”.
Darwin’s Ideas Did Not Develop in a Vacuum
Contributor’s to Darwin’s thinking included:
Charles Lyell – uniformitarianism.
1797-1875
Georges Cuvier – species extinction.
1769-1832
Darwin’s Ideas Did Not Develop in a Vacuum
Contributor’s to Darwin’s thinking included:
Thomas Malthus – struggle for existence.
1766-1834
Jean Baptiste de Lamarck – evolution by
acquired characteristics.
1744-1829
Jean Baptiste de Lamarck
“Inheritance of Acquired Characteristics”
Animals face a need to change.
• Animals make themselves change
• Animals pass on the changes to offspring.
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Darwin observed
animals and plants in
the Caribbean, South
America, Galapagos,
Hawaii, Indonesia,
Africa & Europe.
He wrote three books
about his experiences.
He took massive notes
in several notebooks,
working on Origin of
Species 20 years!
Common Ancestor
The central idea of biological evolution is that all
life on Earth shares a common ancestor, just as
you and your cousins share a common
grandmother.
Darwin’s Voyage of Discovery
A reconstruction of the HMS Beagle sailing off Patagonia.
The Voyage of the Beagle
Galapagos Islands and South American Mainland
Intra-Galapagos Variability:
Darwin’s Finches
Evolution by Natural Selection
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Variation exists in a population. (Darwin
did not know how the variation was
passed…genetics was not yet understood)
Some organisms survive and pass on
genes…some do not.
Causes of this evolution: migration,
“heredity”, mate selection, predators,
disease, famine, drought,
Lamarck and Darwin: How would
they explain these observations?
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In 1960, a pesticide was sprayed at Tybee
Island, GA, killing 97% of all mosquitoes.
In 1979, the same pesticide killed less than
30% of all the mosquitoes.
Lamarck:
Darwin:
Darwin’s Observations and Inferences
Observation 1: Left unchecked, the number of
organisms of each species will increase
exponentially, generation to generation.
Observation 2: In nature, populations tend to
remain stable in size.
Observation 3: Environmental resources are limited.
Inference 1: Production of more individuals than can be supported by the
environment leads to a struggle for existence among individuals, with
only a fraction of offspring surviving in each generation.
Darwin’s Observations and Inferences
Observation 4: Individuals of a population vary extensively in their
characteristics with no two individuals being exactly alike.
Observation 5: Much of this variation between individuals is heritable.
Darwin’s Observations and Inferences
Inference 2: Survival in the struggle
for existence is not random, but
depends in part on the heritable
characteristics of individuals.
Individuals who inherit
characteristics most fit for their
environment are likely to leave
more offspring than less fit
individuals.
Darwin’s Observations and Inferences
Inference 3: The unequal ability of individuals to survive and
reproduce leads to a gradual change in a population, with favorable
characteristics accumulating over generations (natural selection).
Taken together, these three inferences are a statement of Darwin’s
Theory of Evolution.
The Modern Synthesis
1932-1953
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3)
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Mutation and segregation result in
large variability within populations
Individuals pass alleles to offspring
Differential reproductive success
Adaptation increases allelic fitness
How can allele frequency change?
1. Migration
2. Meiosis and crossing over
3. Mutation
4. Natural Selection (predators, disease,
famine, drought)
5. Genetic drift (chance changes to small
populations)
How can evolution occur?
1. Migration
2. Meiosis and crossing over
3. Mutation
4. Natural Selection (predators, disease,
famine, drought)
5. Genetic drift (chance changes to small
populations)
Evidence evolution
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Biogeography
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Homologous or vestigial structures
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Homologous DNA and proteins
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Evolution observed right now
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Fossil evidence
Homologous skeletal structure
Why use the same skeletal plan for these very
different appendages?
Vestigial whale pelvis bones
Vestigial anthers and pollen:
Dandelions are asexual
Why does organism X have a
feature which looks exactly like
a weakened, not fully effective,
overly-complex version of an
organ found in its evolutionary
ancestors, if not for common
descent via natural selection
and mutation?
Why should different organism possess related genes?
Why does the degree of
relationship of genes
match their degree of
relationship established
by other methods?
Evidence for
Evolution
– Evolution Observed
Evolution of pesticide
resistance in response to
selection.
Explain how this is
perceived as an example of
evolution.
Biogeography
Evolution of new species
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A population becomes divided by a physical
barrier (water, mountains, desert, or just a
large distance.
The two populations experience different
selection pressures and will evolve
separately.
Even if the two populations meet again,
they are now so different that they can no
longer interbreed. They are reproductively
isolated and are two distinct species.
Convergent
Evolution –
process by which
unrelated
organisms
independently
evolve similarities
when adapting to
similar
environments.
Nothing in biology
makes sense except in
the light of evolution.
Theodosius Dobzhansky