Developing the Theory of Evolution
Darwin on the HMS Beagle
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When Darwin began his journey, most people thought the world was ~6000
years old, and that animals and plants were unchanging.
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Darwin’s job on the Beagle was to be a naturalist, and to collect biological
and geological specimens during the travels.
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Darwin’s Theory of Evolution by Natural Selection
Developing the Theory of Evolution
The Galápagos Islands
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Darwin noticed that the different islands all seemed to have their own, slightly
different varieties of animals.
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Almost every specimen Darwin collected on the Galápagos was new to
European scientists, though they were similar to species on mainland South
America.
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Island and mainland species should not have been so similar, unless
mainland species had changed since arriving on the islands.
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Darwin’s Theory of Evolution by Natural Selection
Developing the Theory of Evolution
Darwin continued his studies
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Darwin hypothesized that new species could appear gradually through small
changes in ancestral species.
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Darwin inferred that if humans could change species by artificial selection –
directed breeding to produce offspring with desired traits, then perhaps the
same process could work in nature.
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Darwin’s Theory of Evolution by Natural Selection
Developing the Theory of Evolution
Natural selection
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Darwin described the process of natural selection: some competitors would
be better equipped for survival than others, those less equipped would die.
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Darwin’s Theory of Evolution by Natural Selection
Developing the Theory of Evolution
Natural selection
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Principles of natural selection:
• Individuals show variation
• Variations are heritable
• More offspring are born than can survive
• Variations that increase reproductive success will be more common in
the next generation
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Darwin’s Theory of Evolution by Natural Selection
The Origin of Species
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Darwin published On the Origin of Species by Means of Natural Selection in
1859.
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Today, scientists use evolution to mean cumulative change in a group of
organisms through time.
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Natural selection is not synonymous with evolution – it is a mechanism by
which evolution occurs.
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Darwin’s Theory of Evolution by Natural Selection
Support for Evolution
Comparative anatomy
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Similar structures inherited from a common ancestor are called homologous
structures.
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Evolution predicts that an organism’s body parts are more likely to be
modifications of ancestral body parts than entirely new structures.
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Evidence of Evolution
Support for Evolution
Comparative anatomy
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Analogous structures can be used for the same purpose and be superficially
similar in construction, but are not inherited from a common ancestor.
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Analogous structures show that functionally similar features can evolve
independently under similar conditions.
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Evidence of Evolution
Adaptation
Types of adaptations
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An adaptation is a trait shaped by natural selection that increases an
organism’s reproductive success.
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Fitness is a measure of the relative contribution an individual trait makes to
the next generation.
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The better an organism is adapted to its environment, the greater its chances
of survival and reproductive success.
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Evidence of Evolution
Adaptation
Types of adaptations
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Camouflage is a suite of morphological adaptations that allow an organism to
blend into its environment.
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Mimicry is a type of morphological adaptation where a species evolves to
resemble another species.
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Antimicrobial resistance develops in some bacteria in response to sub-lethal
exposure to antibiotics.
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Evidence of Evolution
Mechanisms of Evolution
Genetic drift
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Any random change in the allelic frequencies in a population results in
genetic drift.
The genes of the next generation will be the genes of the "lucky" individuals,
not necessarily the healthier or "better" individuals.
It happens to ALL populations — there's no avoiding the vagaries of chance.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Genetic drift
The founder effect results when a group of individuals with a different allele
frequency than the original population becomes isolated.
• Alleles that were infrequent in the original population may be common in
the new population.
• Happens when a subset of organisms settles in an area separated from
their original population – Pennsylvanian Amish
• Can result in large genetic variations in the separated population
Ellis-van Creveld Syndrome
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Shaping Evolutionary Theory
Mechanisms of Evolution
Genetic drift
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A bottleneck results when population declines to a very low level and then
rebounds.
Reduced genetic variation: population may not be able to adapt to new
selection pressures, such as climatic change or a shift in available
resources, because the genetic variation that selection would act on may
have already drifted out of the population.
Ex: Extreme hunting of the elephant seal
Reduced its numbers to only 20 individuals
at the end of the 19th century. Though their
population has increased dramatically, they
Have less genetic variation than elephant
Seals who were not so intensely hunted.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Gene flow
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A population in genetic equilibrium experiences no gene flow – it is a closed
system.
Uncommon in natural populations – organisms migrate/move between
populations
Random movement increases genetic variation within a population and
decreases the genetic variation between populations.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Nonrandom mating
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A population in genetic equilibrium must exhibit random mating.
This rarely occurs in populations
• Promotes inbreeding – mating occurs between individuals in close
proximity (e.g. neighbors)
• Changes allelic frequencies, favoring individuals that are homozygous
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Shaping Evolutionary Theory
Mechanisms of Evolution
Mutation
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A mutation is a random change in genetic material.
The cumulative effect of mutations in a population might shift allelic
frequencies.
Most mutations are harmful, but some may be beneficial and become more
common.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Natural selection
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Natural selection acts to select the
individuals that are best adapted
for survival and reproduction.
Three types of natural selection:
• Stabilizing
• Directional
• Disruptive
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Shaping Evolutionary Theory
Mechanisms of Evolution
Natural selection
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Stabilizing selection operates to eliminate extreme expressions of a trait
when the average expression leads to higher fitness.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Natural selection
•
Directional selection increases the expression of an extreme version of
a trait in a when it increases fitness.
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Shaping Evolutionary Theory
Mechanisms of Evolution
Natural selection
•
Disruptive selection removes individuals with average traits, but retains
individuals expressing extreme traits .
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Shaping Evolutionary Theory
Mechanisms of Evolution
Natural selection
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Sexual selection drives change in the frequency of a trait based on the
ability to attract a mate.
Common in populations where males and females look significantly
different
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Shaping Evolutionary Theory
Speciation
Allopatric speciation
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Allopatric speciation occurs when populations are divided by a physical
barrier.
Most scientist think allopatric speciation is the most common type of
speciation.
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Shaping Evolutionary Theory
Speciation
Sympatric speciation
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Sympatric speciation occurs without a physical barrier; populations
remain in close proximity.
Common in plants due to polyploidy
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Shaping Evolutionary Theory
Patterns of Evolution
Convergent evolution
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Unrelated species evolve similar
traits even thought they live in
different parts of the world.
Occurs in environments that are
geographically distinct but
ecologically similar
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Shaping Evolutionary Theory
Patterns of Evolution
Rate of speciation
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Evolution proceeds in small, gradual steps according to a theory called
gradualism.
Punctuated equilibrium states rapid spurts of genetic change causing
rapid speciation punctuate long periods of little change.
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Shaping Evolutionary Theory