Transcript The theory of evolution

THE THEORY OF
EVOLUTION
Natural Selection & Speciation
Fossils shape ideas about evolution
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Scientists used fossils as a
basis for evolutionary concepts
When geologists provided
fossil evidence of the age of
Earth, biologists began to
suspect that life changes
slowly over time
Evolution = the change in
populations over time
Charles Darwin
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English Scientist
 Lived
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from 1809 – 1882
Took a job as a naturalist on the
HMS Beagle (1831)
 Sailed
to South America & the South
Pacific on a 5 yr journey
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Darwin studied & collected
biological specimens every time the
ship stopped
 He
became curious about the possible
relationships between species.
The Galapagos Islands
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A group of small islands
~ 1000 miles off the
coast of S. America
 Near
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the equator
Darwin studied species
unique to the islands
 Similar
to species
elsewhere
 Could not explain how
such changes would occur
Figuring out why
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Thomas Malthus – English economist
 Ideas
about human population growth
 Said that the human population grows faster than
Earth’s food supply
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Darwin related this to his studies:
 Many
species produce more offspring than can survive
 They compete for resources like food, space, predation,
mating, and shelter
 Only some individuals survive
Two Kinds of Selection
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Natural Selection
a mechanism for change in populations – “survival of the
fittest”
 Organisms with certain variations survive & reproduce to pass
on their variations to offspring
 Organisms without beneficial variations are less likely to survive
and reproduce
 Result = each generation inherits beneficial variations and
others die out
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Artificial Selection
Organisms are purposely bred for desirable traits
 Examples: Horses, dog breeds
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Types of Natural Selection
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Stabilizing Selection = favors average individuals
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Directional Selection = one extreme is favored
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Spiders: Large individuals may be easier prey, small individuals
may not be able to get enough food
Woodpeckers: short or average beak cannot reach deep enough
to get food, long beaks can feed on insects deeper within the tree
trunk
Disruptive Selection = either extreme is favored
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Dark brown & white marine limpets: white limpets have an
advantage on light rocks, brown limpets have an advantage on
darker/wet rocks. Tan limpets are easily seen and preyed upon
by birds - tan limpets have the least favored adaptation.
Alfred Russell Wallace
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Reached a similar conclusion
Darwin & Wallace presented their ideas to the
scientific community
Darwin published On the Origin of Species in 1859
There have been many advances since Darwin:
 Any change in the gene pool of a population is now
considered part of evolution
Adaptations
Structural Adaptations:
 Adaptations = any structure, behavior, or internal
process that makes an organism more likely to survive
 develop over generations
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Examples: Giraffe height, mole-rats
Physiological Adaptations
 Develop more quickly than structural adaptations
 Examples: antibiotics (penicillin) & pesticides
Other Adaptations
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Mimicry = one species
resembles another
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“warning” coloration = red,
orange, and yellow
Camouflage = organisms
are able to blend into
surroundings
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Insects
Other Evidence of Evolution
1. Fossils – provide a record of early life and
evolutionary history
2. Embryology
 Embryos
at early stages of development are very
similar in appearance
3. Biochemistry
 Comparisons
of DNA or RNA of different species
Other Evidence of Evolution
4. Anatomy

Homologous structures – similar structures with different
functions
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Example: animal forelimbs have the same bone structure even though
they have different functions
Analogous structures – similar functions with different
structure
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Example: wings of a butterfly & wings of a bird have the same
function but different structure
15.2
Population Genetics
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Darwin developed his theory
without an understanding of
genes & inheritance
Population genetics = studies of
the behavior of genes in
populations of plants or animals
Natural selection acts on a range
of phenotypes in a population
If a variation of a phenotype is
beneficial, it will be more
common
Evolution of Populations
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All of the alleles of a population’s
genes = its’ gene pool
Genetic Equilibrium
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Genetic equilibrium = the frequency of alleles
remains the same over generations
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Populations in genetic equilibrium are not “evolving”
Disruptions to genetic equilibrium:
Mutations
 Genetic drift (chance alterations to allele frequency)
 Movement of individuals in or out of the population
 Size of population – can cause recessive alleles to become
more common
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Evolution of a species
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Speciation = evolution of a new species
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Occurs when members of similar populations no longer
interbreed to produce fertile offspring
Causes of speciation:
Geographic isolation – physical barriers separate populations
& each develops its own gene pool
 Reproductive isolation – formerly interbreeding organisms can
no longer mate (may be caused by behavior such as breeding
seasons)
 Chromosomal change – mistakes during cell division may result
in polyploidy
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Speciation can occur at different rates
Gradualism = species originate through
gradual changes and adaptations
 Punctuated equilibrium = change occurs in
rapid bursts with period of genetic equilibrium
in between
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 May
be caused by environmental changes or
competitive species
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Scientists agree that either method can occur,
depending on the circumstances
Patterns of Evolution
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Adaptive radiation = when a species evolves into an
array of species
Example: Hawaiian Honeycreepers
 Common on islands
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Divergent evolution – a diverse group of species
share a common ancestor
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A type of adaptive radiation
Convergent evolution – distantly related species
develop similar traits
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Similar environmental