Transcript How Does Evolution Work?

HOW DOES EVOLUTION
WORK?
By: A Good Student
What is Evolution?

Evolution is descent with modification.
On a small scale, this can be changes in the gene
frequency of a population over time.
 On a large scale, this can be the descent of different
species from a common ancestor

How Does Evolution Work?
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
Evolution depends on genetic variation and the
resulting phenotypic variation in populations.
Sources of genetic variation
 Mutations
 Recombination
 Sex
Mechanisms of Evolution
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Natural Selection
 Sexual
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Selection
Mutation
Genetic Drift
Gene Flow (Migration)
Natural Selection
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Variation exists in populations
Variation is heritable
Variation influence reproductive success
Helpful variations are passed on to the next
generation in greater abundance
Natural Selection: an example
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Variation in beak size in Galapagos finches
Beak size is a heritable trait
After a drought, individuals with bigger beaks
survived better and reproduced more often
The next generation had a larger average beak
size
Sexual Selection
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A “special case” of natural selection
Occurs because of nonrandom mating
Acts on an individual’s ability to attract a mate
 Example:
Male guppies have bright spots to attract
females
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Can work in the opposite direction of natural
selection
 Example:
Predators can see bright males more easily
Mutation
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A mutation could cause parents with genes for one
trait to produce children for genes with another trait
Example:
 Marfan
Syndrome
 Spontaneous
mutation causes a connective tissue disorder
 This is passed on to offspring
Genetic Drift
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Due to random factors or chance (not selective
pressures), individuals with one genotype produce
more offspring
Occurs in small population sizes
 Founder
effect: a small group breaks off from the
large, limiting the gene pool
 Bottleneck effect: population size reduced, genetic
variation decreases
Genetic Drift: an example
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Northern elephant seals were hunted by humans
 Decreased
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population size and genetic variation
Population size has rebounded
Genetic variation has not
 Display
less variation than southern elephant seals that
were hunted less
Gene Flow
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When organisms from one population migrate to
another population, they change the composition of
the gene pool of both populations
Example:
 If
all the red headed people left Scotland, the next
population would have less people with the trait. The
place where they migrated to, would have more.
Speciation
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Mutation, Natural Selection, and Genetic Drift
would all act to increase the likelihood of speciation
because they increase genetic diversity between the
two populations.
Gene flow would reduce the likelihood of speciation
because it makes populations more genetically
similar.
Speciation
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Speciation occurs when two or more species (group of interbreeding
individuals) come from a single species. The general causes of
speciation are genetic drift and gene flow.
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Allopatric speciation: occurs when a specific barrier exists to isolate two
populations geographically, preventing them from interbreeding
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Peripatric speciation: a type of allopatric speciation that occurs when one of
the isolated populations is very small
Parapatric speciation: occurs when there is no specific geographic
barrier present; organisms are more likely to reproduce with individuals
closer to them than further away in the geographic range of the species
Sympatric speciation: occurs when there is no geographic distance or
barrier present; instead some individuals might start to exploit a new
niche, creating a divide between the population
Sources
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http://evoultion.berkeley.edu/evosite/Evo101.html
http://evolution.berkeley.edu/evosite/search/searc
h_lessons.phpsort_by=audience_rank&topic_id=&k
eywords=teach+about+homologies&Submit=Searc
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