Transcript Evolution - Valhalla High School

Evolution
Chapter 16
regents
How Common Is Genetic Variation?
Many genes have at least two forms, or alleles.
All organisms have genetic variation that is “invisible”
because it involves small differences in biochemical
processes.
An individual organism is heterozygous for many genes.
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Variation and Gene Pools
A population is a group of individuals of the same species
that interbreed.
A gene pool consists of all genes, including all the different
alleles, that are present in a population.
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In genetic terms, evolution is any change in the
occurence of alleles in a population.
Sources of Genetic Variation
The two main sources of genetic variation are mutations
and the genetic shuffling that results from sexual
reproduction.
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Mutations
A mutation is any change in a sequence of DNA.
Mutations occur because of mistakes in DNA replication
or as a result of radiation or chemicals in the
environment.
Mutations do not always affect an organism’s phenotype.
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Gene Shuffling
Most heritable differences are due to gene shuffling.
Crossing-over increases the number of genotypes that
can appear in offspring.
Sexual reproduction produces different phenotypes, but
it does not change the relative frequency of alleles in a
population.
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16-2 Evolution as Genetic
Change
Natural selection affects which individuals
survive and reproduce and which do not.
Evolution is any change over time in the relative
frequencies of alleles in a population.
Populations, not individual organisms, can
evolve over time.
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Natural selection on single-gene traits can lead to
changes in occurences and thus to evolution.
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Genetic Drift
What is genetic drift?
• A random change in occurrence
Genetic drift may occur when a small group of
individuals colonizes a new habitat.
Individuals may carry alleles in different relative
frequencies than did the larger population from
which they came.
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Genetic Drift
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Descendants
Genetic Drift
Population A
Population B
When the occurrence of a trait changes due to migration of a small subgroup of a
population it is known as the founder effect.
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16-3 The Process of Speciation
Natural selection and chance events can change
the relative frequencies of alleles in a population
and lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that breed
with one another and produce fertile offspring.
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What factors are involved in the formation of new
species?
The gene pools of two populations must become
separated for them to become new species.
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Isolating Mechanisms
As new species evolve, populations become
reproductively isolated from each other.
When the members of two populations cannot
interbreed and produce fertile offspring,
reproductive isolation has occurred.
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How natural selection works
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
mutation!
How natural selection works
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
Generation 4: 0.12 not resistant
0.88 resistant
Testing Natural Selection in Nature
Studies showing natural selection in action involve
descendants of the finches that Darwin observed in the
Galápagos Islands.
The finches Darwin saw were different, but he
hypothesized that they had descended from a common
ancestor.
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Speciation in Darwin's Finches
Speciation in the Galápagos finches occurred by:
•
•
•
•
•
founding of a new population
geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
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Founders Arrive
A few finches—species A—travel
from South America to one of
the Galápagos Islands.
There, they survive and
reproduce.
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Geographic Isolation
Some birds from species A cross
to a second island.
The two populations no longer
share a gene pool.
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Changes in the Gene Pool
Seed sizes on the second island
favor birds with large beaks.
The population on the second
island evolves into population
B, with larger beaks.
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Reproductive Isolation
If population B birds cross back to the first island, they
will not mate with birds from population A.
Populations A and B are separate species.
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