Download Activating Strategy AP Lesson #65

2/27/2011
Activating Strategy
• Think-Pair-Share
– Would you expect natural selection to favor prezygotic or post-zygotic isolating mechanisms
between sympatric species (ex. Donkey and
Horse)?
AP Lesson #65
EQ: At what rate does speciation
occur?
• Earths history shows periods of slow evolution
punctuated by periods of rapid evolution
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What factors govern the rate of speciation?
• Mutation Rates
– a positive force, providing new genetic material
– could change 1 gene or many
• Selection Rates
– a negative force, removing mutations that are not
beneficial
• Genetic Drift
• Once structures and/or
behaviors evolve it is
difficult to change them
radically
Where can we study speciation?
• Hybrid Zones
– Reproductive barriers have been removed
– Members of different species meet and mate
How quickly does speciation occur?
• Current debate:
Does speciation happen gradually or rapidly?
– Gradualism
• Charles Darwin
• Charles Lyell
– Punctuated equilibrium
• Stephen Jay Gould
• Niles Eldredge
Niles Eldredge
Curator of American
Museum of Natural History
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What are the patterns of speciation?
• "Normal"
– new species form in a "filled" environment
– leads to normal rates of evolution
• Explosive
– new species form in an empty environment
– competition between species is minimal but
increases with time
– differences are selected for
What is gradualism?
• Gradual divergence
over long spans of time
– assume that big
changes occur as the
accumulation of many
small ones
– Supported by transition
species
2006 Fossil Discovery of Early Tetrapod
• Tiktaalik
– “missing link” from sea to land animals
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What is punctuated equilibrium?
• Rate of speciation is not
constant
– rapid bursts of change
– long periods of little or no
change
– species undergo rapid
change when they 1st bud
from parent population
• Often occurs as a result of
genetic drift or natural
selection
What are some examples of Punctuated
Equilibrium at a species level?
• Polyploidy
– Errors during cell division
that result in extra sets of
chromosomes
• Resulting species have
a different number of
chromosomes than the
previous generation
• Occurs more often in
plants
What are the different types of polyploidy?
• Autopolyploid
– individual with more than two chromosome sets,
derived from one species
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
4n
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
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• Allopolyploid
– species with multiple sets of chromosomes derived
from different species
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Hybrid
with 7
chromosomes
Normal
gamete
n=3
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Viable fertile
hybrid
(allopolyploid)
2n = 10
What are some examples of Punctuated
Equilibrium on a larger scale?
• Cambrian explosion
• Diversification of Animals
– within 10–20 million years most of the major phyla of
animals appear in fossil record
543 mya
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Diversity of life & periods of mass extinction
Cambrian
explosion
Evolution is not goal-oriented!
Evolution is not always the survival of the fittest.
Rather it is the survival of the just good enough.
Surviving species
do not represent
the peak of
perfection. There
is compromise &
random chance
involved as well
Remember that for
humans as well!
Summarizing Strategy
• Quick Write:
– In 3 sentences or less explain the following:
– What is the difference between gradualism and
punctuated equalibrium?
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Assessment
• HW: Read Chapter 25
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