Natural Selection and Evolution

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Natural Selection and Evolution
The central, unifying tenet of modern biology
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The Darwin myth revisited
Darwin was a product of his time and benefited
from the ideas of
Malthus
Lyell
Lamarck
Even his own grandfather
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He also took into account artificial selection
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Artificial selection is also called selective
breeding
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And he mulled over comparative physiology
Comparative embryology
Comparative anatomy homologous structures
Vestigial structures
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Then he went sailing
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And he saw fossil and living evidence of
biogeography
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But he DID miss some clues
The Galapagos has had a huge impact on
evolutionary science, but Darwin missed many of
the clues while he was there.
http//www.abdn.ac.uk/zoohons/lecture5/sld004.htm
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Returning home, he processed all hed learned
Correspondence with Alfred Russell Wallace
Insightful deductions
Careful research on barnacles and pigeons
Meticulous preparation
And the threat of Wallaces manuscript
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All of these led to The Theory of Natural
Selection
Because
Populations contain natural diversity,
AND
Many of those different traits are heritable
AND
Each population overproduces offspring
AND
Populations are limited by resources
THEN.
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A summary of natural selection
Individuals within a population would experience
differential survival, based on inherited traits.
AND
Small changes in the proportion of a trait in a
population could accumulate over time and lead to
entirely new species.
He called this descent with modification
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Support came pouring in

• Mendel, and his evidence of genetic crosses
  explained the mechanism of inheritance Darwin
  couldnt explain.

• Plate tectonics (1960s) verified ideas of
  bio-geography.

• Molecular evidence cell structures, proteins
  and particularly DNA all confirm common ancestry
  and strengthen the mechanism of natural
  selection mutations and inheritance.

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More support came pouring in

• Experimental evidence documented both natural
  selection and species change antibiotic
  resistance, pesticide resistance, sticklebacks,
  finches and more

• Exotic and invasive species have provided
  important field evidence of the mechanisms of
  species change.

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Evolutionary theory today The Modern Synthesis
Natural selection, Darwin style
PLUS
Genetic mechanisms that explain mutations and
patterns of inheritance
PLUS
Population biology

• An overarching principle of biology that is
  powerful, predictive, and elegant.

http//www.pbs.org/wgbh/evolution/darwin/origin/in
dex.html
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The fuel for natural selection
Natural selection depends on natural diversity.
The source of natural diversity is mutations in
the population.
Sexual reproduction reshuffles mutations, and
results in a wider array of traits than if the
population reproduces asexually.
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The fuel for natural selection, cont.
Imagine that you have 4 alleles for a gene.
That means one gene has mutated 3 times.
Since you have two copies of each gene, sexual
recombination means there are 16 different
combinations of genes and at least as many traits.
D-A D-B D-C D-D
C-A C-B C-C C-D
B-A B-B B-C B-D
A-A A-B A-C A-D
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If there was no recombining of genes in sexual
reproduction, there would be 4 different traits
in the population, not 16. That extra diversity
means many more chances for the population to
have a beneficial trait that may be selected by
the environment.
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Some definitions
A population a group of individuals of the same
species living in the same place at the same
time a breeding group.. The unit of evolution!
A species a group of populations whose
individuals have the potential to interbreed and
produce fertile offspring. Its a reasonable
working definition, but isnt perfect
Alleles different versions (mutations) of a
gene allele frequencies describe variations in
traits within a population.
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Natural selection is one of several mechanisms
When Darwin proposed Natural Selection, it was
offered as the only mechanism of evolution. As
we explore the ideas more, we have identified
many more mechanisms.
Also, we have separated evolution into two
categories when there are changes in gene
frequencies but no new species, that is
microevolution evolution that leads to new
species is called macroevolution. (see
evolution powerpoint 2)
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Microevolution change in a populations gene
pool

• If, for example, the proportions of the gene for
  long necks in a population of giraffes changes
  over a few generations, that is microevolution.
  (allele frequency changes)

• or if the proportion of genes for big beaks
  changes in Galapagos finches after a period of
  drought, that is microevolution. (allele
  frequency changes)

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• or if the proportions of genes for antibiotic
  resistance changes over time, that is
  microevolution. (allele frequency changes)

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When does microevolution become macroevolution?
If the changes in gene frequency are big enough
to prevent two populations from breeding, then
you will have arrived at the doorstep of
macroevolution, the creation of new species.
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Hardy-Weinberg what would it take for no
change?
Hardy and Weinberg were instrumental in defining
all of the mechanisms of evolution when they
identified 5 conditions under which evolution
would NOT occur. The first four all apply to
microevolution isolation is the key to
macroevolution.

• large population

• no new mutations

• random mating

• no natural selection

• isolated population (no migration)

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But no populations conform to Hardy-Weinberg, so
This identified the mechanisms of microevolution
as
mutations
Immigration/migration
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Mechanisms of microevolution, cont.
Genetic drift (random events)
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Mechanisms of microevolution, cont.
Mate choice (sexual selection)
And, finally, Natural selection
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More information on the mechanism of genetic drift
One dramatic insight from Hardy Weinberg was
that random events can influence allele
frequencies.
e.g. Bottleneck effects when plain old bad luck
reduces your gene pool the resulting genes
arent necessarily related to fitness.
e.g. Cheetahs Elephant seals
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Still more information on genetic drift
A final random mechanism is
Founder effect when a new population is
established from a small sub-set of the original,
the allele frequencies in the new population are
most often different. The 30 South American
finches that were swept to the Galapagos did not
represent the allele frequencies of the S.
American population.
www.umanitoba.ca/.../ 121/fyde/founder.html
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Outcomes of the mechanism of Natural Selection
Differential survival based on TRAITS, can lead
to different patterns
Stabilizing selection occurs when extreme
traits (and their genes) at both end of a bell
curve are selected against.
http//www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_St
abilizing.html
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Outcomes of natural selection, cont.
Another option is directional selection
When one extreme is favored, the allele frequency
may shift toward that extreme.
http//www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_Di
rectional.html
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Outcomes of natural selection, cont.
Lastly, selection may cause two extremes to
become more common by acting against the middle
trait. Diversifying (or disruptive) selection
http//www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_Di
sruptive.html
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A further mechanism sexual selection
Mate choice and different selective pressures for
each sex can result in sexual dimorphism and
exaggerated traits.
Elaborate traits only work if selective pressure
(natural selection) is low.
Study John Endlers guppies http//www.pbs.org/w
gbh/evolution/sex/guppy/index.html
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Important parting words on microevolution
These mechanisms all act TOGETHER to create the
complex interactions that lead to evolution.

• Organisms are constrained by their genetic
  history

• Adaptations are compromises

• Selection can only edit existing variation new
  alleles do not arise on demand (Campbell, p.
  277)

• Natural selection doesnt result in the best,
  just in better than the rest species

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Natural Selection is commonly misunderstood

• There is no PERFECT design. Each population has
  different mutations and different environments
  that act as selective agents on favored or
  unfavored traits.

• Survival of the fittest does not mean survival
  of the strongest. Being fit may, in some
  environments mean being slow, or sneaky, or
  watchful or camouflaged or it may result from a
  relationship with another organism.

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More misunderstandings

• Individuals DO NOT evolve populations do. That
  means individuals do not adapt to their
  surroundings. They are stuck with the traits they
  inherited.

• Mutations DO NOT arise when you need them
  evolution is not goal-driven.

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More web resources
http//www.pbs.org/wgbh/evolution/
http//www.lifesci.ucsb.edu/eemb/faculty/endler/re
search/research.html
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Evolution, part II macroevolution
Stay tuned for The origin of species.. Part II
Coming soon to a powerpoint near you