Evolution and Sex

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Evolution and Sex
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Quote of the Day

• The sight of the peacocks tail makes me sick.
• Charles Darwin

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Whats so disgusting about the peacock?
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Darwins Dilemma

• Many species have characteristics that seemingly
  should be eliminated by natural selection.
• Bright coloring and attention-getting behavior
  tend to attract predators.
• Large body size or highly exaggerated physical
  features require a large amount of energy to
  produce.

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Darwins Solution

• Sexual Selection the theory that competition for
  mates between individuals of the same sex drives
  the evolution of certain traits.
• Seemingly harmful traits can evolve when the
  sexual attractiveness of the trait outweighs the
  problem it presents for survival.
• Whereas natural selection is sometimes called the
  survival of the fittest, sexual selection can
  be thought of as the survival of the sexiest.

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Causes of Sexual Selection Male Competition

• Males have lots of sperm and often reproduce with
  several different females.
• Generally, males compete for the opportunity to
  mate with females.

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Size Differences

• Larger, stronger, and tougher males are typically
  more successful in competitions for females and
  more likely to be able to defend their offspring.
• Over time, great size differences can evolve in
  species when males compete for access to females.
• Examples baboons, lions, moose, crocodiles

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Example Elephant Seals

• Males (bulls) 4000-6000 lbs, up to 20 ft long
• Females (cows) 1300-2000 lbs, 10 ft long

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Causes of Sexual SelectionFemale Choice

• Females have fewer eggs and invest more time and
  energy in reproduction.
• Generally, females hold out for the best genes
  and/or the most helpful mates.

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Exaggerated Physical Features

• When females choose a trait that is closely
  linked to successful genes, that trait spreads
  throughout the population over generations.
• Sometimes these traits can become highly
  exaggerated, as seen in the peacock.

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Exaggerated Physical Features in Males
Lion
Frigate bird
Pheasant
Elk
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Comparison of Natural and Sexual Selection
What is selected for? Who or what does the selecting?
Natural Selection The best survivors Climate, Predators, Competitors, Disease
Sexual Selection The best reproducers Members of the opposite sex
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Runaway Sexual Selection

• When males with a specific trait mate with
  females that prefer that trait, the trait and the
  preference for it become genetically linked.
• This can lead to a "runaway" process, causing the
  male trait to become more and more extreme until
  the survival disadvantages of the trait outweigh
  the sexual attractiveness.
• This runaway sexual selection may eventually
  lead to extinction of a species.
• The Fate of the Irish Elk?

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