13.17 natural selection cannot fashion perfect organisms sexual selection.pdf · 13.17 natural...
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1. Selection can only act on existing variation
– Natural selection cannot conjure up new beneficial alleles
2. Evolution is limited by historical constraints
– Birds arose as the forelimb of a small dinosaur evolved into a wing
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13.17 Natural selection cannot fashion perfect organisms
Wing claw (like dinosaur)
Feathers
Teeth (like dinosaur)
Long tail with many vertebrae (like dinosaur)
3. Adaptations are often compromises
4. Chance, natural selection and the environment interact
Copyright © 2009 Pearson Education, Inc.
13.17 Natural selection cannot fashion perfect organisms
Sexual Selection
13.14 Sexual selection may lead to phenotypic differences between males and females In many animal species, males and females show
distinctly different appearance, called sexual dimorphism
Copyright © 2009 Pearson Education, Inc.
13.14 Sexual selection may lead to phenotypic differences between males and females
Sexual dimorphism
Sexual size dimorphism
13.14 Sexual selection may lead to phenotypic differences between males and females
Intrasexual competition involves competition for mates, usually by males
13.14 Sexual selection may lead to phenotypic differences between males and females
In intersexual competition (or mate choice), individuals of one sex (usually females) are choosy in picking their mates, often selecting flashy or colorful mates
Copyright © 2009 Pearson Education, Inc.
Andersson (1982) Nature 299:818
Male ornaments
Long-tailed widowbird
Experimental manipulation of tail length
Males with unnaturally long tails attract
females away from the nests of “normal”
males or males with shortened tails
Male ornaments
Barn swallow
Experimental manipulation
of tail length
Males with elongated tails:
1. Obtain mates more
quickly
2. Have greater
reproductive success
3. Experience a cost in
tail size the following year
From Moller (1994)
There are several possible answers to explain how these seemingly disadvantageous genes spread through the population, among them:
Runaway selection:
Fisher’s runaway model F
itn
ess
Tail length
Total male fitness
(survival + mating)
Fitness due to survival
Survival Selection
Sexual Selection
Female choice
adaptive for survival
Runaway Selection, ctd.
Once this has happened, the process may run out of control, until the male trait becomes so exaggerated that it is disadvantageous. In other words, female preference, instead of survival advantage, may begin to drive the evolution of ever-longer tails, until males are encumbered by showy plumage that no longer helps them avoid predation.
Natural Selection vs Sexual Selection
Utilitarian, functional Showy, elaborate
Natural Selection vs Sexual Selection
Utilitarian, functional
Solves a problem
Showy, elaborate
Impresses an audience
Natural Selection vs Sexual Selection
Utilitarian, functional
Solves a problem
Sensible
Showy, elaborate
Impresses an audience
Whimsical
Natural Selection vs Sexual Selection
Utilitarian, functional
Solves a problem
Sensible
Economical
Showy, elaborate
Impresses an audience
Whimsical
Wasteful
Natural Selection vs Sexual Selection
Utilitarian, functional
Solves a problem
Sensible
Economical
Constructive
Showy, elaborate
Impresses an audience
Whimsical
Wasteful
Destructive
Natural Selection vs Sexual Selection
Utilitarian, functional
Solves a problem
Sensible
Economical
Constructive
Dull
Showy, elaborate
Impresses an audience
Whimsical
Wasteful
Destructive
Exciting
On the Origin of Species (1859)
Sexual selection disconnected from struggle for existence: « ...what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. »