Download 13.17 Natural selection cannot fashion perfect organisms

13.17 Natural selection cannot fashion perfect
organisms
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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Wing claw
(like dinosaur)
Long tail with
many vertebrae
(like dinosaur)
Teeth
(like dinosaur)
Feathers
13.17 Natural selection cannot fashion perfect
organisms
3. Adaptations are often compromises
4. Chance, natural selection and the environment
interact
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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
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13.14 Sexual selection may lead to phenotypic differences
between males and females
Sexual dimorphism
Weapons
female
male
male
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.
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
Andersson (1982) Nature 299:818
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
Survival Selection
Sexual Selection
Total male fitness
(survival + mating)
Fitness
Female choice
adaptive for survival
Fitness due to survival
Tail length
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  Showy, elaborate
 Solves a problem
 Impresses an audience
Natural Selection vs Sexual Selection
 Utilitarian, functional  Showy, elaborate
 Solves a problem
 Impresses an audience
 Sensible
 Whimsical
Natural Selection vs Sexual Selection
 Utilitarian, functional  Showy, elaborate
 Solves a problem
 Impresses an audience
 Sensible
 Whimsical
 Economical
 Wasteful
Natural Selection vs Sexual Selection
 Utilitarian, functional  Showy, elaborate
 Solves a problem
 Impresses an audience
 Sensible
 Whimsical
 Economical
 Wasteful
 Constructive
 Destructive
Natural Selection vs Sexual Selection
 Utilitarian, functional  Showy, elaborate
 Solves a problem
 Impresses an audience
 Sensible
 Whimsical
 Economical
 Wasteful
 Constructive
 Destructive
 Dull
 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. »