13.17 Natural selection cannot fashion perfect organisms

Transcription

1 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 Copyright 2009 Pearson Education, Inc.

2 Wing claw (like dinosaur) Teeth (like dinosaur) Long tail with many vertebrae (like dinosaur) Feathers

3 13.17 Natural selection cannot fashion perfect organisms 3. Adaptations are often compromises 4. Chance, natural selection and the environment interact Copyright 2009 Pearson Education, Inc.

4 Sexual Selection

5 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.

6 13.14 Sexual selection may lead to phenotypic differences between males and females

7 Sexual dimorphism Weapons male male female

8 Sexual size dimorphism Sexual dimorphism

9 13.14 Sexual selection may lead to phenotypic differences between males and females Intrasexual competition involves competition for mates, usually by males

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11 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.

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13 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

14 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)

15 There are several possible answers to explain how these seemingly disadvantageous genes spread through the population, among them: Runaway selection:

16 Fitness Fisher s runaway model Female choice adaptive for survival Survival Selection Sexual Selection Total male fitness (survival + mating) Fitness due to survival Tail length

17 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.

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19 Natural Selection vs Sexual Selection Utilitarian, functional Showy, elaborate

20 Natural Selection vs Sexual Selection Utilitarian, functional Solves a problem Showy, elaborate Impresses an audience

21 Natural Selection vs Sexual Selection Utilitarian, functional Solves a problem Sensible Showy, elaborate Impresses an audience Whimsical

22 Natural Selection vs Sexual Selection Utilitarian, functional Solves a problem Sensible Economical Showy, elaborate Impresses an audience Whimsical Wasteful

23 Natural Selection vs Sexual Selection Utilitarian, functional Solves a problem Sensible Economical Constructive Showy, elaborate Impresses an audience Whimsical Wasteful Destructive

24 Natural Selection vs Sexual Selection Utilitarian, functional Solves a problem Sensible Economical Constructive Dull Showy, elaborate Impresses an audience Whimsical Wasteful Destructive Exciting

25 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.»