Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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. 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 Copyright © 2009 Pearson Education, Inc. 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 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. »