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Be careful about adaptationist explanations… 5) A feature may be a by-product of selection for another trait.

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Presentation on theme: "Be careful about adaptationist explanations… 5) A feature may be a by-product of selection for another trait."— Presentation transcript:

1 Be careful about adaptationist explanations… 5) A feature may be a by-product of selection for another trait

2 Spandrel = a phenotypic characteristic that is a byproduct of the evolution of some other character, rather than a direct product of adaptive selection. Spandrels may become useful later (exaptations) or not..

3 Why is blood red? Color is a by- product of high iron content -probably selectively neutral

4 Pleiotropy = a single gene influences multiple phenotypic traits

5 Negative (?) By-Product/Pleiotropy Example Spotted Hyaena

6 Spotted Hyena highly social, female-dominated groups

7 Polygynous, but females larger than males Females have masculinized genitalia

8 pseudo-scrotum -derived from labia -same embryonic tissue as male scrotum -filled with two fat pads that resemble testes pseudo-penis -modified clitoris same embryonic tissue as male penis

9 male female Display of erect pseudo-penis by subordinate females

10 Cost? 9-18% of females died during their first birth in a captive colony – Survivors lost 60% of first born young Decreased survival at birth not seen in natural colony

11 Fratricide (Siblicide) -pups born with full set of teeth in aardvark burrows -pups fight for dominance; weaker may be prevented from nurse -high levels of androgens

12 females with “male-like” genitalia? (Phallus evolved for display hypothesis) a female reproductive organ that is so hazardous? siblicide? NONE of these features are found in other hyaena species What evolutionary forces could have produced:

13 striped hyena brown hyena -other hyenas are scavengers -do not “hunt” in groups to take big game -rely on carcasses carcasses are rare Keeps group size and overall population density VERY LOW “Usual” behavior and morphology

14 Young nurse for 12-18 mo Need help after weaning (bone cracking is difficult; competition is intense) Selection may favor aggressive mothers Group size variable 5–90 members led by a single alpha female

15 Death Rates In a natural colony, death rate was low during the reproductive period, moderate in dens, but very high after weaning (Does masculinization improve survival at weaning?)

16 alpha all others Matrilines Female Reproductive Success sons daughters Social  female RS Dominant matrilines have big advantage in repro. success 6 alpha-line females had >2.5 times the RS of all 30 lower-ranking females combined

17 access to food controlled by dominant female matrilines

18 Competition-Aggression Hypothesis -spotted hyaena are cursorial predators -probably arose relatively recently (not seen in other hyaena species)

19 Communal hunting in spotted hyaena -available food source greatly increased

20 Group hunting is successful Competition to feed weanlings is intense Androgens may improve competition Pleiotropic effects of androgens may cause other features = spandrels with no benefit  Androgens may cause masculinization of females  Androgens may cause siblicide

21 Summary… Natural selection is a powerful explanation for what we see in nature… In fact, we have come to explain so much of what we see in nature with “evolution via natural selection”, that we must now teach ourselves to be careful to not use such explanations uncritically… (TAS) Complex questions have simple, easy to understand, wrong answers. (Anonymous)

22 The “Adaptationist Program” Gould and Lewontin, The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme The “Adaptationist Programme” is: – Based on faith in the power of natural selection as an optimizing agent. – Breaks an organism into unitary "traits" – Proposes an adaptive story for each trait

23 Gould and Lewontin’s Panglossian Paradigm "It is demonstrable," said he, "that things cannot be otherwise than as they are; for as all things have been created for some end, they must necessarily be created for the best end. Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles. The legs are visibly designed for stockings, accordingly we wear stockings. Stones were made to be hewn and to construct castles, therefore My Lord has a magnificent castle; for the greatest baron in the province ought to be the best lodged. Swine were intended to be eaten, therefore we eat pork all the year round: and they, who assert that everything is right, do not express themselves correctly; they should say that everything is best." “Dr. Pangloss to Candide” by Voltaire, 1700’s

24 Gould and Lewontin Distinguish current utility from reasons for origin (consider exaptation) Consider alternatives to adaptive stories – just because it is plausible doesn’t mean it is right Consider neutral theory Consider that a feature may be a by-product (consider spandrels) TEST ideas!

25 Mechanisms of Evolution Mutation Natural Selection Genetic Drift

26 Natural Selection Genetic Drift

27 Drift

28 Fig. 23-8-1 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 C W C R C R C W C R C R C W

29 Fig. 23-8-2 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 p = 0.5 q = 0.5 C W C R C R C W C R C R C W C W C R

30 Fig. 23-8-3 Generation 1 C W C R C R C W C R C R C W p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W C W C R p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R

31 Genetic Drift is not adaptive Rare, beneficial alleles can be lost Genetic Drift

32 Random walks to fixation Eliminates alleles Reduces genetic variation, especially in small populations

33 Rare, beneficial alleles can be lost Genetic Drift

34 Drift causes genetic divergence between populations in the lab

35 Drift causes genetic divergence among populations in nature Drift reduces genetic variation within populations Habitat destruction  isolated populations in the Ozarks

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