Be careful about adaptationist explanations…

5) A feature may be a by-product of selection for another trait

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 usefullater (exaptations)

or not..

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

• Pleiotropy = a single gene influences multiple phenotypic traits

Negative (?) By-Product/Pleiotropy Example

• Spotted Hyaena

SpottedHyena

highly social, female-dominated groups

Polygynous, but females larger than males

Females have masculinized genitalia

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 tissueas male penis

male

female

Display of erect pseudo-penis by subordinate females

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

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

• 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:

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 andoverall population density VERY

LOW

“Usual” behavior and morphology

Young nurse for 12-18 mo

Need help after weaning (bone cracking is difficult; competition is intense)

Selection may favor aggressive mothers

Group size variable5–90 membersled by a single alpha female

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?)

alpha all othersMatrilines

Fem

ale

Repr

oduc

tive

Succ

ess

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-rankingfemales combined

access to food controlled by dominant female matrilines

Competition-Aggression Hypothesis

-spotted hyaena are cursorial predators

-probably arose relatively recently(not seen in other hyaena species)

Communal hunting in spotted hyaena

-available food source greatly increased

• 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 benefitAndrogens may cause masculinization of femalesAndrogens may cause siblicide

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)

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

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

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!

Mechanisms of Evolution

• Mutation• Natural Selection• Genetic Drift

Natural Selection

Genetic Drift

Drift

Fig. 23-8-1

Generation 1p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

Fig. 23-8-2

Generation 1p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2p = 0.5q = 0.5

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Genetic Drift is not adaptiveRare, beneficial alleles can be lost

Genetic Drift

Random walks to fixation

Eliminates alleles

Reduces genetic variation, especially in small populations

Rare, beneficial alleles can be lost

Genetic Drift

Drift causes genetic divergence between populations in the lab

Drift causes genetic divergence among populations in nature

Drift reduces genetic variation within populations

Habitat destruction isolated populations in the Ozarks