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 useful
later (exaptations)
or not..
Why is blood
red?
Color is a byproduct of high
iron content
-probably
selectively
neutral
• Pleiotropy = a single gene influences multiple
phenotypic traits
Negative (?) By-Product/Pleiotropy Example
• Spotted Hyaena
Spotted
Hyena
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 tissue
as 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
What evolutionary forces could have produced:
• 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
-other hyenas are scavengers
-do not “hunt” in groups to
take big game
-rely on carcasses
striped hyena
carcasses are rare
Keeps group size and
overall population density VERY
LOW
“Usual” behavior and morphology
brown hyena
Group size variable
5–90 members
led by a single alpha female
Young nurse for 12-18 mo
Need help after weaning
(bone cracking is difficult; competition is intense)
Selection may favor aggressive mothers
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?)
sons
daughters
•Social  female RS
Female Reproductive Success
•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
all others
alpha
Matrilines
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 benefit
Androgens may cause masculinization of females
Androgens 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
CR CR
CR CR
CR CW
CR CR
CW CW
CR CW
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Fig. 23-8-2
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CR
CW CW
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
Genetic Drift is not adaptive
Rare, 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
Habitat
destruction 
isolated
populations in
the Ozarks
Drift causes genetic
divergence among
populations in
nature
Drift reduces genetic
variation within
populations