Comparative Methods for Studying Trait Evolution

“Comparative methods” are used to:

1) compare traits across many species to determine if similar traits arose independently by natural selection (i.e., they are adaptations: solutions to the same environmental problem)

2) estimate the rate at which a trait appears or is lost, and when key changes occurred on a phylogeny

3) test if one trait affects the evolution of other traits

- if traits are not evolving independently, this could limit adaptation (a constraint)

- certain “master” traits may be very evolutionarily important (traits linked to speciation or extinction, for example)

Evolution of Cooperation in Birds

Levels of promiscuity and cooperative brood care both vary widely across bird species

- cooperative brood care = dad + kids help mom care for the next brood of chicks

- family doesn’t *have* to do this; in many species, they don’t... it’s a form of “altruistic” behavior

Hamilton’s rule states: behaviors will be favored by kin selection when (B)(r) > C

B = benefit to receiver

r = coefficient of relationship: odds two alleles are identical by descent from a common ancestor (0.5 for siblings)

C = cost to performer

Evolution of Cooperation in Birds

Hamilton’s rule: (B)(r) > C for siblings: 0.5B > C

the more related you are to another individual, the more likely you will be to help them so long as the cost to you is less than half the benefit to them

- if I can help my brother have more than two extra kids for each kid I don’t have myself (due to helping him), then it benefits my inclusive fitness to help out

Theory based on Hamilton’s rule suggests that when promiscuity is low, family members will be more related to one another

- should therefore cooperate for indirect fitness gains

Evolution of Cooperation in Birds

Predicted sequence of evolutionary transitions:

Monogamy(high within-clan

relatedness)

Cooperativebreeding

Promiscuity(low within-clan

relatedness)

Female-onlybrood care

1) kin selectionfavors kids

helping mom

r = 0.5 (you’re raising full sibs)

r = 0.25 (you’re raising half-sibs)

2) help from kids frees mom from relying on dad’s

help; she cansleep around &

he can bail

3) r < 0.5 nowfavors kidswho go off

to raise theirown offspring

= level of ancestral promiscuity

= origin of cooperative societies

phylogeny of 267 bird species

not much in groups thathave veryancestors

1) non-cooperative (selfish) ancestors that gave rise to selfish descendents were estimated to be the most promiscuous

Ancestral state

%ancestral

promiscuity

non-cooperativeboth cooperative

Descendents were..

Ancestral state

%ancestral

promiscuity

non-cooperativeboth cooperative

Descendents were..

2) non-cooperative (selfish) ancestors that gave rise to cooperative daughter species (i.e., that underwent character change) were estimated to be the least promiscuous

lack of promiscuity favored evolutionary switch to cooperation

Ancestral state

%ancestral

promiscuity

non-cooperativeboth cooperative

Descendents were..

3) cooperative ancestors that lost cooperation were more promiscuous than those that stayed cooperative

increased promiscuity led to breakdown in cooperation, as predicted

Evolution of Cooperation in Birds

female promiscuity (polyandry) increased during transition to selfish societies, and decreased during gains of cooperation

Evolution of Cooperation in Birds

Comparative methods revealed that cooperation..- evolved more often when ancestors cheated less- broke down when levels of promiscuity increased

promiscuity may be a generally important force in the evolution of complex vertebrate societies

Comparative analysis teased apart the role different traits played in evolutionary transitions

- a given species may be anywhere on the cycle

monogamy

female-only cooperative care

promiscuous

Phylogeny of 45 species of African starlings

- some species have cooperative brood care : males help females raise young

- males may be an asset worth competing for in cooperative systems, compared to most mating systems in which females don’t compete with each other

Rubenstein & Lovette, Nature 2009

monomorphic: males + females same size and color

dimorphic: different in size + color

% specieswith

dimorphiccolor

% speciesw/ dimorphic

body size(= boys bigger)

mono di

Does cooperative brood-care (males help females) lead to loss of sexual dimorphism, as females evolve “sexy” colors and larger body size due to increased competition?

non-co-op

dimorphic monomorphic

co-op

hypothesized ancestral state

sexy boys sexy boys + girls

Two alternative hypotheses to compare

non-co-op

co-op

1) two traits evolve independently

dimorphic monomorphic

- each trait has its own rate of forward () and reverse () evolution

- state you are in for one trait has no effect on the other trait

- character states are: cooperate/don’t, monomorphic/dimorphic

Two alternative hypotheses to compare

non-co-op

co-op

1) two traits evolve independently

dimorphic monomorphic

- each trait has its own rate of forward () and reverse () evolution

- state you are in for one trait has no effect on the other trait

H1: the traits are uncorrelated (no relationship between them)

Two alternative hypotheses to compare

2) H2: trait evolution is correlated

non-co-op

co-op

1) H1: two traits evolve independently

dimorphic monomorphic

Is L score better for a model with different rates of ?

- one rate (q12) for

- separate rate (q34) for

simplified to only consider forward rates!

2) H2: trait evolution is correlated

non-co-op

co-op dimorphic monomorphic

Is L score better for a model with different rates of ?

- one rate (q12) for

- separate rate (q34) for

Two alternative hypotheses to compare

1) What does it mean if q34 is much greater than q12 ?

2) What does it mean if q34 is much greater than q43 ?

non-co-op

co-op

dimorphic mono- morphic

H2 = 7.1, P < 0.0001

H1

strong support for correlation between sexual ornamentation and degree of cooperativity

- when males are a resouce (because they help out), females must compete for them

- drives sexual selection for female ornamentation (appeal to male choosiness)

- favors evolution of larger female body size (intraspecific physical competition among females)

Pitnick et al. 2006 investigated the relationship between mating system and body mass invested in male testes vs. brains, for 334 species of bats

relativebrainmass

relativetestesmass

female promiscuity?

in promiscuous species where females mate with multiple males:

- males had significantly smaller brains, relative to their body size

- males had larger testes

Suggests trade-off: can make bigger testes or bigger brains, but not both

- limited energy available during development

Pitnick et al. 2006 investigated the relationship between mating system and body mass invested in male testes vs. brains, for 334 species of bats

relativebrainmass

relativetestesmass

monogamous

male promiscuity (polygyny)

female promiscuity (polyandry)

effects were due to female, but not male, promiscuity

Reflects increased intraspecific competition among males

- post-mating sperm competition selects for larger ejaculate size (to flush out the competition)

Comparative analyses of diverse groups thus suggests that mating system can affect...

a) degree of cooperation within families, and among species

- mom’s sluttiness affects your willingness to help her out)

b) level of sexual dimorphism and ornamentation for both sexes

- males can become a limiting resource and fuel competition among females, for male attention and against each other

c) relative allocation of energy to testes vs. brains in males

- sexual competition can make you dumber

Brown et al. 2010, bromeliad-breeding frogs and biparental care:

- they modeled a one-way evolutionary scenario, and found much higher rates for gaining parental care after switching to egg-laying in small pools in plant leaves

Brown et al. 2010, bromeliad-breeding frogs and biparental care:

q12

q13q34 = 0.019

q24 = 0.28

Χ2 = 2(-274 –(-291) = 2(17) = 34

Dependent model: rate at which parental care evolves depends on where eggs are laid (favored!)

no care care

leavesponds

Brown et al. 2010, bromeliad-breeding frogs and biparental care:

q12

q13q34 = 0.019

q24 = 0.28

- force these two rates to be equal- does that make the model fit worse?

compare these two L scores

Χ2 = 1.9; not significant

Brown et al. 2010, bromeliad-breeding frogs and biparental care:

q12

q13q34 = 0.019

q24 = 0.28

- now force these two rates to be equal compare these two L scores:

Χ2 = 9.3; P < 0.01

forcing q34 = q24 makes the model worse; therefore, they are not equal

- biparental care evolves more when you start in a small pool