magic traits · mimulus guttatus normal vs copper-tolerant lines (locus a) a 1a 2 hybrid is...

SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 1

Magic traits

Theory: The story so farPleiotropy between adaptation and mate choice“Automatic” vs “classic” magic traits[Blackboard: population genetics of a magic trait]

Theory: The story so far

n Assortative mating (locus B)q mating groups with full isolation: Bb heterozygotes (hybrids) disappear,

BB and bb are isolated species (but isolation is assumed from the start)

q partial penetrance (α < 1): not full isolation but less Bb than in Hardy-Weinberg


21

1

TTT+

=a

T1 T2 T1

BB+Bbbb

BB+Bb bb

flowering time

0.99

1

1.01

1.02

1.03

1.04

1.05

0 0.2 0.4 0.6 0.8 1

allele frequency pA


n Ecological selection (locus A), heterozygote inferiorityq constant fitnesses: loss of polymorphismq Levene model (contrasting selection in two habitats): frequency-dependent fitness

maintains polymorphism despite heterozygote inferiority


W11 W22

W12


n Reinforcement: avoid inferior heterozygote offspring

n The problem of recombination:Reinforcement needs genetic association between a mating locus and a locus undernatural selection, but recombination destroys any such association


Mimulus guttatus normal vs copper-tolerant lines (locus A)A1A2 hybrid is lethal*: W12 = 0

Avoid hybridization: the copper-tolerant line flowers earlier (locus B)association between tolerance and flowering time

* lethality is due to a tightly linked locus, Wright et al. 2013


n Reinforcement: avoid inferior heterozygote offspring

n The problem of recombination:Reinforcement needs genetic association between a mating locus and a locus undernatural selection, but recombination destroys any such association


Mimulus guttatus normal vs copper-tolerant lines (locus A)A1A2 hybrid is lethal*: W12 = 0

Avoid hybridization: the copper-tolerant line flowers earlier (locus B)association between tolerance and flowering time

* lethality is due to a tightly linked locus, Wright et al. 2013

“Magic traits”ecological & mating loci are one and the same– the problem of reinforcement disappears

divergence of the ecological trait implies reproductive isolation

Magic traits


Trends Ecol Evol 2011


Examples

n Flowering time in plants

Mimulus guttatus: mine dump dries faster– direct selection on early flowering

Agrostis tenuis: flowers earlier on copper mine spoilthan on adjacent pastures


Examples

n Phenology in Rhagoletis

apple maggot fly

hawthorn race

apple race


Examples

n Colour of flowers: selection by pollinators

Aquilegia formosahummingbirds Aquilegia pubescens

hawkmoths


n Beak size and song in Darwin finches

Examples

Geospiza magnirostris

Geospiza fuliginosa

0.5 s

kHz


Examples

Heliconius melpomene H. cydnohybrid

prop

ortio

nat

tack

edby

bird

s

ME F1 CY Merrill et al. (2012)

n Colour pattern in Heliconius


Examples

n Haplochromine cichlids in lake Victoria

female species

pref

eren

ceto

war

dsre

dm

ale

Seehausen & van Alphen 1998


Examples

n Body size in stickleback (Gasterosteus aculeatus)Ph

oto

byH

atfie

ld

limne

ticm

ale

pref

eren

cefo

rlim

netic

(sm

all)

fem

ales

Difference in female size(benthic - limnetic)

sympatric

allopatric (limnetic only) males(naively preferslarge females)

Albert & Schluter 2004

Limnetic male chooses betweena benthic and a limnetic female:


Magic traits

n Magic trait: a trait subject to divergent selection andcontributing to assortative mating

q a trait subject to divergent selection and a trait contributing tononrandom mating that are pleiotropic expressions of the samegene(s) [“magic gene”] – Servedio et al. (2011, TREE)

e.g. beak size – song

q divergent selection (excludes increased philopatry, choosiness)

q contributes to nonrandom mating: may need other components( e.g. preference)


Magic traits

n “Automatic” magic trait: reproductive isolation is aby-product of divergence

q phenology (flowering time, Rhagoletis)

q habitat choice(if under disruptive selection)

up, dark, acetaldehyde,early eclosion

down, light, ethanol,late eclosion

START

Rice & Salt 1990


Magic traits

n “Classic” magic trait: mating cue + preference

q mating cue under divergent selection:song in Darwin finchescolour pattern in Heliconius butterfliesbody size in sticklebacks (and many other examples)

q preference under divergent selection: sensory drivecolour perception in haplochromine cichlids

n Bolnick & Fitzpatrick (2007): most putative cases ofsympatric speciation are based on magic traits

Magic traits

n Direct selection on the mating locus (=magic trait) isnecessary to maintain the mating groupsq Equal penetrance: without selection, constant allele frequency

q Unequal penetrance: without selection,


21

1

TTT+

=a

T1 T2 T1

BB+Bbbb p’ = p

BB+Bbbb B spreads to fixation

BB+Bbbb b spreads to fixation

magic traits · mimulus guttatus normal vs copper-tolerant lines (locus a) a 1a 2 hybrid is...

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