magic traits · mimulus guttatus normal vs copper-tolerant lines (locus a) a 1a 2 hybrid is...
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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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 2
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
Theory: The story so far
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 3
W11 W22
W12
Theory: The story so far
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 4
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
Theory: The story so far
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 5
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 6
Trends Ecol Evol 2011
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 7
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 8
Examples
n Phenology in Rhagoletis
apple maggot fly
hawthorn race
apple race
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 9
Examples
n Colour of flowers: selection by pollinators
Aquilegia formosahummingbirds Aquilegia pubescens
hawkmoths
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 10
n Beak size and song in Darwin finches
Examples
Geospiza magnirostris
Geospiza fuliginosa
0.5 s
kHz
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 11
Examples
Heliconius melpomene H. cydnohybrid
prop
ortio
nat
tack
edby
bird
s
ME F1 CY Merrill et al. (2012)
n Colour pattern in Heliconius
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 12
Examples
n Haplochromine cichlids in lake Victoria
female species
pref
eren
ceto
war
dsre
dm
ale
Seehausen & van Alphen 1998
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 13
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:
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 14
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)
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 15
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
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 16
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,
SPECIATION / EVA KISDI / 2018 FALL / LECTURE 9 17
21
1
TTT+
=a
T1 T2 T1
BB+Bbbb p’ = p
BB+Bbbb B spreads to fixation
BB+Bbbb b spreads to fixation
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