Reproductive isolation
Reproductive isolating barriers
Extrinsic and intrinsic isolation: how to tell apart
[Blackboard: population genetics of assortative mating]
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Reproductive isolating barriers
n
Prezygotic isolation: no hybrid zygote is formed
q
Behavioural isolation: song, pheromone, courtship, size, ...
Darwin's finches, Podos 2001
Geospiza magnirostris
Geospiza fuliginosa
kHz
0.5 s
q
Mechanical isolation: insect genitalia, flower morphology
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Reproductive isolating barriers
n
Prezygotic isolation
q
Gametic isolation: cross-fertilization impossible or
conspecific sperm / pollen precedence
Sea urchin spp: sympatric broadcast spawners
Echinometra
bindin: released to enter the vitelline envelope of the egg,
rapidly diverges between species
(high replacement / silent substitution ratio)
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Reproductive isolating barriers
n
Prezygotic isolation
q
Ecological isolation (byproducts of adaptation)
n
n
Habitat isolation: breed in different habitats within cruising range
Allochronic isolation: breed at different times
hawthorn race
apple race
apple maggot fly
Rhagoletis pomonella
n
Aquilegia formosa
(vs A. pubescens)
Pollinator (floral) isolation:
different insect spp or different individuals (!)
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Reproductive isolating barriers
n
Postzygotic isolation: hybrid inviability or sterility
q
Extrinsic postzygotic isolation: environment-dependent
n
n
q
Ecological: hybrids are selected against in nature (no niche)
Behavioural: hybrids cannot find appropriate mates
Intrinsic postzygotic isolation: in any / in stressful environment
n
n
Hybrid inviability: death during development or reduced survival later
Hybrid sterility: no functional reproductive organs or incapable of
courtship/parental care
Agapornis roseicollis x A. personata fischeri
female hybrids fail to build a nest / do not incubate
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Extrinsic postzygotic isolation
H. cydno
hybrid
proportion attacked
by birds
Heliconius melpomene
ME
F1
CY
Merrill et al. (2012)
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
Extrinsic vs intrinsic isolation
How to tell apart extrinsic and intrinsic postzygotic isolation?
Threespine stickleback (Gasterosteus aculeatus)
invaded freshwater lakes in Canada after the ice age
Benthic and limnetic morphs differ in
morphology (large/small; jaws, gill rakers)
diet (benthic invertebrates/zooplankton)
Trade-off between habitats:
Schluter (1995)
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Extrinsic vs intrinsic isolation
Hypothesis: hybrids are not efficient in either habitat
= extrinsic postzygotic isolation
1. F1 hybrids are viable and fertile in the laboratory
Hatfield & Schluter 1999
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Extrinsic vs intrinsic isolation
2.
F1 hybrids do poorly in both natural habitats
benthic
limnetic
F1 hybrid
Hatfield & Schluter 1999
- but is this because they are not adapted to either niche (extrinsic)
or because they have intrinsic genetic incompatibilities that manifest
only in stressful environments?
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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Extrinsic vs intrinsic isolation
3. Backcrosses: F1 x benthic ® BB, F1 x limnetic ® BL
phenotypically closer to the parental forms:
if extrinsic, BB should do better in the littoral habitat
BL should do better in open water
equal genetic incompatibilities:
if intrinsic, BB and BL should do equally badly in both habitats
Rundle 2002
SPECIATION / EVA KISDI / 2014 FALL / LECTURE 2
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