1. Aphids from alfalfa prefer to settle on alfalfa, and aphids from clover prefer to settle on clover. 2. Aphids from alfalfa have higher fitness in alfalfa, and aphids from clover have a higher fitness on clover. 3. Hybrids have lower fitness on both alfalfa and clover. 4. Genetic data suggest a trade off: genes for high fitness on alfalfa confer low fitness on clover, and vice versa. The preference genes seem to be linked to these fitness genes. Local adaptation: ADAPTATION to DIFFERENT PLANT SPECIES BY PEA APHIDS (by Sarah Via et al. see Freeman&Heron)
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1. Aphids from alfalfa prefer to settle on alfalfa, and aphids from clover prefer to settle on clover.
2. Aphids from alfalfa have higher fitness in alfalfa, and aphids from clover have a higher fitness on clover.
3. Hybrids have lower fitness on both alfalfa and clover. 4. Genetic data suggest a trade off: genes for high fitness on alfalfa confer low fitness on clover, and vice versa. The preference genes seem to be linked to these fitness genes.
Local adaptation: ADAPTATION to DIFFERENT PLANT SPECIES BY PEA APHIDS (by Sarah Via et al. see Freeman&Heron)
Experimental Coevolution. Bacterial pathogens (which started from the same ancestral stock) became adapted to nematode host population in only 30 generations. Asterisks indicate the parasite is signiCicantly better at infecting its local host population. Morran et al. (2014)
Local adaptation by parasitic worms to their local host population
4 mm
1. Host: the snail, Potamopyrgus antipodarum.
2. Parasite: a sterilizing trematode worm, Microphallus sp.
3. The hosts show a high degree of genetic differentiation among populations, but the parasite does not, perhaps due to parasite dispersal by ducks.
Microphallus sp., This digenetic
trematode is the most common parasite P. antipodarum.
Infected snails are
sterilized.
About the parasite
This is a similar life cycle as show by human Schistosome worms
Pathology of Microphallus
Infection in P. antipodarum
Photos: Gabe Harp
Lake Poerua Lake Ianthe
Lake Wahapo Lake Mapourika
Lake Ellery
Lake Alexandrina
Lake Paringa
Reciprocal cross-infection experiments
Three-way reciprocal cross-infection experiment: -->local adaptation.
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10
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60
70
80
MAPOURIKA (M) WAHAPO (W) PARINGA (P)
Parasite population
Perc
ent
infe
cte
dM host
W host
P host
S S S
S = Same lake (sympatric)
The parasites are adapted to infect their local (sympatric) host population. This is called “local adaptation.”
Percent infected
The geographic mosaic of coevolution (John Thompson) 1. Populations are structured into semi-isolated demes. 2. In some areas, reciprocal selection is strong, and the populations are coevolving. These are called Hotspots. 3. In other areas, selection is not strong, or one of species is not evolving in response to the other species. These are called Coldspots 4. The meta population (all the demes together) is a patchwork of hotspots and coldspots, between which some migration can occur by one or both species.
Geographicmosaicalongadepthcline
ducksforageintheshallowwater,buttheydisperseeggsintheshallowanddeepwater.Infectionishigherintheshallowthandeep
Shallow: a coevolutionary hotspot?
Deep: a coevolutionary coldspot?
Hypothesis: there is more infection in the shallow water due to coevolutionary interactions with parasites. Shallow-water snails are more susceptible.
Alternative hypothesis:
Experimentaltestofahotspot
Shallow
Deep
Lake Alexandrina
Parasites
Lake Kaniere
Parasites
L.Alexandrinasnails
L.Kanierisnails
Lake Alexandrina parasites
*
Deep Shallow Shallow Deep
Host source
Freq
uenc
y of
infe
ctio
n
Results: shallow snails more susceptible to local parasites
King et al. Current Biology (2009)
ALEXNDRINA KANIERI
Lake Alexandrina parasites
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Deep Shallow Shallow Deep
Host source
Freq
uenc
y of
infe
ctio
n
Results: Reject inherent susceptibility hypothesis.
King et al. Current Biology (2009)
ALEXNDRINA KANIERI
Lake Alexandrina parasites
*
Deep Shallow Shallow Deep
Host source
Freq
uenc
y of
infe
ctio
n
Results: Local deep snails appear foreign.
King et al. Current Biology (2009)
ALEXNDRINA KANIERI
Lake Kaniere parasites
*
Freq
uenc
y of
infe
ctio
n
Host source King et al. Current Biology (2009)
Deep Shallow Shallow Deep ALEXNDRINA KANIERI
Results: shallow snails more susceptible to local parasites
Lake Kaniere parasites
*
Freq
uenc
y of
infe
ctio
n
Host source
Results: Reject inherent susceptibility.
King et al. Current Biology (2009)
Deep Shallow Shallow Deep ALEXNDRINA KANIERI
Lake Kaniere parasites
*
Freq
uenc
y of
infe
ctio
n
Host source
Results: Local deep snails no more susceptible than foreign snails
King et al. Current Biology (2009)
Deep Shallow Shallow Deep ALEXNDRINA KANIERI
Geographicmosaicalongadepthcline
ducksforageintheshallowwater,buttheydisperseeggsintheshallowanddeepwater.Infectionishigherintheshallowthandeep
Shallow: a coevolutionary hotspot? YES. And sexual snails dominate in the shallow.
Deep: a coevolutionary coldspot? YES. And asexual snails dominate in the deep
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10 20 60 70% infected: Poerua snails
Alex. parasite
Poerua parasite
A x P parasite
Hybrid breakdown in the locally adapted parasite
Dybdahl et al. (2008, Am Nat)
% infected: Alex snails
Is there an alternative explanation for the breakdown?
What accounts for the hybrid breakdown?
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25
Perc
ent
infe
cted
(95
% C
I)
Poerua Hybrids Alex.
Parasite source
Allo
Allo
Hybridization not possible
Hybridization possible
Speciation: “the mystery of mysteries” (C. Darwin) Questions 1. Can we explain speciation with microevolutionary forces (mutation, selection, drift), or must we appeal to “macroevolutionary forces” like species selection? 2. Can we explain saltation in the fossil record (phenotypic jumps)? (Remember that Huxley said that gradual evolution was an “unnecessary burden”) 3. Is there natural selection for speciation? Species deCined: The biological species concept (BSC, Mayr 1942): Species are groups of actually or potentially interbreeding individuals that are reproductively isolated from other such groups. The tautological species concept (TSC, Lively unpublished): species are the products of speciation events.
Process of speciation Step 1: Population of interbreeding organisms becomes broken into 2, or more, smaller populations (in space or time). The break may occur by i) the establishment of geographical barriers within the population’s range, or ii) by the migration by some individuals across existing barriers. Step 2: Genetic divergence of isolated populations due to either A. Genetic drift B. Natural selection. i.e. different selection pressures on populations that are isolated in space (or time). C. Drift and selection (shifting balance) causing peak shifts on complex adaptive topographies (more than one attractor).
Step 3: Reproductive isolation 1. Prezygotic isolation: isolation prior to zygote formation, perhaps due to mate choice, or timing of mating. (remember the Kirkpatric model of runaway Sexual Selection?) 2. Postzygotic isolation: perhaps due to hybrid sterility or mortality. (Remember the Zeh and Zeh paper? Incompatibilities between species for the resolution of IGF2 and IGFr?) Let’s consider the second step. Genetic divergence. a. Local adaptation to different environments. (see local adaptation model from scanned overheads. Also show graphs from line-cross experiments from hybrids paper) There is genetic divergence due to selection to adapt to different environments (for example on the wet side verses the dry side of a mountain range.
No epistasis for fitness within habitats.G by G by E interaction.
b. Drift or selection in the same kind of environment (Dobzhansky-Muller model). Different alleles go to Cixation in different populations occupying the same niche, where there is complex epistasis between loci. These alleles, which evolved in allopatry, do not work well together, causing hybrid breakdown. c. Drift and selection in the same kind of environment (shifting balance). (show on board. Note the shifting balance could give a pattern that resembles saltation and punctuated equilibrium) One or more, but not all, populations drift across an adaptive valley, and then are selected up a new adaptive peak. Hybrids are in low Citness valleys.
The Dobzhansky-Muller model of genetic incompatability
Epistasis for fitness.
The Dobzhansky-Muller model of genetic incompatability
Step 3. Reproductive isolation. A. There is post-zygotic isolation due to any of the genetical explanations in step 2. Or B. There is prezygotic isolation due to, for example, mate choice evolution in allopatry. Or C. Prezygotic isolation is a evolutionary consequence due to selection not to mate with member of the other subgroup in zones of sympatry. Speciation by re-enforcement.
Evidence for speciation by reinforcement is fruit Clies
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