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SynopsisSynopsis Adaptation to environments?
Why is sex good?
Evolutionary theory of the maintenance of sex
Case studies
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Adaptation to physical & Adaptation to physical & biological environmentsbiological environments
Physical and biological environments differ radically in PREDICTABILITY
Physical environment - reln. between conditions constant between generations
Biological environment - reln. between conditions can vary WITHIN a generation
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The cost of sexThe cost of sex
2-fold cost of meiosis– useless half of the popn. = males– female dilutes her gene pool
mating – cost of ornamentation– mating displays etc.
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Why is sex good?Why is sex good?
Muller's ratchet– accumulation of deleterious alleles
hitch-hiking– breaks up disadvantageous combinations and
preserves bestGroup seln.
– ultimate in altruism - unlikely unless close relatives
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Why is sex good (2)Why is sex good (2)
Balance theory - states that there is an advantage to simultaneous sexual and asexual reproduction because of environmental demands.– little support as most plants/animals serially
sexual/asexual
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The Biological The Biological EnvironmentEnvironment
Capricious - can change within a generationHow can long lived organisms cope with
such challenges?Sex! - produces unpredictable genetic
combinations each generationThe ‘Red Queen’ Hypothesis
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Freshwater Freshwater bryozoan bryozoan Cristatella Cristatella
mucedo mucedo &&
Myxozoan parasiteMyxozoan parasiteTetracapsula Tetracapsula bryozoidesbryozoides
J.R. Freeland, L.R. Noble & B. Okamura (2000) J. Evol. Biol. 13: 383-395
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Cristatella mucedo Cristatella mucedo - - population structurepopulation structure
Popns. linked by dispersal & geneflowRepeated localized extinctions &
recolonizations
Balance by drift & gene flow - levels of popn. differentiation enhanced/diminished
Affect on popn. genetic structure?
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POPULATION STRUCTURINGPOPULATION STRUCTURING
Gene flow resulting in the homogenization of allelefrequencies
Barriers to dispersal resulting in differentiation due to mutation &random genetic drift
Forces reducing differentiation Forces increasing differentiation
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Reproduction in Reproduction in C. C. mucedomucedo
Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules
(statoblasts)Predominatly asexual reproduction,
budding, colony growth &fission, statoblast prodn.
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Reproduction in Reproduction in C. C. mucedomucedo
Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules
(statoblasts)Predominatly asexual reproduction,
budding, colony growth &fission, statoblast prodn.
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Dispersal/gene flowDispersal/gene flow
Some sexual repdn. beginning of seasonLarvae give limited within-site dispersalAsexual statoblasts highly resistant, survive
winter - gas-filled cells allow buoyancy and within-site dispersal - hooks allow long distance dispersal via animals
Survive desiccation and passage through digestive tract
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Genetic strategy?Genetic strategy?
Facultatively sexual animals produce overwintering propagules via sex
Asexual propagules unusual - but can be produced in abundance
Gives greater chance of passive dispersal and survival = max. chance of (re)colonization
Dispersal potential = metapopulation
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Reproductive strategies Reproductive strategies • partitioning genetic variationpartitioning genetic variation
sexual/asexualsexual/asexual
• budding, self/outcrossbudding, self/outcross• good dispersalgood dispersal• few widespread clonesfew widespread clones• fugitive lifestylefugitive lifestyle• novel myxozoannovel myxozoan
host-parasite - host-parasite - Red Queen, Red Queen, host escapes by evolution of host escapes by evolution of resistanceresistance
Molecular Ecology - bryozoan systemsMolecular Ecology - bryozoan systems
bryozoansbryozoans
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The parasite -The parasite -Tetracapsula Tetracapsula bryozoidesbryozoidesMyxozoan – thought related to
cnidarians – now not sure
Kills all colonies it infects – heavy infections wipe out bryozoan popns.
Agent of Proliferative Kidney Disease in trout (PKD)
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SummarySummary
Persistence of high levels of clonality Clones highly relatedClones varied in abundanceCommonest not disproportionately infectedNo evidence for Red QueenHow does C. mucedo survive?
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The Great EscapeThe Great Escape
Metapopulation structure– evidence of sub-division and gene flow– high diversity of clones = dispersal
Asexual statoblasts– produced at end of season - vs. sexual
overwintering propagules
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0 250 500km
A. B.
0 250 500km
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Favouring AsexualityFavouring Asexuality
Asexual repd. favoured when– metapopulation structure– successful dispersal
Big fitness benefits for single clone– e.g. Loriston Loch
Risk of extinction reduced by broad temporal and spatial spread
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SynopsisSynopsis Adaptation to environments?
Why is sex good?
Evolutionary theory of the maintenance of sex
Red Queen – running to stay ahead of parasites
Speciation – separating good genes from bad
Case studies
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Arionid slugs Arionid slugs &&
the nematodethe nematodePhasmarhabditis Phasmarhabditis hermaphroditahermaphrodita
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Reproductive strategies Reproductive strategies • partitioning genetic variationpartitioning genetic variation
sexual/asexualsexual/asexual
slugsslugs• facultative selfersfacultative selfers• poor dispersalpoor dispersal• species complexesspecies complexes• genesis of taxagenesis of taxa• nematodesnematodes
• budding, self/outcrossbudding, self/outcross• good dispersalgood dispersal• few widespread clonesfew widespread clones• fugitive lifestylefugitive lifestyle• novel myxozoannovel myxozoan
host-parasite - host-parasite - Red Queen, Red Queen, host escapes by evolution of host escapes by evolution of resistanceresistance
Molecular Ecology - slug & bryozoan systems.Molecular Ecology - slug & bryozoan systems.
bryozoansbryozoans
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The Large Arions - are The Large Arions - are they really difficult to they really difficult to identify?identify?
Arion ater ater - black?Arion ater ater - black?– Yes - but also red, yellow, whiteYes - but also red, yellow, white
A. ater rufus - orange?A. ater rufus - orange? Yes - but also black and yellowYes - but also black and yellow
A. lusitanicus - Lusitanian distribution?A. lusitanicus - Lusitanian distribution? No! - anything but, prefers drier eastern sitesNo! - anything but, prefers drier eastern sites
A. flagellus - a distinct flagellum?A. flagellus - a distinct flagellum? No! - a matter of taxonomic precedenceNo! - a matter of taxonomic precedence
Yes!!
}
} A.
lusitanicus
A. ater
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How did this diversity How did this diversity arise?arise?
Clues from distributions of selfing vs outcrossing taxa?
Respective levels of genetic polymorphism?
Anatomical similarity?
Legacy of an Ice Age?
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Selfers vs. Selfers vs. OutcrossersOutcrossers
SelfersSelfers OutcrossersOutcrossersFew clutchesFew clutchesEggs few & large Eggs few & large High quality offspringHigh quality offspringLow juvenile mortalityLow juvenile mortalityLess repd. investmentLess repd. investmentOften biennialOften biennialShort protandryShort protandryLate maturationLate maturationHigh altitudes/latitudesHigh altitudes/latitudes
Many clutchesMany clutchesEggs many & small Eggs many & small Low quality offspringLow quality offspringHigher juvenile mortalityHigher juvenile mortalityMore repd. investmentMore repd. investmentAnnualsAnnualsLong protandryLong protandryEarly maturationEarly maturationLow altitudes/latitudesLow altitudes/latitudes
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Distribution of selfers vs Distribution of selfers vs outcrossersoutcrossers
93% of parthenogenic and selfing taxa found at higher altitudes and latitudes than closely related outcrossing taxa.
Biologically simple vs biologically complex environments.
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But…….?But…….?
Selfers also common in the tropics!
Surely a selfing species can easily be overcome by parasites/pathogens in the evolutionary game of the Red Queen?
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Avoidance via speciationAvoidance via speciation
Speciation very rapid and many species complexes
Each species represent markedly different genetic entities
Method of isolating gene complexes – more difficult for parasites to invade than one
species
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Parapatric speciesParapatric species
Have adjacent but non-overlapping distributions.
Reproductive barriers? – sub species
Rapid speciation in the face of environmental change?
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ConclusionsConclusions
Biological environment a driving force in evolution.
Promotes:– Rapid genetic change– Speciation
– Facultative self-fertilization Fugitives - movers Species - shakers