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SynopsisSynopsis Adaptation to environments?
Why is sex good?
Evolutionary theory of the maintenance of sex
Case studies
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
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.
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
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
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
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
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?
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
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.
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.
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
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
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
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)
SummarySummary
Persistence of high levels of clonality Clones highly relatedClones varied in abundanceCommonest not disproportionately infectedNo evidence for Red QueenHow does C. mucedo survive?
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
0 250 500km
A. B.
0 250 500km
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
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
Arionid slugs Arionid slugs &&
the nematodethe nematodePhasmarhabditis Phasmarhabditis hermaphroditahermaphrodita
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
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
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?
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
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.
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?
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
Parapatric speciesParapatric species
Have adjacent but non-overlapping distributions.
Reproductive barriers? – sub species
Rapid speciation in the face of environmental change?
ConclusionsConclusions
Biological environment a driving force in evolution.
Promotes:– Rapid genetic change– Speciation
– Facultative self-fertilization Fugitives - movers Species - shakers