Synopsis Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sex Case studies.

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