Presentation on theme: "Synopsis Adaptation to environments? Why is sex good?"— Presentation transcript:
1Synopsis Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sexCase studies
2Adaptation to physical & biological environments Physical and biological environments differ radically in PREDICTABILITYPhysical environment - reln. between conditions constant between generationsBiological environment - reln. between conditions can vary WITHIN a generation
3The cost of sex 2-fold cost of meiosis mating useless half of the popn. = malesfemale dilutes her gene poolmatingcost of ornamentationmating displays etc.
4Why is sex good? Muller's ratchet hitch-hiking Group seln. accumulation of deleterious alleleshitch-hikingbreaks up disadvantageous combinations and preserves bestGroup seln.ultimate in altruism - unlikely unless close relatives
5Why 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
6The Biological Environment Capricious - can change within a generationHow can long lived organisms cope with such challenges?Sex! - produces unpredictable genetic combinations each generationThe ‘Red Queen’ Hypothesis
8Cristatella mucedo - population structure Popns. linked by dispersal & geneflowRepeated localized extinctions & recolonizationsBalance by drift & gene flow - levels of popn. differentiation enhanced/diminishedAffect on popn. genetic structure?
9POPULATION STRUCTURING Gene flow resulting in thehomogenization of allelefrequenciesBarriers to dispersal resulting indifferentiation due to mutation &random genetic driftForces reducing differentiation Forces increasing differentiation
10Reproduction in C. mucedo Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules (statoblasts)Predominatly asexual reproduction, budding, colony growth &fission, statoblast prodn.
11Reproduction in C. mucedo Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules (statoblasts)Predominatly asexual reproduction, budding, colony growth &fission, statoblast prodn.
12Dispersal/gene flow Some sexual repdn. beginning of season Larvae 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 animalsSurvive desiccation and passage through digestive tract
13Genetic strategy?Facultatively sexual animals produce overwintering propagules via sexAsexual propagules unusual - but can be produced in abundanceGives greater chance of passive dispersal and survival = max. chance of (re)colonizationDispersal potential = metapopulation
15The parasite -Tetracapsula bryozoides Myxozoan – thought related to cnidarians – now not sureKills all colonies it infects – heavy infections wipe out bryozoan popns.Agent of Proliferative Kidney Disease in trout (PKD)
16Summary Persistence of high levels of clonality Clones highly related Clones varied in abundanceCommonest not disproportionately infectedNo evidence for Red QueenHow does C. mucedo survive?
17The Great Escape Metapopulation structure Asexual statoblasts evidence of sub-division and gene flowhigh diversity of clones = dispersalAsexual statoblastsproduced at end of season - vs. sexual overwintering propagules
19Favouring Asexuality Asexual repd. favoured when metapopulation structuresuccessful dispersalBig fitness benefits for single clonee.g. Loriston LochRisk of extinction reduced by broad temporal and spatial spread
20Synopsis Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sexRed Queen – running to stay ahead of parasitesSpeciation – separating good genes from badCase studies
21Arionid slugs & the nematode Phasmarhabditis hermaphrodita
22Reproductive strategies Molecular Ecology - slug & bryozoan systems.Reproductive strategiespartitioning genetic variationsexual/asexualhost-parasite - Red Queen,host escapes by evolution ofresistanceslugsbryozoansfacultative selferspoor dispersalspecies complexesgenesis of taxanematodesbudding, self/outcrossgood dispersalfew widespread clonesfugitive lifestylenovel myxozoan
23} } Yes!! The Large Arions - are they really difficult to identify? Arion ater ater - black?Yes - but also red, yellow, whiteA. ater rufus - orange?Yes - but also black and yellowA. lusitanicus - Lusitanian distribution?No! - anything but, prefers drier eastern sitesA. flagellus - a distinct flagellum?No! - a matter of taxonomic precedence}A. ater}A.lusitanicusYes!!
24How did this diversity arise? Clues from distributions of selfing vs outcrossing taxa?Respective levels of genetic polymorphism?Anatomical similarity?Legacy of an Ice Age?
26Distribution of selfers vs outcrossers 93% of parthenogenic and selfing taxa found at higher altitudes and latitudes than closely related outcrossing taxa.Biologically simple vs biologically complex environments.
27But…….? 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?
28Avoidance via speciation Speciation very rapid and many species complexesEach species represent markedly different genetic entitiesMethod of isolating gene complexesmore difficult for parasites to invade than one species
29Parapatric species Have adjacent but non-overlapping distributions. Reproductive barriers? – sub speciesRapid speciation in the face of environmental change?
30Conclusions Biological environment a driving force in evolution. Promotes:Rapid genetic changeSpeciationFacultative self-fertilizationFugitives - moversSpecies - shakers