Presentation on theme: "Synopsis Adaptation to environments? Why is sex good?"— Presentation transcript:
1 Synopsis Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sexCase studies
2 Adaptation 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
3 The cost of sex 2-fold cost of meiosis mating useless half of the popn. = malesfemale dilutes her gene poolmatingcost of ornamentationmating displays etc.
4 Why 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
5 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
6 The 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
8 Cristatella 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?
9 POPULATION STRUCTURING Gene flow resulting in thehomogenization of allelefrequenciesBarriers to dispersal resulting indifferentiation due to mutation &random genetic driftForces reducing differentiation Forces increasing differentiation
10 Reproduction in C. mucedo Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules (statoblasts)Predominatly asexual reproduction, budding, colony growth &fission, statoblast prodn.
11 Reproduction in C. mucedo Inhabits discrete lakes & pondssex infrequentdisperses via asexual propagules (statoblasts)Predominatly asexual reproduction, budding, colony growth &fission, statoblast prodn.
12 Dispersal/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
13 Genetic 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
15 The 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)
16 Summary Persistence of high levels of clonality Clones highly related Clones varied in abundanceCommonest not disproportionately infectedNo evidence for Red QueenHow does C. mucedo survive?
17 The 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
19 Favouring 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
20 Synopsis 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
21 Arionid slugs & the nematode Phasmarhabditis hermaphrodita
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!!
24 How did this diversity arise? Clues from distributions of selfing vs outcrossing taxa?Respective levels of genetic polymorphism?Anatomical similarity?Legacy of an Ice Age?
26 Distribution 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.
27 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?
28 Avoidance 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
29 Parapatric species Have adjacent but non-overlapping distributions. Reproductive barriers? – sub speciesRapid speciation in the face of environmental change?
30 Conclusions Biological environment a driving force in evolution. Promotes:Rapid genetic changeSpeciationFacultative self-fertilizationFugitives - moversSpecies - shakers