1 / 30

Synopsis

Synopsis. Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sex Case studies. Adaptation to physical & biological environments. Physical and biological environments differ radically in PREDICTABILITY

jalila
Download Presentation

Synopsis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. Adaptation to physical & biological 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

  3. The 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.

  4. Why is sex good? • Muller's ratchet • accumulation of deleterious alleles • hitch-hiking • breaks up disadvantageous combinations and preserves best • Group 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 generation • How can long lived organisms cope with such challenges? • Sex! - produces unpredictable genetic combinations each generation • The ‘Red Queen’ Hypothesis

  7. Freshwater bryozoan Cristatella mucedo &Myxozoan parasiteTetracapsula bryozoides J.R. Freeland, L.R. Noble & B. Okamura (2000) J. Evol. Biol. 13: 383-395

  8. Cristatella mucedo - population structure Popns. linked by dispersal & geneflow Repeated localized extinctions & recolonizations Balance by drift & gene flow - levels of popn. differentiation enhanced/diminished Affect on popn. genetic structure?

  9. Gene flow resulting in the homogenization of allele frequencies Barriers to dispersal resulting in differentiation due to mutation & random genetic drift POPULATION STRUCTURING Forces reducing differentiation  Forces increasing differentiation

  10. Reproduction in C. mucedo • Inhabits discrete lakes & ponds • sex infrequent • disperses via asexual propagules (statoblasts) • Predominatly asexual reproduction, budding, colony growth &fission, statoblast prodn.

  11. Reproduction in C. mucedo • Inhabits discrete lakes & ponds • sex infrequent • disperses 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 dispersal • Asexual 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

  13. 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

  14. Reproductive strategies • partitioning genetic variation bryozoans Molecular Ecology - bryozoan systems sexual/asexual host-parasite - Red Queen, host escapes by evolution of resistance • budding, self/outcross • good dispersal • few widespread clones • fugitive lifestyle • novel myxozoan

  15. The parasite -Tetracapsula bryozoides • Myxozoan – 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)

  16. Summary • Persistence of high levels of clonality • Clones highly related • Clones varied in abundance • Commonest not disproportionately infected • No evidence for Red Queen • How does C. mucedo survive?

  17. The 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

  18. Favouring 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

  19. Synopsis • 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

  20. Arionid slugs &the nematodePhasmarhabditis hermaphrodita

  21. Reproductive strategies • partitioning genetic variation slugs bryozoans Molecular Ecology - slug & bryozoan systems. sexual/asexual host-parasite - Red Queen, host escapes by evolution of resistance • facultative selfers • poor dispersal • species complexes • genesis of taxa • nematodes • budding, self/outcross • good dispersal • few widespread clones • fugitive lifestyle • novel myxozoan

  22. The Large Arions - are they really difficult to identify? } • Arion ater ater - black? • Yes - but also red, yellow, white • A. ater rufus - orange? • Yes - but also black and yellow • A. lusitanicus - Lusitanian distribution? • No! - anything but, prefers drier eastern sites • A. flagellus - a distinct flagellum? • No! - a matter of taxonomic precedence A. ater } A. lusitanicus Yes!!

  23. 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?

  24. Selfers vs. Outcrossers • SelfersOutcrossers Few clutches Eggs few & large High quality offspring Low juvenile mortality Less repd. investment Often biennial Short protandry Late maturation High altitudes/latitudes Many clutches Eggs many & small Low quality offspring Higher juvenile mortality More repd. investment Annuals Long protandry Early maturation Low altitudes/latitudes

  25. 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.

  26. 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?

  27. Avoidance 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

  28. Parapatric species • Have adjacent but non-overlapping distributions. • Reproductive barriers? – sub species • Rapid speciation in the face of environmental change?

  29. Conclusions • Biological environment a driving force in evolution. • Promotes: • Rapid genetic change • Speciation • Facultative self-fertilization • Fugitives - movers • Species - shakers

More Related