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 mating
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 hitch-hiking Group seln.
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 parasite Tetracapsula bryozoides
J.R. Freeland, L.R. Noble & B. Okamura (2000)
J. Evol. Biol. 13:

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. POPULATION STRUCTURING
Gene flow resulting in the
homogenization of allele
frequencies
Barriers to dispersal resulting in
differentiation due to mutation &
random genetic drift
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
Molecular Ecology - bryozoan systems
Reproductive strategies
partitioning genetic variation
sexual/asexual
host-parasite - Red Queen,
host escapes by evolution of
resistance
bryozoans
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 Asexual statoblasts
evidence of sub-division and gene flow
high diversity of clones = dispersal
Asexual statoblasts
produced at end of season - vs. sexual overwintering propagules

19. 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

20. 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

21. Arionid slugs & the nematode Phasmarhabditis hermaphrodita

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

23. } } Yes!! 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!!

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?

25. Selfers vs. Outcrossers
Selfers Outcrossers
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

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 complexes
Each species represent markedly different genetic entities
Method of isolating gene complexes
more difficult for parasites to invade than one species

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

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