THE EVO OF BREEDING SYSTEM

Nota:

• production of ovum via mitosis --> development of unfertilised ovum--> genetically identical daughters--> lineage of genetically identical females

Nota:

• ONLY FEMALES ARE RELATED TO FECUNDITY CZ THEY PRODUCE EGGS asexual - generation t + 3 (64 direct descendants) sexual (t + 3) - 16 direct descendants (produce eggs)

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• • there are barriers to the evolution of asexuality in some taxa • in other taxa, asexual reproduction is possible

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• bulk of asexuals have arisen relatively recently (generally believed to have limited evolutionary potential)

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• e.g. ostracod, Candonocypris novaezelandiae widespread in southern Australia; includes sexual and parthenogenetic lineages; sexual lineages = known only from 2 places asexual lineages = ‘everywhere’

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• e.g. cyclical parthenogenetic cladocerans, rotifers, aphids

1. involving the production and fusion of a male and female gamete
2. genetic re-organisation
3. two individuals
4. production of genetically diverse male and female offspring

1. A. combination of alleles at multiple genes which have high fitness when they occur together (many phenotypic traits are controlled by multiple genes acting in concert)

   Nota:

   • e.g. mimicry of Papilio memnon- recombination of genes breaks down the mimicry

1. A. reorganises successful combinations of alleles and generates offspring with untried combinations

Nota:

• - sexual reproduction is common, yet sexually reproducing individuals incur a significant cost (paradox of sexual reproduction) - implies there must be an advantage(s) to balance the costs

1. EXPLANATION 1. Sex accelerates evolution
   1. hypothesis that argues sexual populations have an advantage over asexual ones because they evolve faster
   2. group selectionist argument - sex benefits populations or species (consequences for individuals are irrelevant)
   3. HOW?

      Nota:

      • increase the rate at which beneficial mutations can be combined in single individuals (A1B1-A2B2) --> sexual populations are able to adapt more rapidly --> less likely to become extinct, particularly in changing environments 
   4. PROBLEMS
      1. 1) does sexual reproduction actually accelerate evolution? (Non-synchronous Mutations)

         Nota:

         • * only if favourable mutations arise relatively frequently (otherwise each beneficial mutation will become fixed before the next one arises) this is unlikely except in very large populations
      2. 2) can group selection override individual selection?

         Nota:

         • i.e., can traits that benefit a group persist even if they are disadvantageous to individuals?
         1. What is Group Selection? - selection operating on groups

            Nota:

            • differential rate of origination/extinction of groups on the basis of differences among them
         2. Q. What if certain characteristics are beneficial both to groups and the individuals that comprise them?
            1. A. expect such characteristics to prosper
2. EXPLANATION 2. Muller’s Ratchet
   1. hypothesis that argues asexual lineages are ultimately driven to extinction by the accumulation of deleterious mutations
   2. BASIS
      1. # of deleterious mutations (mutational load) in an asexual lineage can only increase through time

         Nota:

         • in asexual - can't recreate the normal gene back - no recombination eventually the clone with the least mutations will be lost (via drift) and so on
      2. ultimately leads to a ‘mutational meltdown’
      3. in contrast, sexual reproduction can generate the least mutation-laden genotype classes and so lighten the mutational load and hence lower extinction rates
   3. PROBLEMS (assumes)
      1. mutations are typically deleterious and back mutation is relatively rare (usually true)
      2. group selection (dubious)
      3. small population size (sometimes true)
   4. SUMMARY
      1. the advantages of sexual reproduction - focusing on the disadvantages of asexual reproduction
3. EXPLANATION 3. The Best Man Hypothesis
   1. hypothesis that argues that sexually reproducing individuals have an advantage because they can produce a few progeny with extraordinarily high fitness
   2. Basis
      1. sexual progeny ⇒ lower average but greater variance in fitness
      2. consequently a few sexual progeny may have extraordinarily high fitness
      3. more likely if the environment changes from one generation to the next
   3. PROBLEMS
      1. assumes high fecundity organisms & strong selection
      2. emphasis on fine-tuned adaptation to the physical environment

         Nota:

         • tight link btwn adaptation and phenotype and physical env
      3. theoretical models indicate that environmental change must be capricious to provide a sufficient advantage

         Nota:

         • Capricious is where the sign of the correlation between two adaptively important features is negative e.g. generation t = hot & dry or cold & wet generation t+1 = hot & wet or cold & dry
   4. SUMMARY
      1. generally discounted as a specific explanation of sexual reproduction
      2. focussed attention on the type of environmental change that might favour sexual reproduction
      3. one of the first explanations based on individual advantage
4. EXPLANATION 4. The Red Queen Hypothesis
   1. hypothesis that argues sexual reproduction provides organisms with the means to constantly improve in order to keep pace with their antagonists
   2. BASIS
      1. concerns the biotic environment (predators, competitors, pathogens – anatogists)
      2. any ‘improvement’ in these antagonists represents a deterioration in the environment of an organism
      3. thus organisms need to constantly improve in order to simply keep pace with their antagonists
   3. EARLY VERSION
      1. emphasized predator-prey interactions and fine-tuned adaptation

         Nota:

         • fine-tuned adaptation - matching the phenotype to the circumstances (env) that particular phenotype carries on
      2. predators and prey - have comparable generation times --> evolve on similar time-scales
      3. the advantage is not clear
   4. CURRENT VERSION
      1. emphasizes host-pathogen co-evolution
      2. pathogens typically have much shorter generation times than their hosts
      3. pathogens are under selection to infect hosts and hosts are under selection to evade pathogens
      4. pathogens that infect the most common host genotype will be selected for
      5. if the pathogen can improve its attack much faster than the host can improve its defence, then the best strategy for the host may be based on genotypic diversity
   5. emphasis on generating genetic novelty per se (rather than fine-tuned adaptation) as a defence against pathogens
      1. UNDERLYING ASSUMPTIONS
         1. 1. parasites/pathogens reduce the fitness of the hosts that they infect
            1. e.g. Egypt- human population
         2. 2. sexual individuals should be favoured when the risk of infection is high
            1. e.g. Potamopyrgus antipodarum

               Nota:

               • • includes sexual and asexual lines that co-occur in some habitats * aside from the mode of reproduction, sexual and asexual females appear to have similar life-histories - total infections by trematodes vs freq of males (high infection, high males)- high sexual reproduction - no trematodes - asexual occurs
         3. 3. parasite/host interactions are based on STRONG GENOTYPE X GENOTYPE INTERACTION

            Nota:

            • genetic control whether you can affect or resist or not
            1. e.g. Interactions Between a Rust Parasite & its Flax Host

               Nota:

               • genetic control some type of rust can affect some variety of flax. some cannot.
         4. 4. parasite/host interactions generate negative frequency dependent selection
            1. e.g. Incidence of Fish Predation on Three Colour Forms of a Corixid Bug

               Nota:

               • if survive earlier- selected, grow larger population- become more common- get selected as prey more often (selection pressure) - decline in popn
   6. SUMMARY
      1. predominant explanations of the adaptive significance of sexual reproduction

Nota:

• cannot generate genotypic diversity unless there is genetic polymorphism