BSC 297 Biological Evolution November 17

1
BSC 297Biological EvolutionNovember 17

• Quiz 6 today
• Study Guide available Wednesday
• Reading
• Chapter 12.1 (last assignment)

2
Solving the Paradox of Sex

• Genes for sexual reproduction should be favored
  over genes for asexual reproduction because sex
  decreases linkage disequilibrium at the
  population level.
• Muellers Ratchet
• LD caused by drift.
• Negative consequence Increased Genetic Load
• Sex decreases LD and thus Genetic Load
• Selection in a Heterogeneous Environment
• Red Queen Hypothesis

3
Maintenance of Sexin Changing Environments

• Cause of Linkage Disequilibrium
• Selection on multi-locus genotypes in variable
  environments
• Reason sex is favored
• Varies with the model proposed

4
Maintenance of Sexin Changing Environments

• Temporal Variation Hypothesis - sex favored by
  unpredictable environmental changes
• Lottery Hypothesis (G.C. Williams)
• Spatial Variation Hypothesis
• intraspecific competition heterogeneous
  environment favors sex
• Red Queen Hypothesis (running fast but going
  nowhere) - sex favored by evolutionary arms race
  between hosts parasites
• negative frequency-dependent selection

5
Red Queen Hypothesis

• Host resistance to parasite is often genotype
  specific (trade-off).
• Some host genotypes are more resistant to
  parasite genotype I.
• Other host genotypes are more resistant to
  parasite genotype II.
• Frequency of the different host genotypes changes
  depending on which parasite genotypes are most
  frequent.
• Frequency of parasite genotype I or II changes in
  response to genetic changes in the host
  population.

6
Red Queen Hypothesis
7
Lottery vs. Red QueenSex in Snails

• Lively. 1987. Nature 328 519-521
• Parthenogenetic and sexual forms
• Populations occur in both lakes and streams
• Larger proportion of snails in lakes are
  parasitized
• Red Queen predicts more sex in lakes

8
Lottery versus Red Queen
stable unpredictable lottery A gt S S gt A red
queen S gt A A gt S
Snails in streams vs. lakes What best explains
the maintenance of sex?
9
Arms Race and Sexual Reproduction

• Populations with a greater frequency of males and
  sexual females would be better able to resist the
  snails due to increased genetic recombination
  (red queen).
• Sampling of 66 lakes found increased presence of
  males with increased parasite infections.
• Sexual reproduction leads to greater genetic
  recombination thus resistance to infections.
• Asexual reproduction leads to unique genetic
  lineages (increased linkage disequilibrium) and
  greater reproduction if worms are few.

10
Lottery vs. Red Queen Sex in Snails
4