Lecture 14

• Asexual reproduction


  Reproduction without sex, which may happen via: Vegetative propagation – generation of offspring from somatic tissue (e.g., buds); Parthenogenesis – "virgin birth" (offspring develop from unfertilized eggs)

  
• Offspring are genetically identical to parents (if you ignore mutation)
• Lineages of asexually reproduced offspring are clones
• Most parthenogenetic lineages are recent (= higher risk of extinction?)
• Sexual reproduction


  Two genomes combine to produce offspring genome; Includes the possibility of selfing (self-fertilization)
• Anisogamy


  Defines the sexes: Female – sex that produces the larger gametes (eggs); Male – sex that produces the smaller gametes (sperm)
• Isogamy


  No true sexes, only mating types (often denoted +/– or a/α)
• Sexually reproducing species


  • Dioecious – each individual is either male or female; Monoecious – each individual has both male and female organs
• Intersex individuals have either intermediate sexual characteristics or a combination of genetics, gonads, and genitals that is not uniquely male or female
• Benefits of sexual reproduction


  • Recombination can increase genetic variation; Sexual selection; Competition for mates (esp. by ♂) and mate choice (esp. by ♀) remove deleterious mutations from the population; Low-quality males don't mate
• Costs of sexual reproduction

  • Recombination breaks up existing adaptive combinations of alleles; Time, effort, and risk of mating (incl. STIs); Twofold cost of sex; "The cost of males"
• Twofold cost of sex

  Each sexual couple produces four offspring, while each asexual individual produces four offspring
• The sexual individuals should eventually be outcompeted and disappear
• Sex determination


  The sexes are defined by gamete production; The means of sex determination differ across taxa; The only foolproof way to identify an individual's sex is to discover what gametes the individual produces
• Sex vs. gender


  Sex is biological: sex is binary because it is defined based on gamete production; Gender is a social construct unique to humans; therefore, gender is not necessarily binary
• Sex-determination systems


  • XX/XY; ZZ/ZW; XX/X0; Temperature-dependent; Haplodiploidy
• Haplodiploidy


  Females are diploid; males are haploid; Fertilized eggs à female; Unfertilized eggs à male; Affects relatedness

  
• An unequal sex ratio is (usually) not evolutionarily stable
• Evolutionarily stable strategy (ESS)


  Strategy that cannot be invaded by "cheaters" if everyone adopts it; Here, the only ESS is for every individual to have a 50:50 chance of producing a son or a daughter
• Natural selection acts on individuals, not for "the good of the species"
• Sex-ratio adjustment in fig wasps

  Females can detect the presence of previous females' eggs in the fig; The more females who have previously laid eggs in the fig, the more male eggs a female lays; Females can adjust the sex ratio of their offspring by choosing how many of their eggs to fertilize with the stored sperm

  
• Explanations for the evolution of sex

  • Fisher–Muller effect: Sex allows beneficial mutations in different genes from multiple lineages to combine; Muller's ratchet: Asexual populations accumulate deleterious alleles, sex could cause the reconstruction of fitter individuals; Red Queen hypothesis: Coevolution of hosts and parasites results in a perpetual, cyclical arms race, sex regenerates rare combinations of alleles that provide immunity to common parasites
• Sex restores Hardy–Weinberg equilibrium and linkage equilibrium through allelic segregation and recombination