Evolution Lecture 9-10

• Diploid organisms


  The first step in sexual reproduction is the production of gametes, each of which has half the chromosomes of the parent
• Crossing over


  Occurs within chromosomes, creating chromosomes that differ from chromosomes in the parents
• Fertilization creates individuals that have genotypes different from either of their parents
• Siblings differ in genotype from one another
• Bacteria and viruses


  Although sexual reproduction occurs only in eukaryotes, they have other mechanisms that result in new combinations of alleles
• Sexual reproduction is an evolved trait
• Modes of sexual reproduction


  • Isogamy
  • Anisogamy
• Isogamous species


  Produce gametes of the same size and form
• Anisogamous species


  Produce gametes that differ in size and form
• Hermaphroditic vs separate sexes


  • Most plants are hermaphroditic
  • Most animals have separate sexes
• Genetic sex determination


  In humans and many other species with separate sexes, sex is determined by genotype (XX females, XY males)
• Environmental sex determination


  In some species of plants and animals, sex is determined by the environment (e.g. temperature in turtles and crocodilians)
• Reproductive capabilities


  • Capable of asexual reproduction (budding, fission, apomixis, parthenogenesis)
  • Incapable of asexual reproduction
• Some species never engage in sexual reproduction (e.g. several species of whiptail lizards)
• Many species can reproduce both sexually and asexually, typically undergoing several generations of asexual reproduction followed by one generation of sexual reproduction
• Self-fertilization differs from parthenogenesis/apomixis as siblings produced by self-fertilization have genotypes that differ from one another and from their parent, while offspring produced by parthenogenesis/apomixis are identical to one another and to their parent
• Two-fold cost of sexual reproduction

  Females who reproduce asexually will on average produce twice as many daughters as females who reproduce sexually
• The fact that many species retain the capacity for both sexual and asexual reproduction tells us that sexual reproduction could be eliminated by natural selection
• Why sexual reproduction persists


  • Genetically diverse offspring are better able to survive in different conditions
  • Asexually reproducing species are not able to eliminate deleterious mutations as efficiently as sexually reproducing species
• Sexual selection


  Darwin's theory that explains sexual dimorphism - features that reduce chances of survival but increase reproductive success
• Darwin recognized that the struggle for existence was both a struggle to survive and a struggle to reproduce
• Darwin noted that sexual dimorphism was associated with male-male competition and female choice
• Examples of sexual dimorphism


  • Peacock's tail
  • Antlers of a deer
  • Horn of a stag beetle
• Examples of male-male competition


  • Within group dominance
  • Female-defense polygyny
  • Territorial polygyny
  • Lekking
• Categories of female choice

  • Resource based (female chooses male that brings most resources)
  • Non-resource based (female chooses male based on inherent property of male)
• Examples of female choice


  • Bush cricket (male provides energy-rich spermatophore)
  • Gray Tree Frog (female chooses male with longer call)
• Leks


  Aggregations of males which females visit only for the purpose of mating
• Sensory bias/exploitation hypothesis


  Latent preferences in females are used by males to gain greater reproductive success. The female sensory bias should evolve before the male trait that exploits it.
• Examples supporting sensory bias/exploitation hypothesis


  • Evolution of chuck call in Physaleumus frogs after female sensory bias for chucks evolved
  • Evolution of swords in Swordtail fish after female preference for swords evolved