asexual reproduction - creates genetically identical offspring

• all chromosomes from a single parent
• animal asexual reproduction involves budding off of a mass of cells
• parthenogenesis - development of an adult from an unfertilized egg
  • common form of reproduction in arthropods
  • diploid female bees, haploid male bees
• more advantageous than sexual reproduction (recombination does more harm than good in evolution)

sexual reproduction - multiple theories on its origin 

• no other process makes diversity more quickly, speeds up evolution
• asexually reproducing organisms tend to live in isolated, demanding habitats where natural selection doesn’t favor change
• sexually reproducing organisms tend to favor versatility, best supported by genetic recombination

DNA repair hypothesis - diploid cells can repair chromosome damage  

• protists only use sexual reproduction in times of stress
• synaptonemal complex may have evolved as a way to fix double-stranded DNA damage
• undamaged homologous chromosome used as template to fix damaged DNA

contagion hypothesis - mobile genetic elements infected eukaryotes  

• elements w/ genes for fusion w/ uninfected cells and synapsis can quickly copy itself onto homologous chromosomes
• idiomorph - genes in homologous positions on chromosomes but are so different that they can’t be of homologous origin
• explains the mating type “alleles” in fungi

red queen hypothesis - saves recessive alleles useful in the future  

• keeping alleles allow organism to keep up w/ changing environment
• sexual species can’t get completely rid of recessive traits in heterozygotes

Muller’s ratchet - can keep mutation level down 

• asexual populations can’t get rid of mutations
• sexual populations get rid of mutations through natural selection