Bio1151 Chapter 24 9
  1. A               species is a group of organisms whose members can interbreed in nature and produce            offspring under natural conditions.
    • A mule is the robust but sterile offspring (hybrid) between a male donkey and a female horse.

      Since their offspring is sterile, donkeys and horses are considered distinct biological species.


    • A liger is the sterile offspring of a male lion and a female tiger.

     
  2. Reproductive             can prevent interbreeding and result in reproductive             .
     
    • Prezygotic barriers prevent the formation of           , and include           ,            ,              ,              , and            isolation.
       
    • Postzygotic barriers prevent the           from developing into a fertile adult, and include reduced hybrid             , reduced hybrid             , and hybrid             .
     
  3. Speciation can occur by               ("Other Country") speciation, or by              speciation.
    • Allopatric speciation due to geographic separation of the Grand Canyon. Harris's antelope squirrel (Ammospermophilus harrisi) inhabits the canyon's south rim (left). Just a few miles away on the north rim (right) lives the closely related white-tailed antelope squirrel (Ammospermophilus leucurus).

      Another example is the Tassel-eared Squirrel, where Abert's Squirrel (Sciurus aberti) lives on the south rim and the Kaibab Squirrel (Sciurus aberti kaibabensis) lives on the north rim.


    • • Sympatric speciation.

      The apple maggot fly's original habitat was native hawthorn trees, but some populations colonized apple trees introduced by European settlers.

      Apples mature more quickly than hawthorn fruit, leading to habitat differentiation.

      The apple-feeding flies have been selected for rapid development, resulting in temporal isolation and reduced gene flow between the populations, even though they both live in the same geographic area.

     
    • Sympatric speciation may also result from
       
      • chromosomal changes such as               that leads to reproductive isolation.
        • Autopolyploidy occurs when an individual has more than two chromosome sets derived from one species. A diploid cell that fails in mitosis can become tetraploid. Meiosis yields diploid gametes; a new species may arise if the gametes produce fertile tetraploid offspring.

        • Allopolyploidy occurs when an individual has multiple sets of chromosome sets derived from different species. Nondisjunction of species B can yield diploid gametes which produce a sterile hybrid with species (A). If its gamete fuses with a haploid gamete from species A, a fertile allopolyploid offspring may result - a new species.
         
      •           selection that leads to nonrandom mating.
     
  4. Some isolation barriers may weaken over time and result in           and allow two species to merge into one.