Bio1151 Chapter 24 The Origin of Species
  1. A             species is a group of organisms whose members can interbreed in nature and produce          offspring under natural conditions.

    Evolution of a Galapagos bird.

    How did the flightless cormorant (Nannopterum harrisi) come to reside isolated Galapagos Islands?

    DNA evidence suggests that these birds descended from flying cormorant species in South America: descent with modification.

    Biological species.

  2. Similarity between different species. The eastern meadowlark and western meadowlark are similar in appearance, but are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding in the wild.
  3. Diversity within a species. Though diverse in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed and produce fertile offspring.

    Some species are able to interbreed, but if the hybrid offspring is sterile, the parental species are considered separate biological species.

      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.

  4. Reproductive           can prevent interbreeding and result in reproductive            .

    Reproductive isolation.

    A mutation in one gene causes the shell of this snail to spiral in the opposite direction from wild type.

    This single mutation prevents mating between the two phenotypes, resulting in reproductive isolation.

    • Prezygotic barriers prevent the formation of          , and include          ,           ,             ,             , and          isolation.

      Prezygotic barriers impede mating or hinder fertilization if mating does occur. Two species of garter snakes: one lives mainly in water while the other is mainly terrestrial. The eastern spotted skunk mates in late winter; the western spotted skunk mates in late summer. Blue-footed boobies of the Galapagos perform a courtship display unique to the species. Boobies: These 2 species of snails have opposite spirals in their shells so their genital openings are not aligned.
    • Postzygotic barriers prevent the         from developing into a fertile adult, and include reduced hybrid            , reduced hybrid            , and hybrid            .

    Postzygotic barriers prevent a hybrid zygote from developing into a fertile adult. Gametes of red and purple sea urchins are released into the water, but are unable to fuse.

    Some salamander subspecies of the genus Ensatina can hybridize, but hybrids do not complete development or are frail. A mule is the robust but sterile hybrid between a male donkey and a female horse. Hybrids of two rice strains are vigorous and fertile, but the next generation (center) may be sterile.

  5. Speciation can occur by             ("Other Country") speciation, or by            speciation.

    Allopatric speciation: a population forms a new species while geographically isolated from its parent population, reducing gene flow. Sympatric speciation: a population becomes a new species by habitat differentiation without geographic separation.

      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.

        Sympatric speciation: polyploidy.

        Polyploidy is the presence of extra sets of chromosomes due to accidents of cell division.

      • autopolyploidy
      • allopolyploidy

          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.

    Sympatric speciation: sexual selection.

    Under normal light, two cichlid species in Lake Victoria are different in coloration, and females mate only with males of their own species.

    Under orange light, the two species appear identical, and females mate with males of either species, yielding fertile hybrids.

    Intersexual selection by females, based on coloration, is the reproductive barrier here.


  6. Some isolation barriers may weaken over time and result in         and allow two species to merge into one.

    Fusion: weakening reproductive barriers.

    Pollution in Lake Victoria over the past 30 years has resulted in cloudy waters that make sexual selection by female cichlids difficult.

    The fertile hybrids produced may increase gene flow between the parent species.

    Over time, the parent species can fuse into a single species.

    Cichlid speciation: