An adult axolotl salamander Ambystoma mexicanum retains the
     An adult axolotl salamander (Ambystoma mexicanum) retains the juvenile characteristics of external gills and a tail fin.  allometry  hybridization  preadaptation  paedomorphosis  adaptive radiation  Which best describes a population that forms a new species within the same geographical region as the original species?  allopatric speciation  sympatric speciation  parapatric speciation  autologous speciation  heterogeneous speciation  What kind of organisms likely perished due to the increase of oxygen in the atmosphere three billion years ago?  Heterotrophs  cyanobacteria  obligate aerobes  obligate anaerobes  facultative anaerobes   
  
  Solution
4. D paedomorphosis
It means that the axolotl salamander in adult phase retains its external gills and tail fins which are juvenile characteristics. This allows salamander to be aquatic

