• A species can be defined as:

A group of organisms that can interbreed to produce?fertile offspring

• The ability to breed and produce fertile offspring is a useful method of distinguishing species for organisms that reproduce sexually but?can be difficult to apply?in some situations
  • Organisms that reproduce by?asexual reproduction, such as bacteria, cannot be classified using this method
  • On rare occasions,?animals of different species breed together and produce fertile offspring, such as the so-called 'wholphin'; the fertile offspring from a cross between a melon-headed whale and a common bottlenose dolphin
    • According to the species rule above the wholphin would be a new species, but while scientists do believe that hybridisation can lead to new species it needs to be a frequent event for this to occur, and wholphins are rare
    • Note that the melon-headed whale is actually a species of dolphin, so the name 'wholphin' is a bit inaccurate!
• The imperfect nature of this method of classifying species means that other characteristics are often used at the same time
  • Organisms of the same species share?similar?morphology
  • DNA sequences?can be compared, with a certain level of similarity indicating that organisms are the same species

• A population can be defined as:

A group of organisms of the same species living in an area at one time

• A?population?can be isolated?from other populations of the same species due to living in a different area
• This isolation means that members of the separate populations?cannot breed together?and?gene exchange?or?gene flow?cannot take place between them
• As long as these isolated populations could, in theory, interbreed to produce fertile offspring, they are the?same species
• If the environmental conditions affecting each population are different, then?natural selection could act differently?on each population and eventually lead to?speciation
  • Genetic drift?can also lead to speciation
• Once speciation has taken place, the?two species?can no longer produce fertile offspring; they are?reproductively isolated

Reproductive isolation of two populations of trees can lead to speciation