IB Biology SL

Revision Notes

4.1.1 Species & Population


  • 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

Allopatric speciation in trees (1), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (2), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (3), downloadable AS & A Level Biology revision notesSpeciation, downloadable IGCSE & GCSE Biology revision notes

Reproductive isolation of two populations of trees can lead to speciation

Exam Tip

Make sure that you can state the definition for a species – organisms belong to the same species if they can interbreed to produce fertile offspring

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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