A level Biology exam revision resources written by A level Examiners


A species can be defined as a group of organisms which are capable of interbreeding and producing fertile offspring. The examples of natural selection discussed so far (the giraffes neck and the peppered moth) show changes within a species. Speciation occurs when natural selection causes a new species to be generated

There are two types of speciation that can occur.

  • Allopatric speciation
  • Sympatric speciation

Start with an interbreeding population of one species.

Figure : One Species

The population becomes divided by a physical barrier, such as water in the case of an Island breaking off the mainland.

Figure : Divided Population

If the two environments are different e.g. different climates, then the two populations will experience different selection pressures. Variations which are beneficial to one group will be passed on within that group, whilst different variations beneficial to the other group will be passed on within that group. After a long period of time the differences between the two groups will have become so large that speciation will have occurred

Figure : Speciation

Then if the two populations are then mixed, they are so different that they can no longer interbreed. They are therefore reproductively isolated, and are two distinct species.

Figure : Two distinct Species

This type of speciation occurs when the original population is not geographically separated, but is reproductively separated within the same area. Reproductive isolation can occur when organisms adopt different patterns of behaviour, or occupy different ecological niches.

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