A species is a population, or groups of populations, whose members have the potential to interbeed to produce fertile, viable offspring

  • Members of a species are unable to produce fertile and viable offspring with members of a different species
  • Hybrids are individuals produced by cross-breeding between two different species, and are reproductively sterile (e.g. ligers, mules)

There are a number of caveats to this definition of species, including:

  • Certain organisms do not reproduce sexually (e.g. bacteria) but can transfer genetic information via the exchange of plasmids (conjugation)
  • Some past organisms are known only through fossils, with no living representatives, so breeding capacity cannot be established
  • Geographically isolated organisms may never come into contact, so there is no information regarding their capacity to interbreed
  • It may be physically impossible for members of the same species to mate and hence reproduce (e.g. certain different breeds of dogs)
  • Some species spread around an area to form interlinked breeding populations whereby the population 'ends' cannot interbreed (ring species)

Reproductive Isolation Barriers

There are two main categories of reproductive isolation barriers which prevent members of different species from interbreeding:

  • Pre-zygotic isolation (before fertilisation occurs)
  • Post-zygotic isolation (after fertilisation occurs)


Speciation is the formation of a new species from pre-existing species

  • There are two main mechanisms by which speciation can occur:
    • Allopatric speciation (geographic isolation)
    • Sympatric speciation (reproductive isolation

Allopatric Speciation

  • Allopatric speciation occurs when a geographic barrier physically isolates populations of an ancestral species
  • The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
  • Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)


Allopatric Speciation

Sympatric Speciation

  • Sympatric speciation is the reproductive isolation of two populations as a result of genetic abnormalities
  • Non-disjunction events result in polyploidic organisms that can no longer interbreed with similar organisms that did not inherit the extra chromosomes
  • Sympatric speciation occurs more commonly in plants, due to their capacity for self-fertilisation
  • It is considered to be the opposite of allopatric speciation as speciation does not require geographic isolation


Extinction is the cessation of a species or higher level taxon, reducing biodiversity

  • It can result gradually, as one population of organisms evolve into something else (phyletic extinction)
  • Alternatively, a species may not leave identifiable descendents and simply cease to exist (abrupt extinction)

Extinctions can be caused by a range of factors, including habitat degradation, predation, disease and environmental disasters

  • Mass extinction events, which are characterised by an unusually large number of species dying out in a relatively short period, can be triggered by global catastrophes
  • Over 99% of species that have ever existed on Earth are now extinct