IB Biology A3.1 Notes

Further species

Whilst you learned about the biological species concept in the SL syllabus, you need to be aware of its limitations in the HL syllabus. The concept works well in the case of sexually reproducing species because they satisfy the criterion of being a group of interbreeding organisms that are reproductively isolated.

However, species that reproduce asexually, such as bacteria via binary fission and yeast via budding do not fit this concept well. This is for several reasons:

1. They do not have reproductive barriers that apply to the definition.
2. The genetic uniformity in offspring produced by asexual reproduction greatly limits variation. This makes it impossible to distinguish a species based only on genetic differences.
3. Asexually produced organisms often evolve rapidly. They are also readily subject to adaptations; this makes delineating species boundaries challenging.
4. Bacteria transfer genes via horizontal transfer as opposed to vertical transfer like other organisms. This removes barriers to reproductive isolation as each bacteria does not undergo evolution by reproduction
5. Lastly, species hybridize. This is common in plants and many significant crops such as wheat and potatoes, or ducks such as the Mallard duck. This presents the problem of species overlap as these hybrids easily breed with other species.

Thus, it can be seen that reproductive isolation and evolution are not independent and delineated processes in these organisms, presenting issues. However, the difficulty of the biological species concept also applies to extinct species and those species for which available data is limited. This is because interbreeding populations are not possible amongst extinct species. Thus classification becomes more difficult.