Biological Classification (DP IB Biology)

Classifying Organisms

Why is biological classification needed?

• The diversity of life on Earth is vast, and is known as global biodiversity

  • The extent of global biodiversity is such that scientists can only estimate the total number of species present on Earth, and it is likely that there are many species yet to be discovered

• For biologists to make sense of the huge array of species, organising them into logical groups is essential; this process of putting organisms into groups is known as classification

  • The science of classification is known as taxonomy, and scientists working in the field of taxonomy are taxonomists

  • Classifying an organism involves deciding which biological group, or taxon (plural taxa), it fits into best, and then naming it according to its taxon

  • Historically an organism's biological group was determined on the basis of its observable characteristics, and today this information is combined with genome sequencing data for more accurate classification

• Correct classification allows scientists to:

  • Accurately determine the number of known species

    • Without classification there would be a risk of recording species more than once, or incorrectly grouping multiple species together as one

  • Learn about the evolution of species

    • Accurate classification allows scientists to determine the evolutionary relationships between species, i.e. which species share common ancestry, and how recently

  • Ensure that conservation is carried out when needed

    • It is not possible to conserve a species if we don't know that it exists, e.g. if several species are incorrectly classified as one species, then the extinction of what might be thought of as one local population could actually result in the extinction of an entire species

  • Carry out medical research more quickly

    • E.g. if one plant species is known to have medicinal properties, then it is possible to quickly identify others by looking at its close relatives; this is much faster than just selecting plants at random for research

  • Identify and treat new diseases more quickly

    • E.g. the virus that causes COVID-19 was quickly classified as being a coronavirus, providing medics with information about how it might affect the body, and how to go about developing a vaccine

Classifying Organisms: Challenges

The traditional hierarchy of taxa

• Biological classification involves putting organisms into groups, or taxa (singular taxon)

• The taxa form a hierarchy

  • A hierarchical system is one in which larger groups contain smaller groups with no overlap between groups

• The taxonomic hierarchy contains the following taxonomic groups in descending order of size:

  • Domain

  • Kingdom

  • Phylum

  • Class

  • Order

  • Family

  • Genus

  • Species

Examples of classification table

Difficulties with classification

There are multiple challenges when it comes to accurately classifying organisms into the hierarchy of taxa described above; such difficulties include:

Morphology

• Historically, organisms have been classified on the basis of their morphology, but this can lead to errors; similarities in observable characteristics do not always mean that organisms share a recent common ancestor

  • E.g. dolphins and sharks could in theory be grouped together as they are both groups of aquatic animals that share a similar body shape, but they in fact belong to different classes

    • Dolphins are mammals and sharks are fish

    • Their streamlined body shapes evolved separately rather than originating in one common ancestor

• One solution to this difficulty is to use genome sequencing data; this helps to avoid the difficulties with misleading morphology, but does not solve all of the challenges

Taxonomic rank

• In the hierarchy system described above, each level of classification fits into an established taxonomic rank, i.e. kingdom, phylum, class etc.

• Classification can be complicated if a group of organisms falls across taxa, or needs to be moved from one taxon to another

  • Plant species in distant taxa can sometimes breed together to produce fertile hybrids; the resulting offspring will technically be a new species, but will be very difficult to classify under the hierarchy of taxa

  • Moving a group of organisms between taxa runs the risk of needing to shunt all of the groups currently in a taxon into a different rank to make room for the new grouping

Species

• The point at which two populations are classified as different species can be highly subjective

• The fertile offspring of a cross between two species may then go on to breed only with members of one of its parent species; this is known as introgression

  • Introgression demonstrates how difficult it can sometimes be to neatly apply species classification; the resulting offspring after several generations do not fit completely into either species, but neither does it seem to make sense to classify them as a new species

  • E.g. hundreds of thousands of years ago an early human bred with a Neanderthal and the offspring of this cross then went on to breed only with early humans; the result of this is that some groups of modern humans have some Neanderthal genes in their genomes

Introgression diagram

Several generations after an interbreeding event has occurred, the species X individuals with some genes from species Y do not fit perfectly into either species X or Y, but cannot correctly be considered a new species; this is introgression

NOS: A fixed ranking of taxa is arbitrary because it does not reflect the gradation of variation

• The hierarchy of taxa described here has been arbitrarily set up by humans because it is a neat way of organising life into groups, not because it fits with the patterns that we see in the natural world

• In order to fall neatly into the taxonomic ranks of the traditional classification system, species need to be clearly distinct from each other, and they need to obey reproduction rules such as not interbreeding, and not producing fertile offspring; this is far from the reality of the biological world where the differences between organisms are on a gradual scale, and neat breeding rules do not always apply

• A successful classification system needs to follow the evidence rather than seeking to fit the natural world into a human-designed system

  • New evidence comes from genome sequencing, and often leads to the reclassification of species

  • The newer system of cladistics uses unranked groups based on evolutionary relationships alone to produce evolutionary trees