Classification & Taxonomic Tools (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Classification
There are millions of different species that currently exist on Earth
Biologists and ecologists can manage and organise this enormous diversity of species by putting similar species together into groups
This process is known as classification
It involved organising and categorising species based on their similarities and differences
Species are grouped into a hierarchy of different categories according to the biological characteristics that they share
Classifying species in this way allows us to quickly identify them and predict their characteristics
The hierarchical classification system—the higher ranks contain more organisms with less similarity between them and the lower ranks contain fewer organisms with more similarity between them
The first step to classifying a species is to put it into a group known as a genus
A genus is a category in the classification hierarchy that includes one or more species that are closely related and share common characteristics
Every species gets a two-part name in the following format: the name of the genus, then the name of the species
The genus name is always written first and with a capital letter
The species name does not have a capital letter
Both genus and species should be either italicised or underlined.
For example, humans are called Homo sapiens, where Homo is the genus and sapiens is the species
This could also be written as Homo sapiens
Although a genus usually contains more than one species, we are the only species in our genus that exists today
However, in the past, there were other species within the Homo genus, such as Homo neanderthalensis (Neanderthals) and Homo erectus, among others
An example of a well-known genus is the genus Canis
This genus includes several species of canids or members of the dog family, including:
Canis lupus (Grey wolf)
Canis familiaris (Domestic dog)
Canis latrans (Coyote)
Canis aureus (Golden jackal)
Canis simensis (Ethiopian wolf)
Canis mesomelas (Black-backed jackal)
Canis anthus (African golden wolf)
Canis adustus (Side-striped jackal)
Canis lupaster (African wolf)
These species share common characteristics, such as similar body structures, behaviours and genetic traits
While they may have distinct features and habitats, they are all grouped together under the genus Canis due to their close evolutionary relationship and shared ancestry
Taxonomic Tools
Taxonomists use various tools to identify an organism and to help them decide how to classify it
Identification in this context means determining which species an individual organism belongs to
Comparison of specimens with reference collections
Taxonomists can compare unknown specimens with well-documented reference collections
These reference collections contain a large number of similar organisms that have already been identified and classified
This method involves physically comparing the specimen to known samples
It relies on the taxonomist's expertise and the quality of the reference collection
For example, a botanist could identify an unknown plant specimen by visually comparing it with a large collection of known plant species at a botanical garden
Today there are apps that identify unknown species by comparing a photo to thousands of photos of different species in an online database (a virtual reference collection)
DNA surveys
DNA surveys involve analysing an organism's DNA to determine its species
This method compares the DNA sequence of the specimen with known sequences from a very large number of species, stored in very large computer databases
It provides precise and reliable identification, especially for closely related species
For example, in a wildlife conservation project, researchers could use DNA surveys to distinguish between similar-looking species of butterflies
Dichotomous keys
Dichotomous keys are tools used to identify organisms based on their characteristics
The keys consist of a series of paired statements or questions with two possible answers
Each pair offers two choices, leading the user to another pair of statements or questions, eventually resulting in the identification of the organism
Below is an example of a dichotomous key that can be used to identify eight species in the Serengeti ecosystem:
Serengeti Dichotomous Key
1 | a | Animal covered in black and white stripes | Zebra (Equus quagga) |
| b | Animal not covered in black and white stripes | go to 2 |
2 | a | Animal is a large cat | go to 3 |
| b | Animal is not a large cat | go to 4 |
3 | a | Animal covered in spots | Cheetah (Acinonyx jubatus) |
| b | Animal not covered in spots | Lion (Panthera leo) |
4 | a | Animal has horns | go to 5 |
| b | Animal does not have horns | go to 7 |
5 | a | Horns meet in the middle of the head | Cape buffalo (Syncerus caffer) |
| b | Horns do not meet in the centre of the head | go to 6 |
6 | a | Horns are long and curved | Grant's gazelle (Nanger granti) |
| b | Horns are not long and curved | Oribi (Ourebia ourebi) |
7 | a | Animal has a long neck | Giraffe (Giraffa camelopardalis) |
| b | Animal does not have a long neck | African elephant (Loxodonta africana) |
There are limitations to using a dichotomous identification key:
Limited scope:
Dichotomous keys are typically designed to identify a limited number of species and may not be comprehensive enough to identify all organisms in a given ecosystem
Inaccuracies:
Dichotomous keys are only as accurate as the information provided
If the key is not designed properly or lacks important distinguishing characteristics, the identification may be inaccurate
Variability:
Organisms can exhibit variability in their physical characteristics, which can make it difficult to accurately identify them using a dichotomous key
Time-consuming:
Using a dichotomous key can be a time-consuming process, especially for beginners who are not familiar with the organisms in question
Expertise required:
Dichotomous keys require a certain level of expertise and familiarity with the organisms in question
Beginners may find it difficult to use the key without assistance from an expert
Limited to physical characteristics:
Dichotomous keys are limited to the physical characteristics of organisms and may not take into account other important factors, such as behaviour or habitat, which can be important in identifying certain species
Examiner Tips and Tricks
You should try to practice using dichotomous keys with different examples to familiarise yourself with the process, as it can be a little confusing at first, especially when trying to identify closely related species.
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