Modern Classification

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More recent classification systems

Fungi are difficult to classify - they do not move about, their hyphae grow into the surrounding substrate in the same way as roots. However they do not photosynthesise. They digest organic matter and absorb the nutriends like animals.

Resulted in the five-kingdom classification.

Most recent research uses a wider range of techniques, and has produced even more detailed evidence. Physiology is the study of how living things work - how muscles contract, how oxygen enters the blood. Several more detailed branches of scientific study have grown out of physiology, including the study of biochemistry.

Using biochemistry in classification

Evidence from biochemistry can help to determine how closely related one species is to another. Certain large biochemical molecules are found in all living things. But they may not be identified in all living things. The differences reflect the evolutionary relationships.

Cytochrome C

A protein called cytochrome c is used in the process of respiration. All living organisms, except chemosynthetic prokaryotes, must respire, therefore all organisms must have cytochrome c. But cytochrome c is not identical in all species. Proteins are large molecules made from amino acids. The amino acids in cytochrome c can be identified. If we compare the sequence of amino acids in samples of cytochrome c from two different species, we can draw two certain conclusions:

if the sequences are the same, the two species must be closely related

if the sequences are different, the two species are not so closely related

the more differences found between the sequences, the less closely related the two species.

DNA

Certain biochemicals are found in all living organisms. All organisms use DNA or RNA. DNA always provides the genetic code - the instructions for producing proteins. That code is the same for all organisms. This means that a particular piece of DNA or RNA codes for the same proteins in a bacterium as in any other organism.

Comparison of DNA sequences provides a way to classify species. The more similar the sequence, the more closely related the two species. This is probably the most accurate way to demonstrate how closely related one species is to another. Such evidence has largely backed up the evolutionary relationships that have already been worked out. We can also use it to clarify or correct relationships we are unsure about.

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