4.6.4Classification
Traditionally, living things have been classified into groups depending on their structure and characteristics, in a system developed by Carl Linnaeus.
Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species. Each level is a subdivision of the one before it, so the groups get smaller and the organisms in them more alike as you go down.
Organisms are named by the binomial system of genus and species — Homo sapiens, Panthera leo. The genus comes first with a capital letter, the species second in lower case.
Classification systems have changed as the evidence available changed. As evidence of internal structures became more developed, due to improvements in microscopes, and as the understanding of biochemical processes progressed, new models of classification were proposed.
That is the impact of developments in biology on classification systems, and it is the point of this sub-topic. Linnaeus could only classify by what he could see; later biologists could look inside cells, and later still could compare the chemistry. Better evidence produced a better grouping — which is how scientific methods and theories develop over time.
Due to evidence available from chemical analysis, there is now a 'three-domain system' developed by Carl Woese. In this system organisms are divided into:
• archaea — primitive bacteria usually living in extreme environments • bacteria — true bacteria • eukaryota — which includes protists, fungi, plants and animals.
The striking part is that archaea and bacteria look almost identical under a microscope, and Linnaeus would have had no way to separate them. Their chemistry shows they are as different from each other as either is from us — which is why the new evidence forced a new system rather than a small adjustment.
Evolutionary trees are a method used by scientists to show how they believe organisms are related. They use current classification data for living organisms, and fossil data for extinct ones.
To interpret one: work from the bottom up. Each point where the tree splits is a common ancestor of everything above it. Two organisms that branch apart near the top shared an ancestor recently and are closely related; two that split near the bottom have been separate for far longer. It is the branching pattern that matters, not how near the tips happen to be drawn to each other.
Common mistake
An evolutionary tree does not show organisms evolving into the ones next to them. Humans did not evolve from chimpanzees — the tree shows that humans and chimpanzees share a common ancestor at the point where their branches meet. Reading it as a ladder rather than a branching tree is the usual error.
- Binomial system
- the naming of organisms by genus and species, with the genus first and capitalised and the species second in lower case
- Classification
- the grouping of living things by their structure and characteristics, in Linnaeus's system into kingdom, phylum, class, order, family, genus and species
Before you practise
- Linnaeus: kingdom, phylum, class, order, family, genus, species.
- The binomial name is genus then species — Homo sapiens.
- Better microscopes and biochemistry gave evidence Linnaeus never had, so new models were proposed.
- Woese's three domains: archaea, bacteria, eukaryota.
- Evolutionary trees use classification data for living organisms and fossil data for extinct ones.
- A branch point is a common ancestor — not one organism turning into another.