Sorting life by its family tree

A3.2.1 Why we classify

Why sort life at all?

There are more kinds of living thing than anyone can hold in their head. Biologists have named well over a million species, and many more are still waiting to be found. Nobody can learn them one by one. So we sort them. Classification turns the diversity of life from a heap into a map.

A good map does two jobs. It helps you name and find an organism: once you know which group a beetle or a fern belongs to, you know where to look it up and what it is probably like. And it makes further study possible. When a new species is found, its group tells a biologist what to ask about it, such as what its relatives eat, how they breed and how best to protect them, instead of starting from nothing. Medicine, farming and conservation all lean on groups in this way.

• Classification is needed because there are too many species to study one at a time.
• Once organisms are classified, a broad range of further study becomes possible.
• Different rules give different groups, so the rule matters.

A3.2.3 Classification that follows relationships

Groups that are family trees

Think of your own family. Cousins tend to resemble each other more than strangers do, because they share grandparents. Living things are the same, only the family is far bigger and the shared ancestors are further back. The ideal classification follows that family tree, so that every member of a group has evolved from one common ancestor, and none of that ancestor's descendants are left out.

The great strength of such groups is prediction. A feature that appeared in the common ancestor is passed to all its descendants, so it is shared across the group. If you know that one member of the group has hair and feeds its young on milk, you can predict the same of a newly found member without testing it. The figure follows one feature through a tree.

The predictions reach beyond looks, to biochemistry, life cycles, behaviour and disease. Plants in one group often make similar chemicals, so a chemical that proves useful in one plant is first looked for in its relatives. A group made from looks alone cannot promise this, because look-alikes with different ancestors need not share anything underneath.

• The ideal classification follows evolutionary relationships: all members of a group descend from one common ancestor.
• Features of one member can be predicted for the rest, because they are shared within the group.

A3.2.5 The molecular clock

Differences that count the years

Mutations change a gene's sequence at random. When one species splits in two, each line collects its own mutations from then on. So the longer ago two species split, the more differences you find between the same gene in each (you met this with sequences in A4.1). If changes arrive at a roughly steady rate, the differences work like a clock.

To turn a count into a date you need a calibration. Take pairs of species whose split is dated from fossils, and plot their differences against time. A straight line through the points is the molecular clock. Now a pair with no fossil date can be placed: read its number of differences across to the line and down to the time.

The result is an estimate, never a date. The rate at which changes build up is not the same everywhere, because it is affected by the length of the generation time, the size of the population, the intensity of selection and other factors. In the figure's last step, a line that collected changes faster gives a quite different answer from the same count.

A3.2.7 Analysing cladograms

Reading a cladogram

A cladogram is a map of splits, so you read it from the base up to the tips. The base is the root. Each fork is a node, which stands for a hypothetical common ancestor: it must have existed to explain the split, but it need not be a known fossil. Each line out to a tip is a terminal branch. Tap the parts.

To deduce a relationship, follow two tips back until their lines meet. That node is their most recent common ancestor. The nearer it is to the tips, the closer the relatives. Two groups that branch from the same node are sister groups. In the figure the human and chimpanzee meet at a node that the gorilla has not yet joined, so a human is closer to a chimpanzee than to a gorilla.

Any node and everything beyond it is a clade, so you can read clades straight off the tree. Then use the clade to deduce what its members share.

A3.2.9 The three domains

A new level above kingdom

For a long time life was sorted into a few kingdoms: animals, plants, fungi, and two kinds of single-celled life, the ones with nuclei and the ones without. The cells without nuclei, the prokaryotes, were treated as one group. Then, in 1977, Carl Woese and George Fox compared rRNA base sequences from many organisms, and the picture changed.

Why rRNA? Every living cell has ribosomes, so every organism has rRNA genes to compare. They can be lined up across the whole of life, from a bacterium to a whale. The comparison showed that the prokaryotes were not one group. Some, such as the methane-makers, had rRNA as different from ordinary bacteria's as bacteria's is from ours.

Life falls into three great branches, now called domains: Bacteria, Archaea and Eukarya (organisms whose cells have nuclei). Archaea look like bacteria under a microscope, but they share a more recent common ancestor with Eukarya. This was a revolutionary reclassification, with an extra level above the kingdoms.