Fishy Cladistics

Vertebrates used to be so simple. They came in five common-sense categories: amphibians, birds, fish, mammals and reptiles. Birds were the winged and feathered ones, reptiles the scaly, cold-blooded ones. And so on. A place for everything and everything in its place.

That was before cladistics, a more rational system of taxonomy initiated by the German entomologist Willi Hennig in the 1960s. It analyses shared characteristics and genetic relationships to group species according to their evolutionary ancestry. That sounds fair enough - but strict rationality, it turns out, plays havoc with those familiar groupings. ²

But pity the poor reptiles. The traditional group Reptilia - things like lizards, crocodiles, snakes, tortoises plus many extinct groups - is not a true clade, because the common ancestor of all those animals also gave rise, at different points, to mammals and birds. According to cladistics, you can clump these three groups together in a mega-grouping, known as the amniotes, but you cannot hack off a single, consistent reptile branch (see diagram).

Amphibians fare a little better, but only the living ones: frogs, toads, newts, salamanders and worm-like caecilians. Include the extinct ones, and you encounter the reptile problem on bigger scale: the relevant clade includes all tetrapods, the four-limbed vertebrates. And don't even start on fish. ³

If you think this is cladistic correctness gone mad, you have a point. For everyday purposes, most biologists are happy to use traditional, common-sense classifications based on obvious characteristics. You won't ever hear them refer to "non-avian, non-mammalian amniotes" when "reptiles" would do. But the term is really a hangover from a less well-informed age. ⁴

Synapomorphies are the basis for cladistics

Cladistics is a particular method of hypothesizing relationships among organisms. Like other methods, it has its own set of assumptions, procedures, and limitations. Cladistics is now accepted as the best method available for phylogenetic analysis, for it provides an explicit and testable hypothesis of organismal relationships.

The basic idea behind cladistics is that members of a group share a common evolutionary history, and are "closely related," more so to members of the same group than to other organisms. These groups are recognized by sharing unique features which were not present in distant ancestors. These shared derived characteristics are called synapomorphies.

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What assumptions do cladists make?

There are three basic assumptions in cladistics:

1. Any group of organisms are related by descent from a common ancestor.

2. There is a bifurcating pattern of cladogenesis.

3. Change in characteristics occurs in lineages over time.

The first assumption is a general assumption made for all evolutionary biology. It essentially means that life arose on earth only once, and therefore all organisms are related in some way or other. Because of this, we can take any collection of organisms and determine a meaningful pattern of relationships, provided we have the right kind of information. Again, the assumption states that all the diversity of life on earth has been produced through the reproduction of existing organisms. ⁵

HOW TO CONSTRUCT CLADOGRAMS

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2. Determine the characters (features of the organisms) and examine each taxon to determine the character states (decide whether each taxon does or does not have each character). All taxa must be unique.

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5. Work out conflicts that arise by some clearly stated method, usually parsimony (minimizing the number of conflicts).

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To accomplish the task of creating a good cladogram, you must use your judgement [sic]. Ask yourself the following questions and answer them carefully.

• Could a supposed synapomorphy be the result of independent evolutionary development?
• Are your characters chosen well?
• Should you consider other characters?
• Should you consider additional taxa? ⁶