Science at a Distance

CLAS The Impact of Evolution

Darwin changed everything. The publication of his work on The Origin of Species in 1859, threw the whole of biological science into a new paradigm, including the study of classification theory and the principles of taxonomy.

While using logic as the basis of their work, both Aristotle and Linnaeus had developed their classification schemes on taxonomic principles that were fundamentally arbitrary. Their groups, while logical, were not based on any obvious relationships of a biological nature. They were convenient groups that humans could quickly see, identify and use.

This was acceptable because (a) no one could think of anything better, and (b) most people at the time believed in the 'fixed species' concept in which organism had been created in their current form and could never change.

After Darwin it was realized that organisms could indeed change, and that all current forms of living things had arrived at that form by change and natural selection, the mechanism of evolution. Scientists began to construct phylogenies, lists or diagrams that showed the evolutionary paths taken by populations of organisms through many generations and over long periods of time.

These phylogenetic diagrams quickly started to look like trees, as it was realized that ancestral stocks occasionally broke up, branched and became two or more different species, which could later branch again and again. A phylogenetic tree was a bit like a family tree, showing who the nearest relatives were and who shared a common ancestor, and when.

Organisms were related to one another, and these relationships could form the basis of a new type of taxonomy; on based on evolutionary origin and evolutionary relatedness.

CLAS Two main types of taxonomy have arisen from this principle. They look alike, but there are serious differences. Evolutionary systematics makes an attempt to construct trees that accurately show phyletic lineages (proper branching on the family tree), along with a consideration of when and how new species arose and moved into new habitats and niches (established a 'new' way of life as opposed to some trivial character change).

CLAS Cladistics, however, ignores when and where a branch occurs, tries to use purely objective criteria, and defines each branch point by a fundamental character of evolutionary significance. Both methods have their strengths and weaknesses.

Cladistics gets its name from the branches on the family tree, which are called clades. A cladogram is a stylized diagram that looks like a series of Y's or forks in a road. At each branch, or "Y" junction, novel characters of evolutionary origin are used to separate off one group from the rest.

Cladograms can be constructed for any group of organisms. For example, the following organisms are a set from which a cladogram can be made; kangaroo, earthworm, amoeba, lizard, cat, sponge, and salmon. Each of these creatures has an evolutionary relationship to one another. They all share a common origin, and their current forms are all derived from branching events somewhere in the phylogenetic past. The question is, when did these branches occur?

The process of constructing a cladogram begins with data; a table of traits or characteristics that have evolved or been derived by the evolutionary process.

Derived Characters
segmented jaws hair placenta multicellular limbs
kangaroo + + + - + +
earthworm + - - - + -
amoeba - - - - - -
lizard + + - - + +
cat + + + + + +
sponge - - - - + -
salmon + + - - + -

In the next step each of the organisms are compared to see if they share a trait or derived character. For example all but the amoeba share the common derived trait of 'multicellularity', but only the cat and the kangaroo share the derived trait of 'hair'.

Using these patterns of shared derived characters, a cladogram can be constructed as a series of Y's or branches. At every branch, one of the organisms that does not share a common character with the rest of the group is "branched off" into its own clade. The order, or sequence, of these branches depends on how many characters are left within the larger group.

See for yourself. Move your cursor over the control buttons beside the group diagram and reconstruct the cladogram generating process:

CLAS Cladograms are a useful way of organizing, in a visual way, the relationships between creatures that share and do not share derived characters. In practice, when cladograms are constructed, many hundreds of characters may have to be considered, and computers are needed to sort out the best fit between the branches and who should be on them.

Cladograms emphasize the sequence or order in which derived characters arise from a central phylogenetic tree. That is their main strength. However, nothing in a cladogram indicates how strong or profound the derived character is, and its evolutionary importance. Equal weight is given to all the characters used. This can sometimes lead to unusual groupings which may be technically correct, but questionable.

Evolutionary Systematics uses a taxonomic principle that is also based on the splitting of phyletic lineages, and constructs family trees, but in contrast the pure cladistic method, a weighting system is used that favors some derived characters over others. For example, a derived character or trait such as giving birth to young alive (as seen in the marsupials and mammals) is considered more significant than the variety of colors seen in bird feathers.

However, and it is a big 'however', evolutionary systematists do find it necessary to use a lot of subjective judgment when deciding which factors to weight the most heavily. Whenever humans substitute their own 'feelings' over provable biological fact, the results will always have to be viewed with care and constant revision. Even with this caution, however, evolutionary systematics appears, at the moment to, give a picture of biological relationships between species that is based on the soundest taxonomic principles.


Science at a Distance
© 1998 Professor John Blamire