Cladistics

CLADISTICS

Cladistics is another way of classifying organisms and is the most commonly used method in today s time. It is an overall classification system that categorizes groups of plants or animals into clades, which is based on the most recent common ancestor of the group. It is determined by looking at the derived characteristics that can be seen in the group s most recent common ancestor. Figure 2 shows the clade involved in human skull formation:

Figure 2.

Willi Hennig was an entomologist who first wrote a book on cladistics in 1966. It has been used to some degree since 1901, when it was applied to birds. It was later applied to insects and to plants. It was originally used to describe differences in structural characteristics but, when the polymerase chain reaction or PCR testing was developed in 1990, the use of DNA characteristics to define relationships between organisms became more popular. Computers are used to identify DNA differences and similarities.

What gets created when cladistics is used is called a cladogram. This looks like a tree that best hypothesizes the evolutionary relationship between organisms. As mentioned, genetic relationships are now more commonly used to do this than morphological characteristics. We spoke of parsimony in the creation of phylogenetic trees. Because of the accuracy of DNA sequencing, parsimony is no longer completely followed and cladograms can be more sophisticated. Interestingly, when different pieces of data are used, the cladograms produced can be different and confusing.

There are certain terms used to describe the character states within members of a clade. A plesiomorphy is the ancestral state that has been retained by the descendants of the ancestor. A symplesiomorphy is a character state that is different among members of a clade, even though they are related. Cold-bloodedness in reptiles and warm-bloodedness in birds are different phenomena, even though these are closely related organisms. In such situations, warmbloodedness is a new feature, called a symplesiomorphy.

An apomorphy is a new innovation or derived state within a clade. It can be used to name a clade. Clades are identified by the synapomorphies, which are shared within a clade, while autapomorphies are traits that do not show a relationship between clade members. A synapomorphy can be something like having digits that helps to define the clade.

Any characteristic that is seen in at least two descendants but is not seen in the common ancestor is called a homoplastic state. Mammals and birds are both warm-blooded but the common ancestor was not warm-blooded. They also represent different smaller clades but are part of a larger clade. Because it was developed independently from one another, warmbloodedness in mammals and birds is not a synapomorphy. Instead, warm-bloodedness in these two groups is considered a homoplasy that does not identify these two organism types as part of a clade.

So, basically, an apomorphy is something that each member of a clade has in a monophyletic clade, unless the organism subsequently lost the apomorphic trait. If there is at least one plesiomorphy in a clade that was not inherited by all of the descendants, it is called a