Artificial classifications are a key or classification based on a particular organ. This forms a System, one that can predict or mimic a natural classification.
Taxonomists, systematists and biogeographers often use artificial classifications or Classification Systems in order to identify and classify taxa. People around the world use classification systems everyday. This is one that many learn at school:
Classification systems are helpful in identifying taxa but they only mimic real relationships. In the case above only mammals and birds are natural (monophyletic) groups, but the classification system for birds may also apply to taxa that are categorized as reptiles. In other words, the system above only mimics the natural group (i.e., birds), but it does use the homologies that define that group.
- Fish have scales and no limbs.
- Amphibians lay eggs on land and live in water.
- Reptiles lay eggs, have scales and live on land.
- Birds lay eggs and have feathers.
- Mammals have skin and hair, mothers feed their young milk.
Linnaeus was the first person to define a classification system that attempts to mimic natural groups. The system can still be used today in order to identify plants. What Linnaeus’s, or any classification, does not do is purport to be a natural method.
A method is a key or classification based on all of the organs of a taxon; methods are sub-divided into artificial and natural depending on their purpose.Classification methods not only mimic, they also may predict. In either case they attempt to generate classifications that are near the mark. Phenetics uses a method in order to generate a classification that mimics a natural group. The method for doing so can be useful in order to work out similarities between taxa, but the method is only a mimic. Phenetics becomes problematic when it starts getting closer to the mark. In some cases a phenetic analysis can replicate a true relationship – a homology – without the need for homologies. Although these methods are praiseworthy, they do not actually find homologies. A mimic only replicates something, it does not actually discover. A phenetic analysis may for instance replicate a monophyletic group perfectly, using an assortment of homologues, but since the method uses similarity (i.e., non-relationships) it cannot, by definition, discover homologies, even though it replicates them perfectly.
An analogy would be to state that anything that lives in water and lays eggs on land is an amphibian. Although this behavioural trait is more likely to be common amongst toads, frogs, salamanders and newts, it is not a homology as it is something not unique to that group. Birds may lay eggs and bear feathers, but so do a number of therapod groups. Similarity is not a relationship, only a measurement of likeness based on one or more hypotheses.
Phenetics becomes problematic when it confuses the mimic for the real thing. Certainly phenetics can create a classification system using a method of similarity, but it does not discover natural groups. Therefore the term Natural System is a contradiction. A system cannot be natural as it is based on a single characteristic or assumption and not relationship. Natural groups, as pointed out in the post Phenetic "Natural" Classifications, are not based on a priori assumption:
"... system of classification is the more natural the more propositions there are that can be made regarding its constituent classes" (Sokal & Sneath 1963: 19).Sokal and Sneath (1963) have turned the mimic into natural group.
Phenetics as purveyor of natural groups is erroneous and prophetic. Stating that natural groups can be reached through a system of quantification and similarity is appealing to those that rely on statistical programs. Most systematists and biogeographers rely on such programs and have swallowed the “phenetic prophesy” hook, line and sinker. Natural groups, it seems, is just a matter of quantity.
Wag the Dog
The phenetic prophesy states that similarity* is relationship, and can discover natural groups. This is wagging the dog.
Taxonomists, systematists and biogeographers can only discover patterns, homologies that give us insight into relationship. Before we do this we may impose a system of beliefs, hypotheses and theories about our own groups and their relationships. Some times we test these assumptions by discovering homologies and find that we were right. That is the nature of a robust scientific discipline. Once we turn that around and impose our own “natural” law, then we can only formulate more hypotheses in differing ways, never discovering only generating. Molecular systematics is now in a unique position to learn from 300 years of systematic theory that has discovered time and time again that homology is not similarity. Unfortunately many in the field ignore the past systematic literature and read that of the phenetic prophesy.
One day someone bent over a PCR machine may come to realise that they are part of a 300 year cycle of wagging.
*There are two forms of similarity. One is that of simile “That kangaroo looks like a rat”. The other is quantifiable and is born from statistics (i.e., divergence and possibility) “The ape is 22% banana”. We refer to the latter form throughout this post.
Sokal R.R. & Sneath P.H.A. 1963. Principles of Numerical Taxonomy. W. H. Freeman, San Francisco.