Blackwell Publishing

Classification and evolution - Is phenetics natural and objective?


Phenetics is subjective

There is a lack of objectivity in the phenetic system. The phenetic classification represents the morphological similarity for large numbers of characters. Any consistency in a classification of this sort does not follow from the phenetic system itself; there is no hierarchy of morphological similarity which exists in nature. A phenetic classification is imposed subjectively by the taxonomist.

This is shown by the diagram. There are five species with two characters. To form figure (b) we group each species with its phenetically nearest neighbor. Two clusters of species 1 and 2, and 4 and 5 are created. But to which of these clusters should we join species 3? The nearest species is 2 and so we put it with cluster A. However, if we calculate average distance of each cluster as a whole, the answer is the opposite - cluster B has the nearest average neighbor to species 3 and we form figure (c).

Phenetic classification, therefore, even in its modern numerical form, is not objective. It can produce classifications, but classifications that lack a deep philosophical justification.

Figure: The phenetic similarity between species can be shown. In (a), five species have been measured for two characters, length of wing vein and length of tibia. The distance between the two species on the graph is the phentic distance between them. (b) The phenetic cluster by the single nearest neighbor technique puts species 3 with the group (cluster A) which has the nearest individual neighboring species (species 2). But if it uses the nearest average neighbor technique (c), it puts species 3 with the group (cluster B) that has the nearest average for all its species.

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