Everything about Phenetics totally explained
» Phenetics shouldn't be confused with phonetics, the study of speech sounds, despite the similarity in pronunciation.
In
biology,
phenetics, also known as
numerical taxonomy or
taximetrics, is an attempt to classify organisms based on overall similarity, usually in
morphology or other observable traits, regardless of their
phylogeny or evolutionary relation.
Phenetics has largely been superseded by
cladistics for research into evolutionary relationships among species. However, certain phenetic methods, such as
neighbor-joining, have found their way into cladistics, as a reasonable approximation of phylogeny when more advanced methods (such as
Bayesian inference) are too computationally expensive.
Phenetic techniques include various forms of clustering and ordination. These are sophisticated ways of reducing the variation displayed by organisms to a manageable level. In practice this means measuring dozens of variables, and then presenting them as two or three dimensional graphs. Much of the technical challenge in phenetics revolves around balancing the loss of information in such a reduction against the ease of interpreting the resulting graphs.
Difference from cladistics
Phenetic analyses don't distinguish between
plesiomorphies - traits that are inherited from an ancestor (and therefore phylogenetically uninformative) - and
apomorphies - traits that
evolved anew in one or several lineages. Consequently, phenetic analyses are liable to be misled by
convergent evolution and
adaptive radiation. A typical error occurring in phenetic analysis is that
basal evolutionary grades - which retain many plesiomorphies compared to more advanced lineages - appear to be
monophyletic.
Consider for example
songbirds. These can be divided into two groups -
one that retains ancient characters in
phenotype and
genotype, and
one that has more modern traits. But only the latter are a group of closest relatives; the former are numerous independent and ancient lineages which about are as distantly related to each other as each of them is to the more modern songbirds. In a phenetic analysis, the large degree of overall similarity found among the former will make them appear to be
monophyletic too, but their shared traits were present in the ancestors of
all songbirds. It is the loss of these ancestral traits rather than their presence that signifies which songbirds are more closely related to each other than to other songbirds.
But the two methodologies need not be mutually exclusive. In general, phenetics is today recognized to provide little if any information about the evolutionary relationships among
taxa. But there's no reason why for example species identified using phenetics can't subsequently be subjected to cladistic analysis, to determine its evolutionary relationships.
Phenetic methods can be superior to cladistics when only the
distinctness of related taxa is important, as the computational requirements are lower. On the other hand, whenever information on the
evolutionary history of taxa is needed for a study, cladistic methods are used today.
Phenetics today
Traditionally there was a great deal of heated debate between pheneticists and cladists, as both methods were initially proposed to resolve evolutionary relationships. Perhaps the "high-water mark" of phenetics were the
DNA-DNA hybridization studies by
Charles G. Sibley,
Jon E. Ahlquist and
Burt L. Monroe Jr., from which resulted the 1990
Sibley-Ahlquist taxonomy for
birds. Highly controversial at its time, some of its findings (for example the
Galloanserae) have been vindicated, while others (for example the all-inclusive "
Ciconiiformes" or the "
Corvida") have been rejected. However, with computers growing increasing powerful and widespread, more refined cladistic
algorithms became available and could put the suggestions of
Willi Hennig to the test; as it turned out, the results of cladistic analyses turned out to be superior to those of phenetic methods - at least when it came to resolving phylogenies.
Many systematists continue to use phenetic methods, particularly in addressing species-level questions. While the ultimate goal of taxonomy includes finding the 'tree of life' - the evolutionary path connecting all species - in
fieldwork one needs to be able to separate one
taxon from another. Classifying diverse groups of closely-related organisms that differ by very subtle differences is difficult using a cladistic approach. Phenetics provides numerical tools for examining overall patterns of variation, allowing researchers to identify discrete groups that can be classified as species.
Modern applications of phenetics are common in
botany, and some examples can be found in most issues of the journal
Systematic Botany. Indeed, due to the effects of
horizontal gene transfer,
polyploid complexes and other peculiarities of plant
genomics, phenetic techniques in botany - though less informative altogether - are also less prone to errors compared cladistic analysis of
DNA sequences.
In addition, many of the techniques developed by phenetic taxonomists have been adopted and extended by
community ecologists, due to a similar need to deal with large amounts of data.
Further Information
Get more info on 'Phenetics'.
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