66 K. Sreedevi et al.
Conventional Taxonomy
- Evolutionary or traditional taxonomy
- Phenetics or numerical taxonomy
- Cladistics or phylogenetic systematics
- Cladoendesis
Evolutionary Taxonomy
Evolutionary taxonomy, originated in early twen-
tieth century, attempts to classify the organisms
based on phylogenetic relationships coupled
with degree of evolutionary changes. It takes
taxon into consideration rather than a species.
The characters differ in information content re-
garding phylogeny and hence have different
weights. Both recency of phyletic splitting and
rate of divergence are given importance. Evolu-
tionary taxonomy evolved through the influence
of theory of evolution on Linnean classification
during post Darwinian period where the tree of
life gained importance in scientific works with
publication of The Origin of Species. In 1930s,
few biologists developed a Mendelian frame-
work for Darwinian evolutionary theory, result
of which was the evolutionary synthesis (Marc
Ereshefsky 2007 ). Theodore Dobzhansky, Ernst
Mayr, and Gaylord Simpson were the few of
evolutionary taxonomists. The school consists of
two tenets, firstly all taxa being a genealogical
lineage and secondly constructing classification
that reflects both cladogenesis (branching) and
anagenesis (divergence). In cladogenesis, specia-
tion occurs with the selection pressure (genetic
revolution) on the isolated population from the
rest of the species where single lineage splits into
two branches whereas in anagenesis, speciation
occurs in a single lineage. As a result, evolution-
ary taxonomists see two types of taxa viz., mono-
phyletic and paraphyletic taxa arising from the
processes of speciation through cladogenesis and
anagenesis, respectively. In brief, evolutionary
taxonomists believe that classifications should
highlight only genealogical taxa, and those taxa
can be either monophyletic or paraphyletic (Marc
Ereshefsky 2007 ).
Phenetics or Numerical Taxonomy
Phenetic systematics determines the relationships
of organisms through a measure of similarity,
considering plesiomorphies (ancestral traits) and
apomorphies (derived traits) to be equally infor-
mative. It aims at natural classification using nu-
meric algorithms like cluster analysis rather than
using subjective evaluation of properties. A priori
every character is given equal weight. From the
twentieth century onward, it was superseded by
cladistics, which considers plesiomorphies to
be uninformative for an attempt to resolve the
phylogeny of earth’s various organisms through
time. Today’s systematists generally make exten-
sive use of molecular biology and computer pro-
grams to study organisms. An alternative to these
matrix methods in phylogenetics and systematics
is cladoendesis.Cladistics or Phylogenetic Systematics
Cladistics got conceptualized in second half of
the twentieth century and is termed as Phyloge-
netic systematics by Willi Hennig (also the title
of his 1966 book). In cladistics, classification is
mainly based on common ancestry and hence, be-
lieves in cladogenesis, where two taxa originated
in the same branching event have a common an-
cestor that is not shared by any other taxon. Thus,
cladistics represents only monophyletic taxa in
their classifications. Those who follow cladistics
perceive that the concepts of phenotypic differ-
ence and adaptive zone are ambiguous and are
applied inconsistently to different types of taxa
(Hennig 1966 ; Eldredge and Cracraft 1980 ). Cla-
dists believe that the concepts of phenotypic di-
versity and adaptive zone are too malleable and
reject them as grounds for classifying taxa (Marc
Ereshefsky 2007 ). A group of cladists developed
the Phylocode—a phylogenetic code of biologi-
cal nomenclature, which is considered alternative
to the Linnaean system (Cantino and de Queiroz
2004 ). The widely used and popular phylogenetic
approach is cladistics.