The theoretical bases and approaches of cladistics and some specific
problems that, directly or indirectly, rely on cladistic analysis for their revo-
lution, are outlined and discussed. Seven sections comprise this paper: a ) the phi-
losophical foundation of cladistics; b) the theoretical tenets of cladistics; c) the
operational procedure of cladisties; d) three schools of classification; e) cladistics
and biogeography; f) cladistics and hybrid recognition; and g) is cladistic sys-
tematics a scientific theory ?
Considerations of scientific methodology involve philosophical questions.
From this point, Popper'falsificationism serves a good foundation. Popper
emphasizes that all scientific knowledge is hypothetical-deductive, consisting of
general statements (theories) that can never be confirmed or verified but only
falsified. The theories, that can be tested most effectively, are preferable.
Cladistics, aiming at generating accurately expressed and strictly testable systematic
hypotheses, is well compatible with this requirement.
The principles central to the cladistic theory and methodology are: the
Principle of Synapomorphy; the Principle of Strict Monophyly; and the Principle
of Strict Parsimony. The first requires forming nested groups by nesting
statements about shared evolutionary novelties (synapomorphy) postulated from
observed similarities and is the primary one. The second is mainly
methodological, subject to modification and compromise. The principle of strict
parsimony specifies the most preferable hypothesis (namely the one exhibiting
the most congruence in the synapomorphy pattern).
The operational procedure that might be followed in formulating and testing
hypotheses of the synapomorphy pattern (the cladogram itself) consists of five
steps. The erections of monophyletic groups, to a greater or lesser extent, rely
on the hypothesis of the previous systematic studies and is the starting point for
cladistic analysis. Character analysis, which focuses on character distribution and
determination of the polarities, decides the reconstructed phylogeny. A detailed dis-
cussion on the methodological principles for identifying transformation sequence is
presented. Many algorithms have been designated to infer the cladogram,
and are basically of parsimony techniques and Compatibility techiques. The thus
yielded cladograms, with their expected pattern of congruent synapomorphies, are
tests of a particular hypothesis of synapomorphy and reciprocally synapomorphies
are tests of cladistic hypothesis (cladogram). Such reciprocity is a strong stimulus
to profound understanding on phylogenetic process and phyletic relationships. The
cladogram and the Linnaean classification have the identical logic structure and
the set-membership of the two can be made isomorphic.
There are three principal approaches to biological classification : cladistics,
phenetics and evolutionary classification. Cladistics is the determination of the
branching pattern of evolution, and in the context of classification, the develop-
ment of nested sets based on cladograms. Phenetics is the classification by overall
similarities, without regard to evolutionary considerations. Evolutionary classifica-
tion attempts to consider all meaningful aspects of phylogeny and to use these for
making a classification. The last approach has been done intuitively, without ex-
plicit methods. An enumeration of their differences and a discussion on their rela-
tive merits are presented.
Three theoretical approaches have been proposed for interpreting
biogeographical history: the phylogenetic theory of biogeography, classical evolu-
tionary biogeography and vicariance biogeography. The former two show some
similarities in that they usually look upon biogeography in terms of centers of
origin and dispersal from the centers. But the first puts a strong emphasis on the
construction of hypotheses about the phylogenetic relationships of the organisms in
question and the subsequent inference of their geographic relationships; the second
advocates a theory which does not have a precise deductive link with phylogenetic
construction and often results in wildly narratative-type hypotheses. The vicariance
approach de-emphasizes the concepts of centers of origin and dispersal and at-
tempts to analyse distribution patterns in terms of subdivision (vicariance) of
ancestral biotas. The development of the theory of plate tectonics and its
universal acceptance enormously stimulate biogeographers to look at the world's
continents and oceans from a mobilist point, which, along with the establishment
of the rigorous tool of the phylogenetic analysis (cladistics), profoundly reshapes
the above three theories.
Hybridization and polyploidy are outstanding features of many plant groups.
But hybridization, or reticulate evolution, is inconsistent with the basic concepts
of cladistics which is an ever-branching pattern. Cladists have suggested several
approaches. One of them analyses all the taxa by a standard cladistic procedure
and closely examines the cladograms for polytomies and character conflicts that
may indicate possible hybrids. Such generated hypothesis of hybridization can be
corroborated or falsified by other forms of data, such as distribution, polyploidy,
karyotype and pollen fertility.
There are three criteria to justify a theory to be scientific: a) whether it is a
theory composed of hypotheses strictly falsifiable; b) whether it has predictive
effect; and c) whether it has a explanatory value. Cladistic systematics aims at
generating cladograms, which are hypotheses of the nested pattern of
synapomorphy, phylogenetic process and phyletic relationships, susceptible to
testing by postulated synapomorphies. The predictive effect of systematics relies on
the acceptance of hypotheses of congruence about the correlation of characters,
which has been well founded. For non-systematic biologists, phylogenetic
classification can be used as axiom to form a preliminary and fundamental
explanation.
