BSC 314 Lecture Notes - Lecture 42: Succulent Plant, Euphorbiaceae, Cluster Analysis
27 Jun 2018
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Types of Classifications
Classifications are orderly ways to present information and, depending upon their
objectives, can be artificial, natural, or phylogenetic (phyletic), which includes phenetic
and cladistic.
Artificial and natural classifications
Classifications that use single or at most only a few characteristics to group plants usually
are artificial classifications—that is, all the plants in a single group share the same characteristics, but
they are not closely related to one another genetically. Popular floras (books to identify plants of a certain
area) sometimes group plants using color of their flowers, or their growth form (trees, shrubs, herbs, and
so on). Although such books are useful in finding the names of taxa, they provide few clues about
relationships among the taxa and hence are not predictive, which means that you can deduce nothing
more about the plant other than that it exhibits the characteristics used to classify
it. Natural classifications group together plants with many of the same characteristics and are highly
predictive. That is, by enumerating the characteristics of a plant, one can predict the natural group to
which it belongs. Taxonomic floras, for example, identify species, genera, and families by listing as many
characteristics as possible concerning anatomy, morphology, cytology, ecology, biochemistry, genetics,
and distribution.
Phylogenetic (phyletic) classifications
Phyletic classifications are natural classifications that try to identify the evolutionary history of natural
groups. When botanists accepted Darwin's theory of evolution near the end of the last century, the
reasons why some groups of plants looked alike became clear: They were related to one another by a
common ancestry. The mission of taxonomy since Darwin has become a quest for evolutionary
relationships, not just at the lower levels of the hierarchy, but at the upper levels as well.
The evolutionary history of a taxon is called its phylogeny. To establish phylogenies, decisions must be
made concerning which characteristics are “primitive” and which “advanced”—that is, which taxon is the
ancestor of the others. Early phylogenetic classifications were based primarily upon plant morphology and
anatomy with great emphasis upon reproductive morphology, which is more stable and less influenced by
the environment than is vegetative morphology. Today, taxonomists additionally use the techniques of
biochemistry and molecular biology to add details of internal organization and mechanisms to the
classifications. But phylogenies, no matter how carefully constructed, are dependent upon someone's
interpretation of data, and herein lies the problem: Systematists frequently differ in their interpretations of
relationships. A phylogenetic classification is a hypothesis, a scientific explanation of the data and, like
any hypothesis, is subject to further testing.
Certain assumptions are necessary in phylogenetic classifications. A taxon should bemonophyletic (all
of the members of the taxon should be descendants of a single common ancestor). The characters or
features used to identify the taxa must behomologous, which means that they must have a common
origin, but not necessarily a common function. For example, all the parts of a flower—petals, sepals,
stamens, and carpels—originate in the same way as leaves from primordia in meristems. Although they
now have different functions in the flower (they're not photosynthetic), some sepals and petals structurally
resemble leaves. Leaves and the parts of the flower are homologous structures.
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