Pipette WORLD OF MICROBIOLOGY AND IMMUNOLOGY438•
be based on the evolutionary history of life, such that it predicts
properties of newly discovered or poorly known organisms.
Phylogenetic systematics is an attempt to understand the
evolutionary interrelationships of living things, trying to inter-
pret the way in which life has diversified and changed over
time. While classification is primarily the creation of names
for groups, systematics goes beyond this to elucidate new the-
ories of the mechanisms of evolution.
Cladistics is a particular method of hypothesizing rela-
tionships among organisms. Like other methods, it has its own
set of assumptions, procedures, and limitations. Cladistics is
now accepted as the best method available for phylogenetic
analysis, for it provides an explicit and testable hypothesis of
organismal relationships.
The basic idea behind cladistics is that members of a
group share a common evolutionary history, and are “closely
related,” more so to members of the same group than to other
organisms. These groups are recognized by sharing unique
features that were not present in distant ancestors. These
shared derived characteristics are called synapomorphies.
Synapomorphies are the basis for cladistics.
In a cladistic analysis, one attempts to identify which
organisms belong together in groups, or clades, by examining
specific derived features or characters that those organisms
share. For example, if a genus of bacteriaforms a specific
color or shaped colony, then those characters might be a use-
ful character for determining the evolutionary relationships of
other bacteria. Characters that define a clade are called
synapomorphies. Characters that do not unite a clade because
they are primitive are called plesiomorphies.
In a cladistic analysis, it is important to know which
character states are primitive and which are derived (that is,
evolved from the primitive state). A technique called outgroup
comparison is commonly used to make this determination. In
outgroup comparison, the individuals of interest (the ingroup)
are compared with a close relative. If some of the individuals
of the ingroup possess the same character state as the out-
group, then that character state is assumed to be primitive.
There are three basic assumptions in cladistics:- Any group of organisms are related by descent from a
common ancestor. - There is a bifurcating pattern of cladogenesis.
- Change in characteristics occurs in lineages over time.
The first assumption is a general assumption made for all
evolutionary biology. It essentially means that life arose on
Earth only once, and therefore all organisms are related in one
way or another. Because of this, scientists can take any collec-
tion of organisms and determine a meaningful pattern of rela-
tionships, provided they have the right kind of information.
The second assumption is that new kinds of organisms
may arise when existing species or populations divide into
exactly two groups. The final assumption, that characteris-
tics of organisms change over time, is the most important
assumption in cladistics. It is only when characteristics
change that different lineages or groups are recognized. The
convention is to call the “original” state of the characteristic
plesiomorphic and the “changed” state apomorphic. The
terms primitiveand derivedhave also been used for these
states, but they are often avoided by cladists, since those
terms have been abused in the past.
Cladistics is useful for creating systems of classifica-
tion. It is now the most commonly used method to classify
organisms because it recognizes and employs evolutionary
theory. Cladistics predicts the properties of organisms. It pro-
duces hypotheses about the relationships of organisms in a
way that makes it possible to predict properties of the organ-
isms. This can be especially important in cases when particu-
lar genes or biological compounds are being sought. Such
genes and compounds are being sought all the time by com-
panies interested in improving bacterial strains, disease resist-
ance, and in the search for medicines. Only an hypothesis
based on evolutionary theory, such as cladistic hypotheses,
can be used for these endeavors.
As an example, consider the plant species Taxus brevi-
folia. This species produces a compound, taxol, which is use-
ful for treating cancer. Unfortunately, large quantities of bark
from this rare tree are required to produce enough taxol for a
single patient. Through cladistic analysis, a phylogeny for the
genus Taxushas been produced that shows Taxus cuspidata, a
common ornamental shrub, to be a very close relative of T.
brevifolia. Taxus cuspidata, then, may also produce large
enough quantities of taxol to be useful. Having a classification
based on evolutionary descent will allow scientists to select
the species most likely to produce taxol.
Cladistics helps to elucidate mechanisms of evolution.
Unlike previous systems of analyzing relationships, cladistics
is explicitly evolutionary. Because of this, it is possible to
examine the way characters change within groups over time,
the direction in which characters change, and the relative fre-
quency with which they change. It is also possible to compare
the descendants of a single ancestor and observe patterns of
origin and extinction in these groups, or to look at relative size
and diversity of the groups. Perhaps the most important fea-
ture of cladistics is its use in testing long-standing hypotheses
about adaptation.See also Bacterial kingdoms; Evolution and evolutionary
mechanisms; Evolutionary origin of bacteria and viruses;
Microbial genetics; Viral geneticsPILI• seeBACTERIAL APPENDAGES
PPipetteIPETTE
A pipette is a piece of volumetric glassware used to transfer
quantitatively a desired volume of solution from one container to
another. Pipettes are calibrated at a specified temperature (usu-
ally 68°F [20°C] or 77°F [25°C]) either to contain (TC) or to
deliver (TD) the stated volume indicated by the etched/painted
markings on the pipette side. Pipettes that are marked TD gener-
ally deliver the desired volume with free drainage; whereas in
the case of pipettes marked TC the last drop must be blown out
or washed out with an appropriate solvent.womi_P 5/7/03 11:09 AM Page 438