XX
HELEN A. STAFFORD
caused by a lack of information concerning certain groups, others are
inherent in the fact and mechanisms of evolution itself. While some
minor differences still remain, geneticists and experimentalists have defined the taxon (or group) called a species in sexually reproducing organisms in terms of the amount of gene exchange still occurring, a
factor which can be tested experimentally. Therefore, a well delimited
population of individuals which possesses a set of distinguishing
morphological, physiological, or biochemical characteristics can be assigned the taxonomic rank of a species if there is still gene exchange
occurring within the group, but not with closely related groups (2-4).
Unfortunately, similar definitions for families, orders, etc., are not
available, and although a higher taxonomic group might be equally
well delimited and represent a natural grouping, the rank assigned to
that group is relatively arbitrary (2-4). Any evaluation of these taxa,
therefore, is determined mainly by its convenience and usefulness, and
might vary with different workers. Families, classes, orders, etc., in different evolutionary lines, therefore, will not be comparable to each
other in terms of evolutionary advancement, size, etc. In other words,
organisms have evolved into different species; after they have become
well defined, biologists arrange them into higher and higher degrees of
order and magnitude to illustrate the interrelationships.
The longer that groups have been separated in evolutionary time,
the easier it is to distinguish between or to delimit the taxonomic
groups involved. However, even here, there will still be "splitters" and
"lumpers" among taxonomists, and indecision as to the rank of even a
well delimited group. On the species level, furthermore, the genetic
definition is often impractical, and taxonomists will separate certain
groups into a number of species although some gene exchange is still
occurring. The synonymous terms indicated in Charts II and III illustrate some of these problems on the phylum level. Smith for instance,
prefers to elevate the ferns (Pterophyta) to the rank of a separate division, while others have grouped them in the subdivision Pteropsida
along with the Gymnospermae and Angiospermae. Hyman, likewise, discusses the splitting and lumping of many of the small groups of the
pseudocoelomate forms within the Bilateria.
The following references are listed as sources of further information
discussed here.
R. S. Breed, E. G. D. Murray, and A. P. Hitchins, eds., "Bergey's Manual
of Determinative Bacteriology," 6th ed. Williams & Wilkins, Baltimore,
Maryland, 1948.
F. A. Brown, Jr., ed., "Selected Invertebrate Types." Wiley, New York,
1950.
HELEN A. STAFFORD
caused by a lack of information concerning certain groups, others are
inherent in the fact and mechanisms of evolution itself. While some
minor differences still remain, geneticists and experimentalists have defined the taxon (or group) called a species in sexually reproducing organisms in terms of the amount of gene exchange still occurring, a
factor which can be tested experimentally. Therefore, a well delimited
population of individuals which possesses a set of distinguishing
morphological, physiological, or biochemical characteristics can be assigned the taxonomic rank of a species if there is still gene exchange
occurring within the group, but not with closely related groups (2-4).
Unfortunately, similar definitions for families, orders, etc., are not
available, and although a higher taxonomic group might be equally
well delimited and represent a natural grouping, the rank assigned to
that group is relatively arbitrary (2-4). Any evaluation of these taxa,
therefore, is determined mainly by its convenience and usefulness, and
might vary with different workers. Families, classes, orders, etc., in different evolutionary lines, therefore, will not be comparable to each
other in terms of evolutionary advancement, size, etc. In other words,
organisms have evolved into different species; after they have become
well defined, biologists arrange them into higher and higher degrees of
order and magnitude to illustrate the interrelationships.
The longer that groups have been separated in evolutionary time,
the easier it is to distinguish between or to delimit the taxonomic
groups involved. However, even here, there will still be "splitters" and
"lumpers" among taxonomists, and indecision as to the rank of even a
well delimited group. On the species level, furthermore, the genetic
definition is often impractical, and taxonomists will separate certain
groups into a number of species although some gene exchange is still
occurring. The synonymous terms indicated in Charts II and III illustrate some of these problems on the phylum level. Smith for instance,
prefers to elevate the ferns (Pterophyta) to the rank of a separate division, while others have grouped them in the subdivision Pteropsida
along with the Gymnospermae and Angiospermae. Hyman, likewise, discusses the splitting and lumping of many of the small groups of the
pseudocoelomate forms within the Bilateria.
The following references are listed as sources of further information
discussed here.
R. S. Breed, E. G. D. Murray, and A. P. Hitchins, eds., "Bergey's Manual
of Determinative Bacteriology," 6th ed. Williams & Wilkins, Baltimore,
Maryland, 1948.
F. A. Brown, Jr., ed., "Selected Invertebrate Types." Wiley, New York,
1950.