96
DAVID D. KECK
Natural Selection. This is universally considered to play a fundamental
role in evolution. Its operation in the higher plants has been inferred
from strong circumstantial evidence, but it was not expected that the
action of so slow a process could be traced within a human lifetime.
Now experimental confirmation of it has been obtained with populations of the fruit fly, Drosophila pseudoobscura,
which can produce a
new generation every two weeks. This rapid reproduction rate makes it
excellent research material for producing artificial hybrid combinations
and testing the selective value of the heterozygotes in nature. Using
statistical methods, natural selection was demonstrated to be operating
in such an experiment by Dobzhansky (1948).
The importance of natural selection seems to be rather obvious when
applied to ecotypes and related ecospecies. It is not always so distinct,
however, when it is applied to progressive mutational changes of negative adaptive value. In such cases, an explanation of the evolutionary
trend may need to be sought in orthogenesis (Cronquist, 1951).
XVIII. CONCLUSION
The taxonomist still faces a tremendous task of completing the inventory of the vegetation of the world. More remains to be done here
than is realized by many, and the body of workers committed to this
work is not large in the aggregate. In order to do the job, much additional field work must be done, particularly in the tropics. Genera
must be revised as important additions to their content are discovered,
and often a reconsideration of their placement in the natural classification of the family, and of the family in the order, may be required as
such new data come to hand. To make the inventory more generally
available as it accumulates, the taxonomist must revise old floras and
write new ones.
A task as formidable as the one just mentioned faces the taxonomist.
It has become obvious that ultimately it is up to him to utilize the
pertinent data supplied by the evolutionist, the biosystematist, the cytogeneticist, the anatomist, the biochemist, and all the rest. It has been
said that a primary function of the theory of evolution is "to account for
natural systematic relationship by postulating that it is a consequence
of actual genetic relationship.
,,
These workers are supplying potent data
for clarifying both the evolutionary processes and the natural classification of the end-products, namely, the present-day biota. The taxonomist
must increasingly give his attention to weighing and coordinating these
data with all others he has, in order to effect as orderly an evolution of
DAVID D. KECK
Natural Selection. This is universally considered to play a fundamental
role in evolution. Its operation in the higher plants has been inferred
from strong circumstantial evidence, but it was not expected that the
action of so slow a process could be traced within a human lifetime.
Now experimental confirmation of it has been obtained with populations of the fruit fly, Drosophila pseudoobscura,
which can produce a
new generation every two weeks. This rapid reproduction rate makes it
excellent research material for producing artificial hybrid combinations
and testing the selective value of the heterozygotes in nature. Using
statistical methods, natural selection was demonstrated to be operating
in such an experiment by Dobzhansky (1948).
The importance of natural selection seems to be rather obvious when
applied to ecotypes and related ecospecies. It is not always so distinct,
however, when it is applied to progressive mutational changes of negative adaptive value. In such cases, an explanation of the evolutionary
trend may need to be sought in orthogenesis (Cronquist, 1951).
XVIII. CONCLUSION
The taxonomist still faces a tremendous task of completing the inventory of the vegetation of the world. More remains to be done here
than is realized by many, and the body of workers committed to this
work is not large in the aggregate. In order to do the job, much additional field work must be done, particularly in the tropics. Genera
must be revised as important additions to their content are discovered,
and often a reconsideration of their placement in the natural classification of the family, and of the family in the order, may be required as
such new data come to hand. To make the inventory more generally
available as it accumulates, the taxonomist must revise old floras and
write new ones.
A task as formidable as the one just mentioned faces the taxonomist.
It has become obvious that ultimately it is up to him to utilize the
pertinent data supplied by the evolutionist, the biosystematist, the cytogeneticist, the anatomist, the biochemist, and all the rest. It has been
said that a primary function of the theory of evolution is "to account for
natural systematic relationship by postulating that it is a consequence
of actual genetic relationship.
,,
These workers are supplying potent data
for clarifying both the evolutionary processes and the natural classification of the end-products, namely, the present-day biota. The taxonomist
must increasingly give his attention to weighing and coordinating these
data with all others he has, in order to effect as orderly an evolution of