De Haan emphasizes the pattern of appearance of higher taxa in which
phylum-level diversity reaches its peak in the fossil record before
class-level diversity, and class-level diversity before that of orders,
etc. He and other critics of macroevolution interpret this apparent
"top-down" pattern as contrary to expectations from evolutionary theory.
However, this pattern is generated by the way in which species are assigned
to higher taxa. When a hierarchical classification is applied
retrospectively to a diversifying evolutionary tree, a "top-down" pattern
will of necessity result. Consider, for example, species belonging to a
single evolving lineage that is given genus-level status. This genus is
then grouped with other closely related lineages into a family. The common
ancestors of these genera are by definition included within that family.
Those ancestors must logically be older than any of the other species
within the family. Thus the family level taxon would appear in the fossil
record before most of the genera and species included within it. The
"top-down" pattern of taxa appearance is therefore entirely consistent with
a branching tree of life. A hierarchical grouping of lineages constructed
from Darwin's own diagram will produce a "top down" pattern of appearance
of taxa. It is simply an artifact of the retrospective way in which the
classification system was contructed beginning with modern extant species.
Another consequence of the classification system is that discontinuity is
emphasized. In fact, the present classification system is unable to
communicate the nature of transitions between higher taxa. Because taxonomy
is superimposed on a branching tree of life, a higher level in the
taxonomic hierarchy does not imply a greater degree of morphologic
distance. For example, two species belonging to different classes are not
necessarily more different from each other than two species belonging to
different genera. When looking backward through time using the fossil
record, it is found that representatives of different higher-level taxa
become more "primitive", that is have fewer derived characters, and appear
more like the primitive members of other closely related taxa. The more
complete the fossil record of the origin and early radiation of higher
taxa the more similar the transitional species, and the more difficult it
is to determine their taxonomic assignments. Species placed into two
different higher taxa may thus have very similar morphologies. Were it not
for the subsequent evolutionary history of the lineages, species spanning
the transitions between families, orders, classes, or phyla would be placed
in the same lower taxon. This is completely consistent with the paradigm
of common descent, and with the origin of higher taxa through evolutionary
processes at the population and species level.
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www.ksu.edu/~kbmill/