Historical Constraints and the Evolution of Development 1171
Thus, the common ancestor of all 40,000 or so modern gnathostome species
already had four Hox sets, and only the handful of agnathan species has fewer sets
among modern vertebrates. Thus, our large clade of 40,000 species evolved under the
general rule featured throughout this section: phenotypic specialization correlated
with Hox deletions" and restrictions of expression. Or, to put the matter somewhat
facetiously, you start with all you will ever get, and work "down" from there—an
optimal formula for the evolutionary importance of historical constraint.
As Coates and Cohn (1998, p. 375) write (see also Fig. 10-30): "During the
period since these gene duplication episodes, jawed vertebrate Hox cluster evolution
seems to have been characterized by gene deletions." Moreover, as Figure 10-30 also
shows, teleost fishes, which did not originate until Mesozoic times, evolve different
patterns of deletion from those found in mammals, a group with a Paleozoic ancestry
from a very different vertebrate lineage—thus "indicating quite separate patterns of
gene loss in tetrapod and teleost lineages" (Coates and Cohn, 1998, p. 375).
The relatively homonomous architecture of the postcranial skeleton of many
early fishes (and many early tetrapods as well) has evolved in the conventionally
"higher" tetrapods, primarily in mammals, into a more complex, specialized and
regionalized axial skeleton with clear and often quite sharp distinctions, in both form
and function, from cervical to thoracic to lumbar, sacral and caudal regions of the
vertebral column. Burke et al. (1995) have demonstrated an interesting basis for
much of this phenotypic complexity and regionalization in the establishment of
definite boundaries of action for particular paralogy groups of the Hox clusters, thus
repeating the general arthropod correlation of Hox regionalization with phenotypic
specialization along the AP axis.
For example, different groups of vertebrates vary greatly in the number of
vertebrae per region, but the boundary between regions may still remain
10 - 30. Following the evolution of four Hox clusters in vertebrates, the major pattern of change
has not resulted in further addition, but rather in elimination —in different patterns in
various groups. From Coates and Cohn, 1998, p. 375.