Biology Reference
In-Depth Information
we would not likely want to construct a compound name from the labels for each one.
For that example, the compound term would be some form of “proportional giantism plus
neoteny” but, by definition, proportional giantism produces a giant replica of the ancestral
morphology whereas neoteny produces an adult that resembles the ancestral juvenile.
The combination of the two is self-contradictory. But the fact that there are no terms for
modifications in size plus shape is problematic, although it is an interesting feature of the
scheme that only two perturbations predictably affect growth or size. The others
affect only development and shape. As a result, one cannot use this scheme to predict
how heterochrony will affect size. Even though ontogenetic scaling is a special case of
heterochrony, there are no terms for perturbations that lead to extensions or truncations of
both size and shape.
Shea (1983a)
introduced terms for perturbations that affect size and shape, including
rate hypermorphosis or hypomorphosis, and time hypermophosis or hypmorphosis. These
are sometimes used in the anthropological literature, but for readers unaccustomed to
them they can be confusing because they combine the feature that is modified (growth
rate or time) with a term for a morphological outcome of a change in developmental
timing (
). The distinction being made is between an increase or decrease in rate (with
no change in the duration of growth) and an increase or decrease in duration (with
no change in the rate of growth). Rather than using a term that means a delayed offset in
development for an increased rate of growth, it seems clear enough, if more verbose,
to say that durations of growth are extended or truncated, or that rates of growth and
development are increased or decreased.
As should be evident by this point, both the clock model and the Alberch et al. formal-
ism can only be applied when the ancestral and descendant ontogenetic trajectories
shape are the same. The trajectories can differ only by extension or truncation. That is not
a limitation of either scheme because, by definition, heterochrony and scaling refer to
extensions or truncations of conserved ontogenetic trajectories of shape. The two schemes
are thus intended to be used solely for the cases in which the ancestral and descendant
ontogenies differ only by extension or truncation. If the two ontogenies differ otherwise,
the hypotheses of heterochrony and scaling are (or should be) rejected. If that hypothesis
of a conserved ontogenetic trajectory of shape is not rejected, the two schemes can be used
to diagnose the heterochronic perturbation. If the hypothesis is rejected, the question is
whether the trajectories are parallel or not.
1
δβ
Parallel Trajectories (Transpositional Allometry)
Parallel trajectories (see
Figure 11.10C
) are at least as intriguing as heterochrony and
ontogenetic scaling because they mean that early development is less conserved than
later. Consequently, adults differ in precisely the same direction that larvae or infants
did. This is surprising in light of the conventional view that early development is more
conservative than later, although the larval stage is still quite late in development. The
conservative developmental phase is the “phylotypic period”
the stage at which
embryos of all members of a phylum look the same (
Seidl, 1960; Sander, 1983; Slack
et al., 1993
). That period begins at onset of neurulation and ends with somitogenesis
