Biology Reference
In-Depth Information
FIGURE 11.22
The impact of variation in attributes of ontogenetic trajectories on the disparity of the nine spe-
cies of piranhas. Levels of disparity are indicated by the height of the bar. The level observed in the data (D) is
compared to disparity produced by fixing juvenile shape and the direction of the ontogenetic trajectory of shape
two attributes to be equal across the three species with shallow-bodied juveniles (S. gouldingi, S. manueli and
S. elongatus). Thus only the length of the ontogenetic trajectory varies. The values for juvenile shape and the
direction of the ontogenetic trajectory of shape are fixed to four values, values observed for P. denticulata (d),
S. elongatus (e), S. gouldingi (g), or S. manueli (m).
focus on the three with the distinctive juvenile body shapes and fix their juvenile shapes,
trajectory lengths or directions either to that seen in the outgroup species (Pygopristis denticu-
lata) or to the value seen in the data for one of these three species. We will let length be the
sole parameter that varies, but the result of that variation clearly depends heavily on the
particular value to which the juvenile shape or direction is fixed (
Figure 11.22
). The lowest
values are found when fixing both juvenile shape and direction in the three shallow-bodied
species to the values observed in either S. gouldingi or S. manueli (both of which have rela-
tively long ontogenetic trajectories). The greatest disparity results from fixing the juvenile
shape of the three shallow-bodied juvenile species to the one seen in P. denticulata,fixing
trajectory length to the large value seen in S. gouldingi, and letting each species follow the
direction of its own trajectory (
Zelditch et al., 2003b
).
Clearly, even when we can dissect the developmental origins of disparity and quantify
the variation in morphology that arises from each modification of ontogeny, it is not
straightforward to answer the question: How much of the disparity arises from variation
in any one parameter? The interactions between them complicate answering that question
because the disparity produced by variation in any one parameter may be countered by
that produced by variation in another.
AGE-BASED COMPARISONS OF GROWTH AND DEVELOPMENTAL
RATES AND TIMINGS
Even after ruling out the hypothesis of heterochrony by finding that species follow differ-
ent ontogenetic trajectories of shape we may still be interested in their rates and timings of
growth and development. Because the species do not follow the same ontogenetic trajectories
of shape, we cannot use the Alberch et al. formalism to compare rates and timings
the
