Evolutionary Developmental Biology (1): The Evolution of Ontogeny - Geometric Morphometrics for Biologists - page 328

FIGURE 11.17 Ontogenetic
scaling and heterochrony. (A)
Superimposed coordinates of juve-
nile shapes; (B) ontogenies of
shape; (C) lengths of ontogenetic
vectors of shape. The two species
have the same shape at the outset
of the measured phase, follow the
same ontogeny of shape, but differ
in the length of their ontogenetic
vectors; the descendant has a trun-
cated version of the ancestral
ontogeny. (D) Superimposed coor-
dinates for showing the ontogenetic
transformation of ancestral shape
(black circles) and the descendant
adult shape (gray squares). The
descendant adult shape is at an
intermediate position along the
ancestral ontogeny.
squared residuals. Piras and colleagues ( Piras et al., 2011 ) offer a modified version of these
tests, using the mean squared error rather than the sums of squares.
A third hypothesis, parallel trajectories, also predicts that the two trajectories point in
the same direction but, in this case, the two species never resemble each other. We would
therefore expect that they have different shapes at the youngest comparable stage
( Figure 11.18A ), but subsequently follow the same ontogeny of shape ( Figure 11.18B ), per-
haps to the same extent ( Figure 11.18C ). To test the hypothesis that only early develop-
ment is labile, we can show that there is a significant difference in shape at the outset of
the measured phase, but the ontogenies of shape do not differ. For the hypothetical species