Simple Size and Shape Variables: Shape Coordinates - Geometric Morphometrics for Biologists - page 56

ontogenetic change ( Figure 3.6D ). The dorsoventral component of shape change has been
removed from all three dorsal landmarks and is expressed as a displacement of every
other landmark away from the dorsal edge. In addition, the elongation of the middle of
the body relative to the rest of the piranha is now expressed as a relative contraction
of the ends towards the middle. If we had used another baseline (landmarks 1 and 3) that
rotates relative to most of the other landmarks, the variation seems to be even further
increased ( Figure 3.6E ) and the piranha's body seemingly rotates as it grows ( Figure 3.6F ).
Clearly, the baselines used in Figures 3.6C,D and 3.6E,F are spectacularly bad choices.
But they simply exaggerate the general problem that the variance of baseline points is
transferred to the other landmarks. It should be intuitively obvious, even if not visibly so,
FIGURE 3.6 The impact of the baseline on variation and the depiction of ontogenetic shape change of
a piranha, S. gouldingi . (A) Coordinates of landmarks relative to a fixed baseline that extends between land-
marks 1 and 7; (B) vectors indicating displacements of landmarks relative to the fixed baseline; (C) coordinates
relative to a fixed baseline that extends between two dorsal landmarks (3 and 5); (D) vectors indicating displa-
cements of landmarks relative to the fixed baseline; (E) coordinates of landmarks relative to the baseline that
extends between landmarks 1 and 3; (F) vectors indicating displacements of landmarks relative to the fixed
baseline.