Biology Reference
In-Depth Information
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PC2
Pygopristis denticulata
Serrasalmus altuvei
S. elongatus
S. gouldingi
S. manueli
S. spilopleura
Pygocentrus cariba
P. nattereri
P. piraya
FIGURE 13.6
Scatter-plot of PC3 on PC2, and the deformations depicting these two dimensions of variation.
implied by similar PC2 scores. Like PC2, PC3 accounts for only a small portion of the vari-
ance (5.6%), but like PC2 it describes a change in location of the pectoral fin relative to the
opercle.
S. manueli
and
S. altuvei
have the highest and lowest scores on PC3, respectively,
which means that their pectoral fins are displaced in opposite directions relative to the
opercle, which needs to be taken into account when assessing their similarity on PC2.
Despite their similar scores on PC2, a feature that might have been judged a morphologi-
cal similarity might not be similar by virtue of the differences along PC1 and PC3. Because
of their different scores on PC1, we would also avoid construing their caudal peduncle
proportions as similar, despite their similar values on PC2. Because species can be similar
along one component and differ substantially along others, we cannot interpret one
component at a time.
The strategy for combining PCs, outlined above, is undeniably tedious, but it might be
successful at finding the features shared by two or more taxa. In cases like our example,
when over 60% of the variation is along a single PC, two or more taxa have high scores
and two or more have low ones, and there is virtually no overlap among the low and high
scores, PC1 points to a character. When the variation is more evenly spread out across
components, it will be necessary to combine many more of them because similarities on
one may be outweighed by differences on the others. An obvious problem is that the com-
parison of vectors in a single plane, such as we used to compare the similarities among
the three shallow-bodied species with respect to their difference from the outgroup, exam-
ines only the differences among them that are in that particular plane. We might prefer to
look at
all
the differences between each species and the outgroup (or any other species
taken as a standard), comparing
those
vectors among taxa.
