Biology Reference
In-Depth Information
0.08
0.06
0.04
0.02
0
0.02
0.04
0.06
0.1
0.05
0
0.05
0.1
PC1
Pygopristis denticulata
Serrasalmus altuvei
S. elongatus
S. gouldingi
S. manueli
S. spilopleura
Pygocentrus cariba
P. nattereri
P. piraya
FIGURE 13.4
Principal components of body shape of nine species of piranhas; data were standardized and
variation is examined among juvenile shapes (at the transition from larval to juvenile phases).
relative to other species with equally high scores on PC1. In effect, PC2 partially “compen-
sates” for PC1.
When interpreting PCs, it is also important to consider that they describe variation
around an average shape. However, the average is not a “typical” piranha; rather, it is the
shape of the consensus, the point having the coordinates 0, 0 (on all PCs). Obviously,
the consensus is not a typical piranha since there are no specimens at the 0, 0 point. The
outgroup (
P. denticulata
) is fairly near it, but if we want to describe differences between
P
.
denticulata
and other species (or to make any other comparisons among species), we
cannot describe changes along one PC, then along another. The direction of the difference
between particular species is often oblique to several PCs.
The importance of considering scores on several PCs becomes evident when comparing
the three shallow-bodied species. All three have high scores on PC1, but they differ in
scores on PC2. One of the three,
S. manueli
, has high scores on PC2 (as do
S. altuvei
and
S. spilopleura
). To see how
S. manueli
differs from
S. gouldingi
and
S. elongatus
with respect
to their differences from other taxa, we can draw the vector extending from those other
taxa, e.g.
P. denticulata
to
S. manueli
(Figure 13.5A) and to
S. elongatus
and
S. gouldingi
(Figure 13.5B). The reason for doing this is to determine what differences from other taxa
are shared by
S. elongatus
,
S. gouldingi
and
S. manueli
. We will then eliminate from the
character the features peculiar to one species. Although we are making this comparison to
the outgroup, we are not assuming that it has the primitive body shape. Comparisons to
other species will also be necessary, and no decisions about polarity are made at this point.
