Biology Reference
In-Depth Information
species have a relatively short mid-body relative to the more posterior body, a feature
hinted at but not so clearly presented when the vectors were drawn from
P. denticulata
to
the species in the PC1
PC2 plane (see Figure 13.5). This particular feature might reflect a
decrease in the length of the dorsal fin relative to the posterior body (dorsally) and the
posterior displacement of the pectoral fin and pelvic fins (rather than changes in propor-
tions of body between them). The three shallow-bodied species appear to vary in the
degree of “mid-body contraction”, but they appear to be consistently more contracted than
the others. The possibility that these three species are similar in having a relatively short-
ened mid-body is worth examining further because, unlike their shallow body, it is not
obvious from a purely qualitative analysis.
To pursue that possibility further, we can compare the vectors of pairwise contrasts to
each other, asking if a more contracted mid-body (compared to that of
P. denticulata
)is
characteristic of the shallow-bodied species but not of the others. This is done by subtract-
ing one of the pairwise vectors from another; where species are identical to
S. gouldingi
(in the differences from
P. denticulata
) the grid is square (Figure 13.8). Large differences
indicate that the direction of change from
P. denticulata
to
S. gouldingi
is not shared
by another taxon. Subtracting each contrast from the contrast between
P. denticulata
to
S. gouldingi
shows that
S. gouldingi
is not much shallower or deeper than either
S. elongatus
or
S. manueli
. All three differ from
P. denticulata
by nearly the same degree, and in that
same direction. Some differences are evident in the relative length of the mid-body, how-
ever. The grid is slightly more contracted in that region, indicating that
S. gouldingi
is
more extreme than the others in that feature. However, the differences are slight. In strik-
ing contrast, the comparisons to the other species indicate not only that
S. gouldingi
is far
shallower than the others, but also that all differ from
S. gouldingi
in either the degree
or the location of mid-body contraction. We could either take these results to mean that
S. elongatus
,
S. gouldingi
and
S. manueli
are all shallow-bodied and contracted in the mid-
body compared to the other species, or we could continue the analysis, doing additional
pairwise contrasts
this time between
P. denticulata
and
S. elongatus
, and also between
P. denticulata
and
S. manueli
to determine that all three species are similarly different
from the others. Of course, we would need additional comparisons to find features that
are more widely shared, or specific to some of the deeper-bodied species.
Unlike the shallow body, which is so evident visually that it requires no detailed
quantitative study, the mid-body contraction discerned in these comparisons is the kind of
subtle feature that justifies the effort of a morphometric analysis.
CODING
Having found a character, we can treat it like any other. That is, if using conventional
cladistic methods, we code the characters according to our preliminary judgments of
homology, include it in the data matrix, and analyze that matrix by parsimony. Coding
methods are a contentious subject; systematists vary considerably in their preferred criteria
for coding. The debates have nothing to do with morphometrics except to the extent that
the methods are applied to quantitative data, and that statistical methods are sometimes
favored to decide whether species are different (and should therefore not be coded as
