Biology Reference
In-Depth Information
CHAPTER
1
Introduction
Shape analysis plays an important role in many kinds of biological studies. A variety of
biological processes produce differences in shape between individuals or their parts, such
as disease or injury, mutation, ontogenetic development, adaptation to local geographic
factors, or long-term evolutionary diversification. Differences in shape may signal differ-
ences in processes of growth and morphogenesis, different functional roles played by the
same parts, different responses to the same selective pressures, or differences in the selec-
tive pressures themselves. Shape analysis is an approach to understanding those diverse
causes of morphological variation and transformation.
Sometimes, differences in shape are adequately summarized by comparing the
observed shapes to more familiar objects such as circles, kidneys or letters of the alphabet
(or even, in the case of the Lower Peninsula of Michigan, a mitten). Organisms, or their
parts, are then characterized as being more or less circular, reniform, C-shaped or mitten-
like. Such comparisons can be extremely valuable because they help us to visualize unfa-
miliar organisms or to focus attention on biologically meaningful components of shape.
However, they can also be vague, inaccurate or even misleading, especially when the
shapes are complex and do not closely resemble familiar icons. Even under the best
of circumstances, we still cannot say precisely how much more circular, reniform, or
C-shaped or mitten-like one shape is than another. When we need that precision, we turn
to measurement.
Morphometrics is a quantitative way of addressing the shape comparisons that have
always interested biologists. This may not seem to be the case because the morphological
approaches once typical of the quantitative literature appeared very different from the
qualitative descriptions of morphology; whereas the qualitative studies produce pictures
or detailed descriptions (in which analogies figure prominently), morphometric studies
usually produced tables with disembodied lists of numbers. Those numbers seemed so
highly abstract that we could not readily visualize them as descriptors of shape differences,
and the language of morphometrics also seemed highly abstract and mathematical. As a
result, morphometrics seemed closer to statistics or algebra than to morphology. In one
sense that perception is entirely accurate: morphometrics
is
a branch of mathematical shape
analysis. The way that we extract information from morphometric data involves mathemati-
cal operations rather than concepts rooted in biological intuition or classical morphology.
