Biology Reference
In-Depth Information
ecological heterogeneity and morphological disparity have become more common (e.g.
Ricklefs and Cox, 1977; Ricklefs and Travis, 1980; Viguier, 2002; Collar et al., 2010; Carlson
and Wainwright, 2010
), but much more work is needed.
Any biological explanation for an empirically documented pattern rests on the assump-
tion that the pattern is real. Whether it is real or an artifact depends partly on how dispar-
ity is measured, and also on the sampling design. Both metrics and sampling designs have
been foci of critical reviews. In particular, a number of critics have taken issue with the
phenetic approach to disparity implicit in the use of a variance as its metric (e.g.
Wills
et al., 1994
). Alternative metrics, which measure change along branches of a phylogeny,
have been recommended, but they are difficult to apply when ancestors have not been
sampled (or are unknown). They also pose an interpretative challenge because they rede-
fine disparity, replacing the idea of variation around an average with that of directed
change away from the ancestor (see
Wills et al., 1994; Wagner, 1997; Smith and Lieberman,
1999
). A second criticism is that measures of disparity typically do not consider the biolog-
ical significance of the contributing variables. It is conceivable that large morphological
changes could have few biological consequences, and some small changes affecting just a
few morphological details could have profound consequences for function. In that light,
weighted measures of disparity that take the biological significance of the changes into
account might seem more justified than measures of disparity per se (see
Wagner, 1995
).
For reviews of the literature, including critical discussions of metrics and methods, and
summaries of empirical studies, see
Foote (1997)
,
Ciampaglio et al. (2001)
and
Wills (2001)
.
Variation
Variation within populations is a major theme in evolutionary biology because it is so
fundamental to evolution
phenotypic variation provides the opportunity for selection to
act, and genetic variation enables selection to effect change. Variation is the raw material
on which selection acts, and its structure can influence the outcome of selection. Because
evolution can be constrained by limited or biased variance, the variance
covariance
matrix is sometimes viewed as an intrinsic constraint on evolution; such limits or biases
arising from developmental processes are developmental constraints (see
Maynard Smith
et al., 1985
). Although that view of variation emphasizes its role as a potential constraint,
the structure of (co)variation itself may be molded by selection. Theoretical models predict
that phenotypic and genetic (co)variance structures evolve to match patterns of develop-
mental and functional integration (e.g.
Lande, 1980; Cheverud, 1982, 1984; Wagner, 1988;
Wagner and Altenberg, 1996
). This matching is expected to result from differential elimi-
nation of pleiotropic effects between members of different functional complexes, combined
with the maintenance (or augmentation) of pleiotropic effects within a complex. There is
much empirical evidence that phenotypic and/or genetic covariances reflect developmen-
tal and functional relationships among traits, a conclusion based on many exploratory
studies (
Olson and Miller, 1958; Berg, 1960; Van Valen, 1962, 1970; Gould and Garwood,
1969
). In addition, many studies have deduced the structure of (co)variation among mea-
surements from developmental and functional theories (e.g.
Cheverud, 1982, 1995;
Zelditch and Carmichael, 1989; Kingsolver and Wiernasz, 1991; Marroig and Cheverud,
