Biology Reference
In-Depth Information
To date, most studies of disparity have focused on its temporal dynamics over a geo-
logical time scale. The chief questions addressed by such studies are:
1.
What is the temporal pattern of disparity?
2.
What evolutionary processes explain those patterns?
Such studies are almost invariably based on fossils because they require sampling
disparity at multiple times in the geological record. Some groups studied in this way
include Cambrian marine arthropods (
Foote and Gould, 1992; Wills et al., 1994
), Paleozoic
blastozoans (e.g.
Foote, 1992
), stenolaemate bryozoans (
Anstey and Pachut, 1995
), crinoids
(e.g.
Foote, 1994; Ciampaglio, 2002
), gastropods (
Wagner, 1995
) and Ordovician trilobites
(
Miller and Foote, 1996
). The growing empirical literature on disparity repeatedly docu-
ments a surprising historical pattern: disparity initially increases and then stabilizes or
even decreases while the number of taxa increases.
Efforts to explain this pattern have focused on two classes of hypotheses: ecological and
developmental. Ecological hypotheses postulate that ecological space is initially open and
then becomes saturated; limits on disparity are thought to arise from the structure of the
ecological space. In contrast, developmental hypotheses propose an intrinsic explanation
for limits on disparity
the acquisition of developmental constraints that stabilize
morphology (see
Wagner, 1995 and Ciampaglio, 2002
for reviews of hypotheses and
approaches to testing them). Whether any explanation is even needed has been questioned
in a profound (if difficult) theoretical analysis (
Gavrilets, 1999
). At present, it is not clear
what we ought to expect from disparity under plausible models; nor is it clear what role
artifacts might play in the patterns detected by empirical analyses. It is also difficult to
isolate causal factors that might explain the temporal dynamics of disparity because of the
multiplicity of uncontrollable factors that can influence those dynamics, including rates of
speciation and extinction, selectivity of extinction or speciation that is non-random with
respect to morphology, the magnitude of change within a lineage, and factors potentially
limiting that magnitude (such as developmental and selective constraints).
Of the various factors that can influence disparity, constraints may be the least under-
stood
partly because they are rarely documented prior to analyzing disparity. Instead,
constraints are inferred from the data, even though it is not clear how either developmen-
tal or selective constraints ought to influence disparity. Both sorts of constraints are
thought to limit disparity, which may seem intuitively obvious; however, like many intui-
tions, it may be faulty. We know little about the impact of either sort of constraint on
disparity, and determining their impacts will require studies that document constraints
independently of such supposed effects. We cannot simply infer constraints from
decreases in disparity when we do not know if they generally decrease disparity. Instead,
we need to determine whether development is constrained or not, and then ask how those
constraints affect disparity. In at least one case, developmental constraints are inferred to
increase disparity (
Zelditch et al., 2003
).
Studies of disparity of living taxa are still relatively rare, but they have been used to
address basic issues in evolutionary biology
such as whether decoupling of integrated
parts increases disparity (
Schaefer and Lauder, 1996
), whether biomechanical and morpho-
logical disparity are related to each other (
Hulsey and Wainwright, 2002
), and whether
developmental constraints might limit disparity (
Zelditch et al., 2003
). Studies relating
