Lr. Godfrey et Mr. Sutherland, FLAWED INFERENCE - WHY SIZE-BASED TESTS OF HETEROCHRONIC PROCESSES DONOT WORK, Journal of theoretical biology, 172(1), 1995, pp. 43-61
Allometric (or size-based)heterochrony is widely used:in drawing infer
ences about heterochronic processes. Whereas problems associated with
substituting size for age in heterochronic analysis are acknowledged,
it is also generally believed that comparisons of ancestral and descen
dant growth allometries often reveal real processual shifts in growth
processes. At the very least, they are presumed to discriminate system
ic perturbations in ''size'' and ''shape'' (''ontogenetic scaling'') f
rom local dissociations of ''size'' and ''shape'' (''neoteny,'' ''acce
leration,'' and so on). When ancestor and descendant follow the same g
rowth trajectory in size (relative to age), it is further presumed tha
t size-based heterochronies will match ''true'' (age-dependent) hetero
chronies. In this paper we argue that growth allometries are a poor ve
hicle for inferring heterochronic process; the actual processual shift
s that produce them can be counter-intuitive. Systemic shifts can resu
lt in dissociated allometries, and overlapping growth allometries can
conceal perturbations in ''shape'' relative to ''size''. Changes in gr
owth allometries are influenced as much by the initial ancestral relat
ionship between ''size'' and ''shape'' as they are by any perturbation
s that may occur during the evolution of the descendant from the ances
tor. Finally, it is emphatically not the case that size-based heteroch
rony will reveal true heterochrony as long as the size trajectories of
ancestor and descendant remain constant relative to age. We urge care
ful consideration of the actual shapes of biological growth trajectori
es, recognition of the fact that they arl rarely simple power function
s of age, and calculation of possible relationships between complex an
d/or lazy-S growth trajectories.