J. Clobert et al., THE EVOLUTION OF DEMOGRAPHIC TACTICS IN LIZARDS - A TEST OF SOME HYPOTHESES CONCERNING LIFE-HISTORY EVOLUTION, Journal of evolutionary biology, 11(3), 1998, pp. 329-364
We analyze, with an augmented data base, patterns of covariation of th
e three primary demographic parameters (age at maturity, fecundity, ad
ult survival, all measured in the same unit of time) in lizards. This
also constitutes a first attempt to use all three of these parameters
for this group of species. We attempt to place these analyses in the f
ramework of recent theories on life history evolution (Ferriere and Cl
obert, 1992; Charnov, 1993). Life history data were collected from the
literature and from our original work, and a composite phylogeny was
assembled, based on a variety of published sources. Using a phylogenet
ically based statistical method (independent contrasts), the allometri
c (log-log) relationship of fecundity (and of clutch size) in relation
to snout-vent length was found to differ significantly between the tw
o major clades of extant lizards, Iguania (43 species in our data set)
and Scleroglossa (47 species). We therefore emphasize analyses done s
eparately for the two clades. Without removing correlations with body
size, the relationships between fecundity and survival, and between fe
cundity and age at maturity, were also found to differ between clades,
which differs from Charnov's (1993) predictions. When correlations wi
th body size were removed statistically, however, the two clades did n
ot differ significantly in these relationships. In a principal compone
nts analysis (PCA) of the three demographic variables plus snout-vent
length, the first axis explained the majority (53-57%) of variation in
both clades, while the second axis explained 27-31% of the variation
and loaded mainly on fecundity. In a PCA of size-adjusted demographic
variables residuals (from log-log regressions on snout-vent length), t
he first axis explained 66-68% of the variation and was clearly interp
retable as the classical ''slow-fast'' continuum, which has been descr
ibed in birds and mammals. The PCA of residuals did not provide clear
evidence of additional significant patterns of covariation. However, t
he rate of evolution of mortality (size-corrected), but not of fecundi
ty or age at maturity, differed significantly between clades. Furtherm
ore, fecundity and age at maturity, both corrected for variation in ad
ult mortality (in addition to body size), were still significantly rel
ated, indicating the existence of other patterns of variation in these
life history traits. In other words, the ratios between age at maturi
ty and adult mortality, or between fecundity and adult mortality, were
not found to be invariant, because the variation not accounted for by
these ratios was significantly associated with variation in another v
ariable. This result contradicts the prediction of Charnov (1993), and
suggests the existence of other directions of evolution in these life
history traits.