Constancy and change of basipodial variation patterns: a comparative study of crested and marbled newts —Triturus cristatus, Triturus marmoratus — and their natural hybrids

To test the concept of stable and lineage‐specific developmental constraints the stability of biased limb skeletal variation patterns was examined by the comparative approach: 384 limbs of crested and marbled newts as well as their natural hybrids were cleared and stained for cartilage and bone. The variation in autopodium was recorded and compared between samples. The frequencies of variations were the same for crested and marbled newt samples. In the hybrids digital variation was significantly elevated, but the basipodium was as conservative as in the parental crested and marbled newts. The tarsus had to be excluded from further statistical analysis, since variation in tarsus was too rare to allow rigorous tests. The carpal variation was highly biased; out of twelve geometrically possible fusions of two neighbouring elements only three were found in the 219 forelimbs. The frequency of the fusions was the same in the parental as well as in the hybrid populations. In one sample of the marbled newt from a marginal population, the frequency of one fusion type was elevated, leading to an even more pronounced bias in phenotypic variation. Hence, neither in the hybrids nor in the marginal population could a breakdown of the constraint be observed. Also the pattern of variation was the same in all Triturus‐samples examined here. The comparison of our results with within‐population variation from other genera showed qualitative differences. The most common variant from our samples was found as a natural variant and even as the typical pattern in some species, but has not been observed in species of other genera. On, the other hand, fusions commonly observed in other genera were not observed in our sample. Even if all populations exhibit a highly bound variation pattern which may restrict the possible directions of morphological evolution, the direction of variation changes from genus to genus. It is suggested that shifts in constrained variation patterns have to be considered as key‐events in the evolution of a clade. The cause of these shifts are unknown.