Further observations on the fate of morphological variation in a population of Smooth newt larvae (Triturus vulgaris)

Sixteen morphometric characters were studied in a population of Smooth newt larvae. Variation was found to decrease in all characters during the course of larval life. This finding is consistent both with earlier work on newt larvae in the same population and with the results of other authors. There is no direct evidence to show that this decline was caused by selection rather than by developmental canalization, although a theoretical argument suggests that canalization may not be very important in animals as fecund as newts. In the case of vertebral number, it seems likely that developmental processes act to reduce variation in very young larvae, but that selection is important in later life. Variation was also studied in this population at and just after metamorphosis, when about 40% of the emerging juveniles were killed by shrews, by scoring 25 morphometric characters. Despite difficulties of interpretation caused by growth and by the morphogenetic changes associated with metamorphosis, a clear case of stabilizing selection was discovered.