Some of the earliest traits subject to natural selection in an oviparous animal's life are those concerning embryonic development and the timing of the first life-history switch point, when eggs hatch (reviewed by Warkentin 2011
). While timing of hatching is often thought to be highly canalized, this has shown to not always be the case in a large variety of taxa, and changes in hatching timing and rates of development can have great effects on the fitness of these organisms (reviewed by Warkentin 2011
). For example, variation in the timing of egg hatching in damselflies may allow some individuals to persist while others die in ephemeral habitats where hydroperiod is in constant flux (De Block et al. 2005
), and hatching timing in monogenean parasitic worms which infect swimming fish is crucial to facilitating successful host infection (Whittington and Kearn 2011
). In amphibians, red-eyed tree frogs (Agalychnis callidryas
) hatch early to avoid predation by egg-eating snakes and wasps, but this also results in individuals hatching at a smaller size and developmental stage, and thus being more vulnerable to larval predators (Warkentin 1995
). The timing and size at which Ambystoma opacum
and Ambystoma talpoideum
salamanders hatch influences their survival and interspecific competitive and predatory interactions as larvae (Boone et al. 2002
), and larvae of the rough-skinned newt, Taricha granulosa
, that are smaller and less developed are more likely to die from predatory attacks by dragonfly nymphs (Gall et al. 2011a
). Thus, the rate and degree of embryonic development that occurs in the egg capsule, and the timing of hatching from that capsule are clearly highly adaptive traits that can have considerable influence on performance during later life stages. However, while clearly adaptive, it is still unclear if these early life-history traits are subject to natural selection, as we have little knowledge of how they vary within a population.
Variation, regardless of its cause, is the underlying raw material for natural selection. If that variation is heritable, it can lead to evolutionary change. Therefore, although rarely completely possible to do, when trying to understand the potential for evolution, it is important to try to distinguish between the different potential sources of phenotypic variation (Berven 1982
; Travis 1983
; Laugen et al. 2005
The rough-skinned newt (T. granulosa, Skilton; Caudata: Salamandridae) () is an amphibian species that is well suited to determining the interfamily variation that may be present in embryonic development and hatching timing. Gravid female newts are easily collected and can be induced to deposit their eggs in the laboratory. Thus, the maternal source of each egg is known, the environment in which the eggs are raised can be controlled, and some morphological characteristics of the female (e.g., size, weight, egg diameter) that might influence embryonic development and hatching timing can be measured and accounted for.
The study organism, Taricha granulosa. (A) Adult newt. (B) Newly hatched larva, developmental stage = 42.
The purpose of this study was to determine if the highly adaptive traits of embryonic development and hatching timing varied significantly among the eggs from different T. granulosa females from a single population (which we hereafter refer to as “families”). As variation is the basis for natural selection, establishing if this variation exists is critical for future studies on the evolutionary biology of amphibian early life-history stages. To achieve our goal, we set out to determine if there was underlying variation in hatching timing, developmental stage, and size at hatching among the newt families, and if this variation was present at different environmental temperatures. As the majority of studies on amphibian early life-history traits have not considered differences among families in any of these traits, we used this consensus view in the literature to establish a testable null hypothesis: that there is no variation in hatching timing and embryonic development among newt families from a single population.