| While evolution is generally thought of resulting from selection acting directly on individual traits, selection also acts indirectly on traits that are correlated with the traits under selection. The results of the studies comprising my dissertation have established several important aspects of the ecology of predation on T. granulosa eggs and how egg predation may be influencing the evolution of toxicity in T. granulosa through a correlation between egg and maternal toxicity. Chapter Two establishes that TTX is likely to be produced endogenously and may have a genetic basis in T. granulosa, and thus toxicity can respond to selection. While caddisfly larvae, the primary predators on eggs of T. granulosa , are resistant to TTX, they still prefer not to eat it (Chapter 3). They will, however, eat newt eggs with high levels of TTX when given no other food source, suggesting they are will opportunistically feed on this highly nutritious resource. Further, predation pressure on eggs appears to be a function of caddisfly abundance, so a combination of high predator abundance and predator preference for low toxicity eggs should lead to stronger selection on egg toxicity in some populations (Chapter 4). Finally, although caddisfly larvae may avoid consuming eggs with higher levels of TTX, developing embryos respond to chemical cues from caddisfly larvae, indicating they are sensitive to the threat posed by the presence their predators (Chapter 5). The combined results of these studies indicate that egg predation is likely to be a significant evolutionary force acting on newt eggs. Egg predators appear able to select on egg toxicity, and this selection should be stronger when predators are more abundant. Selection on egg toxicity indirectly selects on maternal toxicity due to the correlation between egg toxicity and maternal toxicity. These findings are important because the evolution of toxicity in newts has previously only been considered as the result of selection directly on terrestrial newts by snakes. My studies of egg predation bring in another life history stage of newts and another predator, making the evolutionary scenario leading to the evolution of increased toxicity both more complex and more realistic. |
