Evolution in response to environmental unpredictability in Indian tobacco, Lobelia inflata (Campanulaceae).
Date
1999
Authors
Simons, Andrew Michael Benjamin.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
Organisms interact with an environment that is variable over time scales of seconds to millions of years. Although evolution in response to predictable events may be more tractable, unpredictable environmental events also have evolutionary consequences. Under unpredictable environments, so-called "bet-hedging" strategies are expected to evolve. There are two ways in which bet hedging may occur: through the evolution of trait diversification, and through the evolution of conservative traits. These two forms of bet hedging are illustrated by two stages in the life cycle of Indian tobacco, Lobelia inflata : variation in seed germination behaviour is an oft-cited form of diversification, and the "decision" to reproduce is a potential conservative trait.
The timing of germination is found to be influenced by seed size and environmental conditions. Both seed size and the timing of germination within a growing season are found to affect fitness. Little is known, though, about the mechanistic basis of seed trait diversification. The heritability of the diversification in time to germination is found to be nonsignificant. Based on empirical evidence for seemingly random but strong environmental control of germination, I argue that diversification bet-hedging might arise through the evolution of extreme sensitivity of the timing of germination to environmental stimuli, possibly through selection for developmental instability.
L. inflata, with a single opportunity to reproduce, should evolve conservative "rules" governing the timing of reproduction (bolting) if the optimal date changes from year to year. I find that the rules influencing bolting are plastic; they change as the summer progresses. Although the heritability of the reproductive decision is high, no genetic variation for the observed plasticity is detected. Bolting behaviour in the field appears to be suboptimal during the year of the experiment: this behaviour is better accommodated by an empirically-based model if unpredictability in season length is included. Bolting behaviour might thus have evolved as a conservative bet-hedging strategy. Plant survival is dependent not only on season length, but also on other forces of destruction such as herbivores. A bet-hedging model is developed that offers an explanation for plants' ability to compensate for tissue lost to destruction, and some empirical evidence in support of this model is presented.
A major unresolved issue in evolutionary biology is whether the effects of selection operating within populations accumulate to produce the trends observed over macroevolutionary time. I suggest that bet-hedging theory is relevant to evolution over longer time scales, and that recognizing this helps resolve part of the ongoing debate over the continuity between micro- and macroevolution.
Thesis (Ph.D.)--Dalhousie University (Canada), 1999.
The timing of germination is found to be influenced by seed size and environmental conditions. Both seed size and the timing of germination within a growing season are found to affect fitness. Little is known, though, about the mechanistic basis of seed trait diversification. The heritability of the diversification in time to germination is found to be nonsignificant. Based on empirical evidence for seemingly random but strong environmental control of germination, I argue that diversification bet-hedging might arise through the evolution of extreme sensitivity of the timing of germination to environmental stimuli, possibly through selection for developmental instability.
L. inflata, with a single opportunity to reproduce, should evolve conservative "rules" governing the timing of reproduction (bolting) if the optimal date changes from year to year. I find that the rules influencing bolting are plastic; they change as the summer progresses. Although the heritability of the reproductive decision is high, no genetic variation for the observed plasticity is detected. Bolting behaviour in the field appears to be suboptimal during the year of the experiment: this behaviour is better accommodated by an empirically-based model if unpredictability in season length is included. Bolting behaviour might thus have evolved as a conservative bet-hedging strategy. Plant survival is dependent not only on season length, but also on other forces of destruction such as herbivores. A bet-hedging model is developed that offers an explanation for plants' ability to compensate for tissue lost to destruction, and some empirical evidence in support of this model is presented.
A major unresolved issue in evolutionary biology is whether the effects of selection operating within populations accumulate to produce the trends observed over macroevolutionary time. I suggest that bet-hedging theory is relevant to evolution over longer time scales, and that recognizing this helps resolve part of the ongoing debate over the continuity between micro- and macroevolution.
Thesis (Ph.D.)--Dalhousie University (Canada), 1999.
Keywords
Biology, Ecology., Biology, Genetics.