Growth rates are highly variable, both within and among genotypes and populations. The resolution of the trade-off between size and age at maturity has been the study of extensive research by life historians. The fitness advantages of large body size and rapid development time are well supported, leading to two predictions. First, realized growth rates should be maximized. Second, growth rate will be subject to strong stabilizing or directional selection, and consequently, low genetic variability.
In real populations, despite the advantages of rapid growth, animals often, in fact, grow at rates lower than the maximum rate that is physiologically possible, even in the absence of external constraints on growth rate (e.g. resource restriction or risk of predation while foraging). This implies that growth may have direct fitness consequences that are independent of the size and age of maturity, thereby lowering the optimal rate of growth. In addition to inducing plastic declines in growth rate, such costs may also select for lower intrinsic rates of growth.
Despite the strong fitness effects arising from attaining a large body size quickly, variation in growth rate persists at both the phenotypic and genetic levels. The evolutionary and ecological factors contributing to this variation in growth rate are the focus of this thesis. Growth rate variation in insect model species was produced by the manipulation of resource levels during development. By comparing fitness-associated traits and body composition of adults from different treatment groups, I identify direct costs of rapid growth that could explain why animals benefit from growth at submaximal rates. In the second part of the thesis, the relationship between environmental variation and genetic variance in growth rate is investigated by quantitative genetic analysis of body size at different ages and in different growth environments. The results of this analysis suggest that environmental stress can lead to increased genetic variance via decanalization. This has consequences for the evolvability of growth rates in changing environments.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/24343 |
| Date | 15 April 2010 |
| Creators | Dmitriew, Caitlin |
| Contributors | Rowe, Locke |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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