Evolution, functional morphology, and asymmetry of predator defense in stickleback

Threespine stickleback Gasterosteus aculeatus have predator defenses comprised of heritable lateral bony plates and large spines. Morphologically invariant marine stickleback have colonized freshwater habitats across the northern hemisphere, resulting in multiple independently derived freshwater forms highly variable in predator defenses. The islands of Haida Gwaii (the Queen Charlotte Islands British Columbia, contain populations of freshwater stickleback that exhibit defensive variability comparable to the entire species, and vary in defensive asymmetry. The purpose of this thesis is to determine whether plate number reduction is a defensive adaptation to avian predation, and to study functional implications of asymmetry in structural defenses from 115 natural populations.; Experiments showed that plate number reduction in threespine stickleback enhanced fast-start velocity; a possible advantage to fish being pursued by diving birds with similar swimming speeds.; The degree of fluctuating asymmetry (FA) of defensive structures showed a geographical cline across the archipelago, being elevated in lowland stained ponds, and reduced in clear large lakes. FA of juveniles was not correlated with pH, conductivity, light transmittance, or lake size among populations. Plate asymmetry was weakly associated with susceptibility to parasitism, but only where overall plate numbers were low. This supports the hypothesis that FA/fitness correlations are trait and habitat specific, and that sensitivity of asymmetry to developmental instability can be reduced in biomechanically important traits.; In conclusion, reduction in armour in stickleback may be adaptive, but there is only weak evidence of selection by avian predators on lateral plates in the wild. Whether armour reduction is a direct adaptation to avoid capture or a cost-minimization strategy is not clear, but repeatability of reduced armour in habitats with diving birds, and the hydrodynamic benefit it provides, suggest the former. The associations between asymmetry and function suggest that asymmetry should be included in comparisons of divergent populations, as it lends insight into the functional implications of morphological diversity. Lastly, because multiple independent lineages have evolved similar phenotypes in similar habitats in stickleback, this research has reinforced the idea that local adaptation to unique habitats is the driving force of diversification. (Abstract shortened by UMI.)...