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




Bergstrom, Carolyn Ann

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The relative importance of stochasticity and adaptation to biodiversity has long been of interest to evolutionary biologists. Multiple, closely related insular populations provide ideal natural experiments with which to determine die relative strength of these two factors. An example of one such system is the threespine stickleback, Gasterosteus aculeaius. Threespine stickleback 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. Previous studies showed that numbers of defensive lateral plates and plate symmetry are positively correlated with the presence of predatory trout on these islands, but the effect of avian predators, another predator of stickleback, on plate number evolution remains unexplored. 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. Avian injury frequencies increased in populations as plate numbers increased at low plate numbers, but did not increase in populations wife plate numbers greater than ten. Trout injury frequencies decreased as plate numbers increased among populations, probably due to methodological problems. There were no correlations between avian or trout induced injuries and plate number within populations. Experiments indicated that selective predation of lateral plate numbers in stickleback by captive Hooded Mergansers was weak but insignificant, resulting in a very slight reduction in plate numbers after predation. 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. Asymmetric individuals did not have reduced survivorship, contrary to predictions from previous studies of FA. However, asymmetry of lateral plates was negatively correlated wife plate number, and asymmetry of plates that provide structural integrity to the defensive spines was greatly reduced relative to ‘non-structural’ plates, supporting fee hypothesis that biomechanically important traits have greater symmetry. Structural plate asymmetry decreased as water clarity, and the chances of capture by predators, increased, and when the degree of overlap between plates and spine supports increased. 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 Iineages 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.



Sticklebacks, Behavior, Animal defenses