Genetic, morphological and behavioural variation in Scottish three-spined stickleback (gasterosteus aculeatus l.) : insights from differently armoured populations
This thesis addressed a central theme in evolutionary biology, namely understanding the process of adaptive radiation, using the three-spined stickleback, which has become a model system in this context. In particular, the work concentrated on sticklebacks from several unique populations in the Outer Hebrides that have lost the body armour after which this species was named, lacking dorsal spines, pelvic girdle and spines and lateral plates. A first specific aim was to examine an existing hypothesis about the selective force responsible for armour loss in these populations, namely that it is an adaptive response to low calcium levels. This is in contrast to the selective force favouring armour loss in North American populations, where predation by piscivorous fish is thought to favour armour development and predation by invertebrates to favour armour loss. This was studied by relating variability in protective body armour to calcium concentration at 10 sites with calcium concentrations ranging from 1.2mgCA2+/L to 50.5mgCa2+/L and spread over a wide geographical range. The results confirmed previous studies, providing partial support for the low-calcium hypothesis for Hebridean populations. Thus armour reduction is only found in sticklebacks from low-calcium sites, although not all fish from such sites are unarmoured. Piscine, avian and invertebrate predators were present at all sites, ruling out the predation regime hypothesis. A second aim was to relate variability in risk-taking to variable armour expression, both within and between populations of stickleback. 180 wild-caught fish from 7 sites across Scotland (but mainly concentrated in the Hebrides) were screened for risk-taking behaviour using a well-established testing protocol (quantifying rates of exploration of and movement in a novel and potentially dangerous environment) that is broadly predictive of some aspects of the sticklebacks’ response to a predator. No significant relationship was found between individual risk-taking score and the extent of body armour, either across populations or among individuals within populations. The risk-taking test (chosen because it is simple and easily controlled) only reflects one aspect of anti-predator behaviour and may have been too focused to identity subtle and individual differences in risk-taking.