Natural selection by predators on the defensive apparatus of the three-spined stickleback, Gasterosteus aculeatus L.

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Abstract
Three-spined sticklebacks (Gasterosteus aculeatus L.) can receive considerable protection against predators from characteristic dorsal and pelvic spines. The size and structure of this defensive apparatus were examined in marine and freshwater populations located throughout the European distribution of this species and exposed to differing predatory species and levels of predation pressure. Marine populations appear to experience the greatest predation pressure and have the largest defensive apparatus. Predation in freshwater appears to decline towards both the northern and southern distributional extremes of Gasterosteus, a result attributable to the differential distributions of major piscine predators, especially pike and perch; and this pattern produces arched clines in the morphometrics of defensive structures. Local differences in central latitudes can also be attributed to site-specific differences in predation pressure. Evidence is given for both hereditary and environmental determination of variation in dorsal spine number.