Multivariate morphological divergence due to intraguild predation.

Intraguild predation, defined as predation between consumers of a shared resource, is widespread in nature and likely to be an important selective pressure shaping the evolution of defensive and foraging traits in many ecosystems. In many lakes of Southern Greenland, threespine stickleback (Gasterosteus aculeatus) can both compete with- and be preyed upon by Arctic charr (Salvelinus alpinus), meaning that stickleback trait evolution in this system may be influenced by intraguild predation. To test this, we analyse multivariate morphological variation within and across 36 stickleback populations, of which 18 sympatric with charr and 18 without charr. We find evidence of stickleback adaptation to intraguild predation particularly in defensive morphology, via longer spines and larger structural armour, though surprisingly not via more numerous lateral plates. Foraging morphology also responds to the presence of charr, with a shift towards littoral-adapted traits. These differences are revealed beyond single trait comparisons, as we discover diverging patterns of phenotypic (co)variance, with generally lower variation and dimensionality and stronger integration in populations with charr, indicative of selection for efficiency and erosion of variation. These findings corroborate the niche shift hypothesis for the evolutionary consequences of intraguild predation. Further, we find agreement between ecological theory of intraguild predation stability and our results on phenotypic change. Ultimately, our study provides a link between a multi-faceted ecological interaction-intraguild predation-and its high-dimensional evolutionary outcome-the multivariate phenotype-and showcases the power and importance of adopting a multivariate approach in evolutionary ecology studies.