Natural selection favours drought escape and an acquisitive resource-use strategy in semi-arid Mediterranean shrubs

Abstract

Natural selection is the major force driving adaptive evolution in natural populations, varying in strength, direction, and form through space and time, especially in highly variable environments such as Mediterranean ecosystems. Although a conservative resource-use strategy has been hypothesized to be adaptive in Mediterranean taxa, patterns of selection at the intraspecific level, that is, the suite of traits determining individual fitness, are largely unknown. Using a phenotypic selection experiment in natural semi-arid conditions, we measured direct and indirect selection acting through two different fitness components (survival and reproduction), to assess the adaptive value of 20 ecophysiological traits on individuals of two gypsum endemic species, Centaurea hyssopifolia and Helianthemum squamatum, dwelling in environments with contrasting abiotic conditions (south- and north-facing slopes) during two climatically contrasting years (dry and mesic). This allowed quantifying the magnitude and direction of natural selection at different spatiotemporal scales. Our results revealed that different abiotic conditions did not alter selection patterns, being the magnitude of selection more strongly affected by temporal environmental variation. Selection through reproduction indicated consistent selection for early phenology, low water use efficiency, high specific leaf area, low leaf dry matter content, and high leaf N across slopes and years in both species. In contrast, phenotypic trait variation was not linked to survival in either species. Furthermore, while individual reproductive output was higher or similar in environments with higher abiotic stress in both species and years, survival was similar across environmental conditions, and it was neither affected by plant size nor reproductive output. Contrary to our expectations, natural selection via reproductive fitness consistently favoured a drought-escape, acquisitive resource-use strategy in Mediterranean semi-arid plants, rather than a conservative resource-use strategy, even under conditions of higher abiotic stress (i.e. south slopes and dry year). Such acquisitive strategy could allow rapid development by maximizing resource assimilation and reproduction before the most limiting climatic conditions of mid-late summer. Our results shed light on adaptive functional strategies of Mediterranean taxa at the intraspecific level, providing insight on future responses to environmental change, and highlight remarkable differences in selection acting through different fitness components