Abstract. The literature suggests that small genomes promote invasion in plants, but little is known about the interaction of genome size with other traits or about the role of genome size during different phases of the invasion process. By intercontinental comparison of native and invasive populations of the common reed Phragmites australis, we revealed a distinct relationship between genome size and invasiveness at the intraspecific level. Monoploid genome size was the only significant variable that clearly separated the North American native plants from those of European origin. The mean Cx value (the amount of DNA in one chromosome set) for source European native populations was 0.490 AE 0.007 (mean AE SD), for North American invasive 0.506 AE 0.020, and for North American native 0.543 AE 0.021. Relative to native populations, the European populations that successfully invaded North America had a smaller genome that was associated with plant traits favoring invasiveness (long rhizomes, early emerging abundant shoots, resistance to aphid attack, and low C:N ratio). The knowledge that invasive populations within species can be identified based on genome size can be applied to screen potentially invasive populations of Phragmites in other parts of the world where they could grow in mixed stands with native plants, as well as to other plant species with intraspecific variation in invasion potential. Moreover, as small genomes are better equipped to respond to extreme environmental conditions such as drought, the mechanism reported here may represent an emerging driver for future invasions and range expansions.

Species evolve life histories and phenotypes that are adapted to the environments they inhabit. Consequently, latitudinal and elevational gradients in life-history strategies emerged through evolution, shaped by geographic variation in environmental conditions. Since similar life histories co-evolve with suites of convergent physiological adaptations termed pace-of-life syndromes (POLS), corresponding latitudinal and elevational gradients in POLS should also emerge (Ricklefs & Wikelski, 2002).Physiological adaptations are further shaped by other environmental demands, such as thermogenic needs. Research into patterns of physiological variation over large geographical and phylogenetic scales, known as macrophysiology, is therefore essential to our understanding of the mechanisms underpinning global variation in demographic rates and life histories, species distribution and abundance or physiological adaptation under diverse environmental conditions (