Geographic variation in the environment underpins selection for local adaptation and evolutionary divergence among populations. Because many environmental conditions vary across species’ ranges, identifying the specific environmental variables underlying local adaptation is profoundly challenging.

We tested whether natural selection mediated by aridity predicts clinal divergence among invasive populations of capeweed (Arctotheca calendula) that established and spread across southern Australia during the last two centuries.

Using common garden experiments with two environmental treatments (wet and dry) that mimic aridity conditions across capeweed’s invasive range, we estimated clinal divergence and effects of aridity on fitness and multivariate phenotypic selection in populations sampled along aridity gradients in Australia. We show that: (i) capeweed populations have relatively high fitness in aridity environments similar to their sampling locations; (ii) the magnitude and direction of selection strongly differs between wet and dry treatments, with drought stress increasing the strength of selection; and (iii) differences in directional selection between wet and dry treatments predict patterns of clinal divergence across the aridity gradient, particularly for traits affecting biomass, flowering phenology and putative antioxidant expression.

Our results suggest that aridity-mediated selection contributes to trait diversification among invasive capeweed populations, possibly facilitating the expansion of capeweed across southern Australia.

Two common garden experiments (2018 greenhouse; 2019 field) were conducted at Monash University (Clayton, Victoria; latitude: -37.9091, longitude: 145.1411). 

Greenhouse experiment.
In the 2018 experiment, conducted in a greenhouse, we investigated variation in drought resistance traits in six populations sampled along a transect spanning wet-to-dry capeweed range conditions in Victoria. 

Field experiment.
The 2019 experiment was conducted in an open field at Jock Marshall Reserve (Monash University, Clayton) to estimate selection in a semi-natural environment of capeweed. Seeds for this experiment were collected from approximately 30 maternal plants each from eight populations distributed along transects in eastern and western Australia. 

We measured 13 traits that are thought to mediate drought resistance in annual plants. Drought escape traits included growth rate, maximum photosynthetic capacity (F_v/F_m), photosynthetic assimilation rate, final biomass, and days to first flower. Root-to-shoot ratio, leaf size, leaf succulence, specific leaf area (SLA), and stomatal conductance are linked to plants’ dehydration avoidance strategy. We quantified three leaf phenolic compounds that are potentially associated with plants’ drought tolerance strategy (chlorogenic acid, an unknown phenolic acid, and an unknown flavonoid).