Phenotypic Quantitative Divergence Across Heterogeneous Environments in a Widespread Southern South American Tree

Phenotypic and genetic divergence along environmental gradients often reflects local adaptation in broadly distributed species. The Fabaceae family is one of the largest and most ecologically important angiosperm groups; it has a centre of diversity in South America and shows high versatility in arid and disturbed environments. Here, we selected <i>Vachellia</i><i>caven</i>, a native tree with ecological breadth and taxonomic complexity, to investigate whether phenotypic trait variation among populations reflects adaptive divergence. We examined neutral genetic differentiation in six varieties among populations from Argentina, quantified the phenotypic differentiation of quantitative traits by an ANOVA, and performed <i>P</i><i>_ST-F_ST</i> comparisons. We also assessed correlations between phenotypic variation, environmental variables, genotypic variation, and geographic distances. <i>F_ST</i> estimates revealed significant genetic divergence (0.329), in line with isolation by distance and environmental heterogeneity. <i>P_ST-F_ST</i> comparisons showed that all traits were under diversifying selection, supporting the hypothesis of adaptive phenotypic variation. We further detected that fruit width and length were significantly correlated with specific environmental variables like precipitation and temperature. These findings confirm that phenotypic divergence in <i>V. caven</i> is shaped by both geographic and environmental factors. This study offers a preliminary insight into the local adaptation of the examined traits, highlighting how morphological and genetic differentiation has enabled <i>V. caven</i> to thrive in diverse environments and contributing information as to how to face climate change scenarios.