Intraspecific character displacement in oaks

Character displacement refers to the process by which species diverge more in sympatry due to competition for resources. This competition-driven speciation can also occur within populations, known as intraspecific character displacement (ICD). ICD can promote divergence within species by influencing intraspecific competition or encouraging the evolution of alternative phenotypes. Despite its significance, ICD remains understudied and requires further exploration. In this study, we investigate how competition influences genetic and morphological differentiation within species in sympatric and allopatric populations. We focused on Quercus serrata (in China and Japan) and Q. serrata var. brevipetiolata (found only in China), which belong to a small monophyletic group of oak species nested within Section Quercus (white oaks). Using genetic markers, we detected divergence between Chinese and Japanese populations and further diversification within China, with asymmetric historical gene flow primarily from Q. serrata (the earlier diverged species) to Q. serrata var. brevipetiolata (the later variety). Although genetic differentiation did not differ between sympatric and allopatric populations, leaf morphological variation, analyzed through the geometric morphometric method (GMM) and traditional morphological method, revealed greater trait variation in sympatry. In addition, we found an allometric growth relationship between leaf size and leaf mass of Q. serrata and Q. serrata var. brevipetiolata, with the leaf area of Q. serrata var. brevipetiolata decreasing more disproportionately to leaf mass. This suggests a resource trade-off, where Q. serrata var. brevipetiolata, the later diverged variety, adopts more resource-conservative traits in sympatry. Further analysis of trait variation with environmental factors supports these findings, while genetic variation along climate gradients showed significant responses primarily in Q. serrata, regardless of sympatric or allopatric conditions. Although neutral genetic markers are insufficient to capture selection-driven adaptive differentiation, we inferred that Q. serrata var. brevipetiolata is progressing towards ecological divergence from Q. serrata. Overall, our results highlight the role of ICD in driving morphological diversification and resource-use strategies within species in response to competitive pressures.