Abstract:

Background & Aim: The plant species complex, as a central and challenging issue in taxonomic research, comprises morphologically highly similar yet genetically divergent lineages, including cryptic species, hybrids, and polyploids. The taxonomic difficulties associated with these complexes would impact accurate biodiversity assessment and effective resource management. This review aims to systematically synthesize the formation mechanisms of plant species complexes and recent advances in their morphological taxonomy. It also seeks to elucidate the conflict between morphological conservatism and genetic divergence, propose an integrative classification framework, and outline future research directions and methodological trends. The ultimate goal is to establish a foundation for understanding the mechanisms of complex formation, delimiting cryptic species, interpreting interspecific differences and evolutionary relationships, and support germplasm utilization and species conservation. 

Progress: Research findings indicate that the formation of plant species complexes is collectively driven by natural selection, gene flow, polyploidization, and epigenetic regulation. Morphological evidence plays a crucial role in classifying plant species complexes and holds particular advantages during periods of rapid radiative evolution or under strong natural selection pressure. However, its utility is often limited by factors such as phenotypic plasticity, genomic introgression (hybridization), and niche conservatism. Although molecular evidence can compensate for some limitations of morphological data, relying solely on a single-dimensional approach remains insufficient for resolving the full complexity of evolutionary histories, including scenarios like incomplete lineage sorting or reticulate evolution. Addressing classification problems still requires taking morphological interpretation as an essential foundation. Contemporary taxonomy of plant species complexes increasingly relies on integrative methodologies that emphasize multidimensional validation combining morphological, molecular, and ecological evidence. The integration of multi-source data has become the main trend for solving the taxonomic predicament of plant species complexes. 

Prospects: Future research on plant species complexes should prioritize the construction of a robust “morphology-molecular-ecology” tripartite validation system. This involves quantifying phenotypic variation using advanced morphometric techniques and intelligent morphological technologies, elucidating genetic differentiation through genomic phylogenomic and population genetic analyses, and assessing environmental adaptability through ecological niche models. Furthermore, developing more efficient algorithmic models and incorporating phytogeographic regional backgrounds are essential for building a dynamic classification system. Advances in the taxonomy of plant species complexes will enhance our understanding of speciation mechanisms and provide a solid theoretical foundation for precise biodiversity conservation and the sustainable utilization of biological resources.

Key words: plant species complex, formation mechanisms, morphological taxonomy, gene flow, phenotypic plasticity, integrative classification