The article by Nixon and Wheeler (1990) discusses the phylogenetic species concept, which aims to identify the smallest units that can be analyzed using cladistic methods and interpreted as the result of phylogenetic history. The authors define species as the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals. This definition is character-based and pattern-oriented, consistent with contemporary explanations of speciation processes but requiring only the assumption of nested hierarchical patterns. The article reviews various species concepts, including the biological species concept, evolutionary species concept, internodal species, autapomorphic species, and the phylogenetic species concept. It critiques the limitations of these concepts, particularly the need for phylogenetic analysis and the challenges of applying them to asexual or clonal organisms. The authors argue that the phylogenetic species concept is more compatible with cladistic theory and practical cladistic analysis, as it does not rely on specific processes like reproductive isolation. The article emphasizes the distinction between tokogenetic and phylogenetic relationships, where tokogenetic systems are complex reticulate patterns, while phylogenetic systems are hierarchically structured. The phylogenetic species concept is shown to be consistent with various speciation processes, including sympatric and allopatric speciation, and can recognize species that lack autapomorphies. The authors conclude that the phylogenetic species concept is necessary to clarify the differences between phylogeny and tokogeny, macroevolution and microevolution, characters and traits, and systematics and population biology.The article by Nixon and Wheeler (1990) discusses the phylogenetic species concept, which aims to identify the smallest units that can be analyzed using cladistic methods and interpreted as the result of phylogenetic history. The authors define species as the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals. This definition is character-based and pattern-oriented, consistent with contemporary explanations of speciation processes but requiring only the assumption of nested hierarchical patterns. The article reviews various species concepts, including the biological species concept, evolutionary species concept, internodal species, autapomorphic species, and the phylogenetic species concept. It critiques the limitations of these concepts, particularly the need for phylogenetic analysis and the challenges of applying them to asexual or clonal organisms. The authors argue that the phylogenetic species concept is more compatible with cladistic theory and practical cladistic analysis, as it does not rely on specific processes like reproductive isolation. The article emphasizes the distinction between tokogenetic and phylogenetic relationships, where tokogenetic systems are complex reticulate patterns, while phylogenetic systems are hierarchically structured. The phylogenetic species concept is shown to be consistent with various speciation processes, including sympatric and allopatric speciation, and can recognize species that lack autapomorphies. The authors conclude that the phylogenetic species concept is necessary to clarify the differences between phylogeny and tokogeny, macroevolution and microevolution, characters and traits, and systematics and population biology.