The future of human being may rely on biodiversity, and thus depends on how to investigate, conserve, and rationally use biodiversity. Species is the basic unit of biodiversity, and therefore rational delimitation of species is one of the crucial issues for biodiversity pursuits. However, no species concept published until now is both scientific and operative. A tentative species concept is proposed here just for discussion.

One of the diverse species concepts defined before may only perceive one aspect of the mature species like “the blind men feel the elephant” while the mature species at the final speciation stage should have integrated all species concepts. Most “species” in the nature are on the way to the final speciation stage. However, before reaching the final speciation stage, these species undertake further cycles of speciation. Species from the repeated splits of the incomplete divergences show incomplete reproductive isolations, frequent interspecific gene flow and reticulate evolutions. In addition, the earliest divergent gene differs between different pairs of species. Therefore, the divergence orders for different species concepts vary greatly between organisms. Such random divergences lead to the extreme difficulty to define a common and accurate species concept for all “species” on the speciation way. It is better to delimitate species, publish new species and conduct taxonomic revisions based on conditions and approaches of as many species concepts as possible. In addition, incomplete reproductive isolations, limited interspecific gene flow and some ‘abnormal’ individuals not ascribed to any species due to interspecific hybridizations and within-population mutations should be widely acknowledged during species delimitations. Such circumscribed species may be more objective and scientific than previously delimitated based only on one single species concept.

Biologists believe that species is the basic unit of biological diversity, and few people will doubt the fact that there exist distinct species in nature. Nevertheless, despite of ceaseless debating on species concept or species definition among biologists over nearly a century, no consensus has been reached. Species definition concerns many biologists because it has far-reaching practical implications. Therefore, species concepts proposed by various scholars zero in greatly on their operational convenience and practical applicability in research activity such as taxonomic classification. As a result, these concepts and definitions are unavoidably shaped by the visions of the scholars which are further shaped by their insights into the evolutionary process of speciation. Speciation represents a stage in evolutionary divergence, and also different “species” may be at different stages of the speciation process. Since a “definition” is a kind of convention or confining description, every definition would bear some limitations. It would be thus very difficult, if not impossible, to define a perfect species concept that will accord with all species in taxonomic practice. From this perspective, we should confess that while every species concept or definition proposed thus far is reasonable, none is faultless or flawless. It is very important to bear this in mind, because, otherwise, one may stubbornly insist on one single species concept and introduces errors or even triggers chaos and confusion, consciously or unconsciously, in taxonomic and other evolutionary researches.

This is a mini-review on the historical changes in the concept of species. Biologists use different methods or criteria to discriminate species, leading to the formation of different species concepts, e.g. biological species, morphospecies, ecological species, evolutionary species, phylogenetic/cladistic species, or their combinations. These concepts respectively reveal a specific profile of the species’ attributes, as well as reflecting the objective existence of these creatures as different species, but not being satisfied with everyone. For eukaryotes, reproductive isolation (incapable of reproducing fertile offspring) should be the key for two populations to differentiate into two different species, no matter how much they differ morphologically. The mechanisms underlying such isolation might be geographical, behavioral, or otherwise. Reproductive isolation is certainly accompanied by some morphological or genetic changes that are often used as criteria by taxonomists or molecular evolutionary biologists to distinguish species, although these attributes may not be associated with reproductive isolation itself. Extinct species known only from fossils are impossible to be classified taxonomically according to reproductive isolation. The exact definition of the term “species” is still controversial, as a species concept based on reproductive isolation is usually not applicable, but a usable species definition (e.g. morphospecies) is regarded to be artificial.

Both species concept and recognition are fundamental topics in the studies of taxonomy and biodiversity, and also in biological researches. There has never been any unanimous and clear standard for species dilimitation and, thus, taxonomy has more or less been regarded as subjective and often seen as art rather than scientific research. The backgrounds and the current status of three commonly used species concepts, i.e. morphological, biological and phylogenetic species concepts, in the research of fungal taxonomy are briefly reviewed here. The application of the three species concepts are discussed with examples in fungal taxonomy, especially on interlaces and incongruences between the concepts. The materials presented here may provide some useful information for the study on taxonomy and the investigation on species concept.

Very recently, Professor Deyuan Hong, the editor-in-chief of the ongoing Flora of Pan-Himalaya, put forward a series of proposals (which may be conveniently termed Hong’s Ten Points) on raising rationality in species delimitation, a hotly-debated question in systematic biology [Hong DY (2016) Opinion on raising rationality in species delimitation. Biodiversity Science, 24: 360-361]. The present paper is mainly to expound on some of the points. Even after the Flora Reipublicae Popularis Sinicae and its English version, Flora of China, was published, many species-level taxonomic problems of plants from China still remain to be solved. This situation may be due in part to previous unsatisfactory taxonomic practice and in part to our poor knowledge of taxonomic characters of a large number of plant groups from the country. Indeed, a very important stage in plant taxonomy, the biosystematic phase, was basically missing in China, and even the consolidation or systematic phase (one of the stages in alpha-taxonomy) has not yet been accomplished for some groups. Nowadays, it is inappropriate to emphasize the separation of orthodox and experimental taxonomy, and a multidisciplinary approach needs to be adopted in addressing taxonomic problems. To gain a fuller understanding of plant biodiversity in China, creative monographic work is badly needed, which requires the training of young botanical monographers. Moreover, it is suggested that practicing taxonomists should take extreme caution when employing population concept and statistical methods in their routine work. Finally, it is argued that every effort should be made toward a broader biological species concept although botanists have to accept the pluralism in species concept. To revert to a purely subjective species concept, under the pretext of the existence of the so-called lumpers or splitters in systematic biology, is not correct and must be opposed.