Glossary of "Plant Systematics (1st ed.)" by Simpson (2006)
- By one definition, a distinct lineage that, in sexually reproducing organisms, consists of a group of generally intergrading, interbreeding populations that are essentially reproductively isolated from other such groups.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- The base unit of taxonomic classification, consisting of an ancestor-descendant group of populations of evolutionarily closely related, similar organisms. The more narrowly defined "biological species" consists of individuals capable of interbreeding with each other but not with members of other species.
"Phylogenetics" by Wiley (1981), p. 7
- Species-as-taxa are lineages that are independent of other lineages in the sense that they may evolve independently of other such lineages. Species comprise the highest level of taxonomic organization on which the processes of evolution may work.
Glossary of "Ancestors in Our Genome" by Harris (2015)
- a population whose members interbreed under natural conditions, produce fertile offspring, and are reproductively isolated from other such groups.
Glossary of "Environmental Science: Earth as a Living Planet (2nd ed.)" by Botkin & Keller (1998)
- A group of individuals capable of interbreeding.
Glossary of "A Botanist's Vocabulary" by Pell & Angell (2016)
- Taxonomic rank below genus and inclusive of any subspecies, varieties, or forms.
- Botkin, D. B. & Keller, E. A. 1998. Environmental Science: Earth as a Living Planet, 2nd ed. -xxxxii + 649 pp. John Wiley & Sons, Inc., New York.
- Harris, E. E. 2015. Ancestors in Our Genome: the New Science of Human Evolution. - xx + 226 pp. Oxford University Press, New York.
- Pell, S. K. & Angell, B. 2016. A Botanist's Vocabulary: 1300 Terms Explained and Illustrated. 226 pp. Timber Press, Portland.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.
- Simpson, M. G. 2006. Plant Systematics. -xi + 590 pp. Elsevier Academic Press, Oxford.
- Wiley, E. O. 1981. Phylogenetics: the Theory and Practice of Phylogenetic Systematics. -xv + 439 pp. John Wiley & Sons, Inc., New York.