What Is a Species?

Why One of Biology’s Most Familiar Words Is So Difficult to Define

A species is one of the most familiar units in biology, but it is also one of the hardest to define with a single rule. Plants make the problem especially visible. Some species hybridize with close relatives, polyploidy can create reproductive barriers rapidly, and geographically separated populations may look distinctive without having completed the process of evolutionary separation.

Tree TSAR therefore treats a species as a separately evolving lineage that is sufficiently coherent and diagnosable to recognize as an independent taxonomic unit. This follows the general lineage approach developed by de Queiroz (2007), in which reproductive isolation, morphology, genetic structure, ecology, and other familiar species criteria are understood as evidence for lineage separation rather than rival definitions that must all be satisfied in the same way.

The result is a species concept that is evidence-based without pretending that every lineage reaches independence through the same evolutionary path.

Why Isn’t Appearance Enough?

Morphology remains essential to plant identification and taxonomy. Stable differences in flowers, fruits, leaves, stems, anatomy, pollen, seeds, or other traits can provide powerful evidence that lineages are distinct.

Appearance alone can nevertheless mislead. Unrelated plants can converge on similar forms under similar ecological pressures, while recently diverged or cryptic species may remain difficult to distinguish morphologically. Environmental conditions can also alter plant size, leaf shape, pubescence, flowering time, and other traits within a single species.

Tree TSAR therefore asks whether morphological differences are consistent, diagnosable, and congruent with other evidence, not merely whether two specimens look different.

Four Ways Populations Can Diverge

Tree TSAR organizes species-level evidence around four broad dimensions adapted from Oberprieler’s Wettstein Tesseract (Oberprieler 2023):

Genealogical - Genetic clustering, ancestry, genomic differentiation, ploidy, and evidence concerning historical or continuing gene flow.
Morphological - Stable differences in form, anatomy, reproductive structures, or statistically supported quantitative traits.
Geographic - Allopatry, parapatry, contact zones, range discontinuities, and spatial replacement.
Ecological - Habitat, phenology, pollination, reproductive biology, niche differentiation, and local adaptation.

These dimensions are not four mandatory boxes that every species must check. They provide a way to ask whether different kinds of evidence converge on the same biological boundary.

Reproductive Isolation Helps, but Plants Complicate the Rule

The biological species concept made reproductive isolation one of the best-known ways of thinking about species. In plants, however, reproductive barriers are often partial rather than absolute. Closely related species may exchange genes occasionally and still maintain distinct morphology, ecology, and evolutionary trajectories.

Widmer et al. (2009) reviewed the many prezygotic and postzygotic barriers that contribute to reproductive isolation in plants. These barriers can accumulate in different combinations, which means that the presence of occasional hybrids does not automatically collapse two species into one.

Tree TSAR therefore asks whether gene flow is strong enough to prevent the lineages from remaining distinct, rather than demanding complete reproductive incompatibility.

What About Hybrids?

Hybridization is a normal part of plant evolution. It can blur boundaries, transfer genetic material between established species, or contribute to the origin of entirely new lineages. Rieseberg & Willis (2007) and Soltis & Soltis (2009) review the importance of hybridization and polyploidy in plant speciation.

The existence of hybrids is therefore evidence to interpret, not a verdict. A narrow hybrid zone between otherwise coherent species can coexist with strong species boundaries. Continuous intergradation across an entire range, by contrast, can indicate that two proposed taxa are better understood as parts of one variable species.

The pattern matters more than the mere fact that hybridization occurs.

Geography Matters, but It Does Not Define a Species

Geographic separation can promote divergence by reducing gene flow, and long-isolated populations often accumulate genetic and morphological differences. That makes geography valuable evidence.

Yet allopatry alone does not create a species. Populations separated by mountains, islands, or large distances can remain members of one species, while distinct species can coexist in the same habitat. Tree TSAR therefore treats geography as one dimension of evidence rather than a requirement.

Contact zones are particularly informative. When two differentiated forms meet, their behavior in contact can reveal whether they remain distinct, hybridize only occasionally, or merge into a broad continuum.

Species Can Be Real Without Being Simple

The idea that species must be perfectly isolated, perfectly monophyletic, and perfectly diagnosable at every stage is difficult to reconcile with speciation as an evolutionary process. Different properties arise at different times. A lineage may become ecologically distinct before complete reproductive isolation develops, or it may become genetically structured long before obvious morphological differences appear.

The general lineage concept accommodates that reality. Evidence becomes more persuasive when several independent properties point toward the same boundary (de Queiroz 2007).

Tree TSAR uses that principle to avoid both extremes: recognizing every genetic cluster as a species and refusing to recognize a lineage until every possible criterion is complete.

Species, Subspecies, Varieties, and Forms

Not all meaningful plant variation belongs at species rank. Tree TSAR uses infraspecific ranks when a lineage is distinct enough to communicate but the evidence for full evolutionary independence remains incomplete.

A subspecies normally represents substantial, coherent differentiation within a species, often with a strong geographic or ecological component. A variety generally represents a more limited but still biologically meaningful pattern of differentiation. Tree TSAR also distinguishes the formal botanical rank forma, abbreviated f., from the capitalized editorial category Form, which is used for selected stable localized, clonal, or apomictic entities that do not fit comfortably into ordinary formal ranks.

These categories are explained in greater detail in How Tree TSAR Delimits Species. The botanical Code governs how formal ranks are named; it does not prescribe how much biological divergence a taxonomist must require before recognizing them.

Small Species, Big Genera

Tree TSAR’s willingness to recognize a well-supported species does not imply a preference for splitting its genus. Species and genera answer different questions.

A species describes a comparatively terminal evolutionary lineage. A genus organizes related species into a broader, useful group. Tree TSAR can therefore recognize narrowly but defensibly delimited species while retaining a large genus when that genus remains monophyletic, diagnosable, and informative.

When Tree TSAR Disagrees

Species boundaries are hypotheses about evolutionary history, and reasonable specialists can disagree. One treatment may emphasize broad intergradation, another genomic clustering, and another ecological or reproductive separation.

When Tree TSAR adopts a materially different treatment, the goal is to make the reasoning visible. The narrative should identify the evidence, explain the competing interpretation, and state why the selected rank better represents the total pattern.

A taxonomic disagreement is most useful when readers can understand what would cause the conclusion to change.

Species Are Hypotheses We Can Improve

A species name may feel fixed because it appears on herbarium labels, conservation lists, field guides, or garden tags. The biological boundary behind that name remains a scientific hypothesis that can be tested with better sampling and better evidence.

Tree TSAR treats revision as a strength rather than a failure of taxonomy. A species should remain accepted because the evidence continues to support it, not because changing a familiar name would be inconvenient.

At the same time, novelty alone is not evidence. New genomic methods can reveal remarkable structure, but they can also overinterpret population differentiation when used without sufficient biological context. The strongest species treatment is the one in which independent evidence converges on a coherent evolutionary lineage.

Explore Further

How Tree TSAR Delimits Species - The detailed four-axis methodology, scoring heuristic, and difficult cases involving gene flow, polyploidy, and apomixis.
What Is a Genus? - How species are organized into broader evolutionary groups.
Taxonomy and Nomenclature - Why deciding that a species exists is different from determining which scientific name applies to it.
Hybrids, Chimeras, and Synthetic Plants - How Tree TSAR represents reticulate and artificial origins.

References and Further Reading

de Queiroz K (2007) Species concepts and species delimitation. Systematic Biology 56(6): 879-886. https://doi.org/10.1080/10635150701701083

Oberprieler C (2023) The Wettstein tesseract: A tool for conceptualising species-rank decisions and illustrating speciation trajectories. Taxon 72(1): 1-7. https://doi.org/10.1002/tax.12825

Rieseberg LH, Willis JH (2007) Plant speciation. Science 317(5840): 910-914. https://doi.org/10.1126/science.1137729

Soltis PS, Soltis DE (2009) The role of hybridization in plant speciation. Annual Review of Plant Biology 60: 561-588. https://doi.org/10.1146/annurev.arplant.043008.092039

Widmer A, Lexer C, Cozzolino S (2009) Evolution of reproductive isolation in plants. Heredity 102: 31-38. https://doi.org/10.1038/hdy.2008.69