How Tree TSAR Delimits Plant Families

An Integrative Framework for Deep Evolutionary Lineages

Plant families are among the most durable landmarks in botanical classification. They organize genera into broader evolutionary lineages and provide a common scale for floras, herbaria, conservation, ecology, horticulture, education, and comparative biology.

The phylogenetic tree does not automatically tell us which branches deserve family rank. Tree TSAR therefore makes two judgments: whether a proposed group represents a defensible evolutionary lineage, and whether recognizing that lineage as a family produces a useful and stable classification.

Monophyly provides the foundation. Phylogenetic support, morphology, ecology, biogeography, fossils, divergence history, nomenclature, and taxonomic utility determine where the boundary is most informative.

Monophyly Is the Foundation

For ordinary branching lineages, a Tree TSAR family should be monophyletic. A demonstrably polyphyletic family cannot remain unchanged merely because its name is familiar.

This expectation aligns with major modern classification systems. PPG I identifies monophyly as the primary criterion for its classification of extant lycophytes and ferns while also attempting to preserve widely accepted taxa where compatible with phylogeny (Pteridophyte Phylogeny Group I 2016). APG IV similarly revises angiosperm family limits in response to phylogenetic evidence (Angiosperm Phylogeny Group 2016).

The evidence still has to be strong. Weak support, missing critical genera, limited genomic sampling, or major conflict among datasets can make immediate reclassification less durable than a cautious treatment that acknowledges uncertainty.

A Clade Does Not Automatically Become a Family

Every phylogeny contains nested monophyletic clades. If every strongly supported branch became a family, classifications could become progressively fragmented without conveying much more information.

Christenhusz et al. (2015) documented this problem directly in a survey of angiosperm and fern family delimitation. Many taxonomists favored broader family circumscriptions when major internal branches could be represented effectively through subfamilies or tribes.

Tree TSAR therefore asks two questions in sequence: Is the group a clade? and Does family rank improve the classification?

Phylogenomic Support

Family-level changes require broad and appropriate sampling. Tree TSAR gives greatest confidence to relationships strongly recovered across substantial taxon sampling and, where possible, independent genomic evidence.

Nuclear and plastid topologies do not need to agree perfectly, but major conflicts should be investigated rather than hidden. Rapid radiations, introgression, incomplete lineage sorting, extinction, or methodological assumptions can all complicate deep relationships.

A family revision should ideally survive improved sampling rather than require reversal with each new dataset.

Morphology Gives a Family Explanatory Value

Phylogeny identifies shared evolutionary history. Morphology helps explain the lineage to readers and other botanists.

Reproductive structures, floral organization, fruits, seeds, pollen, anatomy, development, chemistry, and vegetative architecture can all contribute to family diagnosis. Tree TSAR strongly favors family boundaries that correspond to meaningful biological characteristics, especially when they reinforce independently supported phylogenetic relationships.

Morphological coherence does not mean every member looks alike. Large families can contain dramatic adaptive radiations while preserving deeper structural features.

Fern classification again illustrates why morphology needs phylogenetic testing: apparently informative characters have sometimes proved convergent or nonhomologous (Christenhusz & Chase 2014).

Ecology, Biogeography, and Fossils

Family-level lineages can correspond with major geographic histories, ecological transitions, reproductive innovations, or distinctive fossil records. These sources add evolutionary context and can strengthen a treatment already supported by phylogeny and morphology.

Fossils are particularly useful when living diversity is sparse. They can reveal that a family now represented by very few species once belonged to a much broader radiation.

Ecology and biogeography remain supporting evidence. Similar habitats can produce convergence, and one coherent family can diversify across many ecological zones.

Divergence Time Is Comparative Evidence

Tree TSAR may use divergence-time estimates to compare the scale of alternative classifications, but it does not apply a fixed temporal threshold for family rank.

A deep split can strengthen the argument for separate family recognition when it aligns with diagnosability and independent history. It cannot substitute for those qualities. Different plant lineages evolve at different rates, and formal ranks have never been temporally standardized across all vascular plants.

Stem and crown ages are distinguished explicitly because they measure different events. Apparent temporal consistency is meaningful only when like values are compared with like values.

Family Size Is Not a Criterion

Family rank is not determined by how many living genera or species survive. Extinction can leave a deep lineage represented by one genus, while a much younger radiation can become species-rich.

Ginkgoaceae is a clear example of a monogeneric family in current treatments. Its small living membership reflects survival history, not an absence of deep evolutionary distinctiveness.

Tree TSAR therefore imposes no minimum diversity requirement for family recognition.

Broad Families Can Preserve Internal Structure

Subfamilies, tribes, and other inframilial groups allow major internal branches to remain visible without automatically becoming separate families.

This is often the best solution when a broad family remains monophyletic and diagnosable while containing several important radiations. Christenhusz et al. (2015) and Christenhusz & Chase (2014) both document the role that broader circumscriptions and lower ranks can play in maintaining stable, informative classifications.

The relevant question is whether a new family communicates something that cannot be represented adequately inside the broader lineage.

Stability Carries Greater Weight at Family Rank

Family names organize enormous amounts of biological information. Changes propagate through floras, herbaria, ecological databases, conservation lists, regulatory systems, horticulture, and education.

Stability never overrides demonstrated polyphyly. It becomes important when several monophyletic alternatives exist. In those cases, a familiar and biologically coherent broader family may be preferable unless splitting produces a clear gain in diagnosability or evolutionary meaning.

Nomenclature Follows the Taxonomic Decision

The Madrid Code governs family names, including typification, priority, conservation, and rejection. It does not determine the biological circumscription that Tree TSAR must adopt.

When families are merged or divided, historical names may become applicable to the resulting clades, and nomenclatural conservation can affect which names are used. Tree TSAR audits these consequences after identifying the defensible biological alternatives.

A convenient name does not determine the phylogeny, but the correct application of names matters once a classification is chosen.

When a Family Is Polyphyletic

Strong evidence of polyphyly requires a solution. Possible responses include expanding a family, transferring genera among existing families, recognizing historical families, or establishing a new family when a well-supported and diagnosable lineage warrants that rank and no appropriate name is available.

APG IV contains numerous examples of transfers, mergers, and changes in family limits made to align angiosperm classification with improved phylogenetic evidence (Angiosperm Phylogeny Group 2016).

Tree TSAR selects among such options by asking which treatment restores monophyly while best preserving diagnosability, biological coherence, nomenclatural feasibility, and stability.

Evidence Used in Family Delimitation

Factor	Role in Family Delimitation
Monophyly	Essential for ordinary branching families; demonstrated polyphyly requires resolution
Phylogenetic Support	Broad sampling and agreement among independent datasets increase confidence
Morphological Diagnosability	Strongly preferred because families should carry explanatory and predictive value
Ecology and Biogeography	Supporting evidence for independent evolutionary history
Fossil Evidence	Important for extinct diversity, character evolution, and lineage history
Divergence Time	Comparative context, never an automatic threshold
Family Size	Not independently determinative
Inframilial Alternatives	Subfamilies and tribes can represent major clades without unnecessary splitting
Nomenclatural Feasibility	Determines which names correctly apply after the taxonomic decision
Stability and Utility	High importance when several monophyletic circumscriptions are possible

The Tree TSAR Decision Process

Establish the phylogeny. Determine whether current and proposed circumscriptions are monophyletic using broad, appropriate sampling.
Investigate conflicting evidence. Compare genomic compartments, morphology, fossils, ecology, and biogeography rather than concealing unresolved relationships.
Identify viable circumscriptions. Consider broad families, narrower families, transfers among existing families, and inframilial alternatives.
Evaluate diagnosability and evolutionary meaning. Determine whether each proposed family corresponds to recognizable biological characteristics and a useful narrative.
Use divergence history comparatively. Examine stem and crown ages where informative without imposing a universal temporal threshold.
Audit nomenclature and stability. Determine the names available and compare the disruption and informational value of each defensible solution.
Adopt the strongest durable treatment. Prefer the classification that restores evolutionary integrity while remaining meaningful, traceable, and stable enough for broad use.

Why Tree TSAR Is Conservative About Families

Tree TSAR is conservative about unnecessary family-level disruption, not about preserving familiar names regardless of evidence.

A distinct lineage can clearly deserve family status, and a polyphyletic family requires correction. Conservatism matters after phylogenetic integrity has been satisfied, when several monophyletic arrangements remain possible.

The preferred treatment is usually the one that retains useful, diagnosable families and represents additional structure at lower ranks unless splitting adds substantial biological information.

When Tree TSAR Disagrees

Family-level disagreement should identify the actual source of conflict. Are two studies recovering different trees? Are they accepting the same tree but assigning family rank to different nodes? Is one treatment more morphologically diagnosable? Does nomenclature make one otherwise defensible solution unusually disruptive?

A reader should be able to tell whether Tree TSAR is rejecting a phylogenetic relationship or choosing a different rank treatment of the same relationship.

Families Should Be Durable Evolutionary Landmarks

The strongest family treatment combines several forms of evidence without allowing any single one to become a universal rule. Monophyly establishes the foundation, phylogenomics establishes confidence, morphology and other biological evidence make the lineage intelligible, divergence history supplies context, nomenclature supplies the name, and stability helps choose among equally defensible alternatives.

That combination makes family rank useful across both scientific research and public understanding.

References and Further Reading

Angiosperm Phylogeny Group (2016) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181(1): 1-20. https://doi.org/10.1111/boj.12385

Christenhusz MJM, Chase MW (2014) Trends and concepts in fern classification. Annals of Botany 113(4): 571-594. https://doi.org/10.1093/aob/mct299

Christenhusz MJM, Vorontsova MS, Fay MF, Chase MW (2015) Results from an online survey of family delimitation in angiosperms and ferns: Recommendations to the Angiosperm Phylogeny Group for thorny problems in plant classification. Botanical Journal of the Linnean Society 178(4): 501-528. https://doi.org/10.1111/boj.12285

Pteridophyte Phylogeny Group I (2016) A community-derived classification for extant lycophytes and ferns. Journal of Systematics and Evolution 54(6): 563-603. https://doi.org/10.1111/jse.12229

Turland NJ, Wiersema JH, Barrie FR, Gandhi KG, Gravendyck J, Greuter W, Hawksworth DL, Herendeen PS, Klopper RR, Knapp S, et al. (eds.) (2025) International Code of Nomenclature for algae, fungi, and plants (Madrid Code). Regnum Vegetabile 162. University of Chicago Press. https://www.iaptglobal.org/madrid-code-online