Cladogram Chloroplast
8.1 What a classification is for #
A classification aims at a stable name, a way to retrieve characters, and a hypothesis of relationship. The third is now primary. A group that contradicts the phylogeny may survive as a convenience (“dicot”, “algae”, “bryophyte” in older books) only if it is labelled as one. Identification matches a specimen to a name with a key. Classification arranges names by relationship. A flora does both for a region. The order of families in an old flora is not, by itself, a tree.
8.2 Reading a tree #
A monophyletic group, a clade, is an ancestor and all its descendants. That is what formal taxa are now meant to be. A paraphyletic group excludes some descendants: “dicots” exclude monocots and are defined by an ancestral pair of cotyledons. Living gymnosperms were suspected of being paraphyletic under the anthophyte hypothesis; current evidence makes them monophyletic (Chapter 6). A polyphyletic group is assembled by convergence and excludes the ancestor that would unite the members. Winged seeds of pine and samaras of maple, grouped as a “flying-seed taxon”, would be polyphyletic.
A character is not a taxon. Vessels evolved more than once. Wood evolved more than once. Possessing either does not define a clade. A character is evidence, weighed with other evidence. Homology is similarity inherited from a common ancestor. Homoplasy is similarity from convergence or reversal. Outgroups distinguish shared derived characters, synapomorphies, from shared ancestral ones. Chlorophyll a does not set land plants off from green algae. An embryo does.
Parsimony, likelihood, and Bayesian algorithms are Bioinformatics. Reading a tree is this course: a node is an ancestor, a branch is a lineage, a polytomy is unresolved, sister group means the immediate relative on that diagram, and rotating a node does not change relationship.
8.3 An outline, without false precision #
Land plants: bryophytes, treated as the likely sister clade of vascular plants, the internal bryophyte order still discussed (Chapter 4); vascular plants split into lycophytes and euphyllophytes; euphyllophytes split into monilophytes and seed plants; seed plants split into living gymnosperms and angiosperms. Inside the gymnosperms, cycads and Ginkgo are early branches, conifers are the bulk, and gnetophytes lie with them rather than with the flowering plants. That is a teaching skeleton. It is not a dated list of divergence years.
Angiosperms are arranged here by major clades only (APG IV, 2016). An early grade, not a clade of “basal dicots”, is Amborella (no vessels; sister to the rest in most analyses), then water lilies (Nymphaeales), then Austrobaileyales. A minority of analyses join Amborella with the water lilies. “Basal” means branching early, not primitive in every trait. The rest are magnoliids, a small Chloranthales line, the monocots, and the eudicots. Do not memorise which two of those three are sisters.
Ranunculales branch early in the eudicots. The two large core groups are rosids (legumes, roses, oaks, mustards) and asterids (asters, mints, nightshades, umbels). Grasses, palms, and orchids are monocots in different orders. The old Liliaceae has been split. An older key can still name a plant. Its sequence is not the tree.
8.4 Names, types, and common names #
A herbarium is a collection of pressed, dried, labelled specimens: locality, date, habitat, collector, and a determination, re-examinable when names change. A DNA sequence without a voucher cannot be re-identified if the name was wrong. Major collections hold the types. An image does not replace the sheet when a fine character must be checked.
Binomials start, for most plants, with Linnaeus’s Species Plantarum (1753). Moss names, except Sphagnum, start with Hedwig’s Species Muscorum (1801). A name is genus plus specific epithet, with the author in formal citation (Quercus robur L.). The author is not part of the name in ordinary use.
The type is the specimen to which a name is tied: a holotype, a later-chosen lectotype, or a neotype if the original is lost. It is an anchor, not “the most typical” plant. If the species is split, the part with the type keeps the name. The Code governs priority and form. Families end in -aceae and orders in -ales. Eight families also keep older names (Compositae, Cruciferae, Gramineae, Labiatae, Leguminosae, Umbelliferae, Palmae, Guttiferae). Both forms are allowed.
Common names fail in two directions. One species has many names, and one name covers many species: “bluebell” is Hyacinthoides in much of Britain, Campanula for the harebell in Scottish usage, and Mertensia in eastern North America. “Cedar” covers Cedrus, Thuja, Juniperus, and Cedrela. Common names show neither relationship nor a type, and no code governs them. They are a field handle, not a scientific claim. When a binomial changes, the epithet follows the type and the genus is the part that moves, because the hypothesis changed.
8.5 Key terms, summary, and self-check #
Key terms. Monophyly; paraphyly; polyphyly; homology; homoplasy; synapomorphy; APG; herbarium; binomial; holotype; type specimen.
Summary. Classification is a hypothesis of relationship, not a key, and only clades are the groups modern taxonomy aims to name. Vessels, wood, and two cotyledons are characters, some of them convergent, and none of them is a taxon by itself. The teaching tree runs bryophytes, lycophytes, monilophytes, gymnosperms, and angiosperms, the last with an early grade (Amborella, water lilies, Austrobaileyales), magnoliids, monocots, and eudicots of rosids and asterids. A binomial is anchored to a type specimen in a herbarium; a common name is not, and it collides both across languages and across unrelated plants.
Self-check.
1. Why is “dicot” paraphyletic, and what kind of group is rejected as a formal taxon?
2. Why does the possession of vessels not define a clade?
3. Where do monocots sit relative to Amborella and to the eudicots?
4. What is a type specimen for, and why is it not “the most typical plant”?
5. Give two different reasons a common name fails, each with an example.
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