Unit 4: Classification

History and Types of Classification

Plant classification systems have evolved from being based on simple, arbitrary traits to complex systems based on evolutionary relationships.

1. Artificial Systems:
   • Based on one or a few, easily observable (but not necessarily related) characters.
   • Example: Linnaeus's "sexual system" (1735), which classified plants into 24 classes based only on the number and arrangement of stamens.
   • Advantage: Easy to use for identification (like a dictionary).
   • Disadvantage: Does not reflect natural relationships (e.g., plants that are unrelated were grouped together just because they had 5 stamens).
2. Natural Systems:
   • Based on overall similarity, using as many morphological characters as possible (e.g., flower, fruit, leaf, stem).
   • Example: Bentham and Hooker's system (1862-1883).
   • Advantage: Groups related plants together, very practical for identification and herbarium organization.
   • Disadvantage: Based on similarity, not necessarily on evolutionary history (pre-Darwinian).
3. Phylogenetic Systems:
   • Based on evolutionary relationships (phylogeny). They try to reconstruct the "family tree" of plants.
   • Example: Engler & Prantl (1887-1915), Takhtajan (1966), and the modern APG (Angiosperm Phylogeny Group) system.
   • Advantage: Scientifically most accurate, reflects evolutionary history.
   • Disadvantage: Can be complex; relationships are often based on non-visible (e.g., molecular) data.

Bentham and Hooker System

Published in Genera Plantarum (1862-1883), this is the most famous natural system of classification.

Outline:

• Divided seed plants (Spermatophyta) into 3 classes:
  1. Dicotyledonae: (Two cotyledons, reticulate venation, taproot).
     • Sub-class 1. Polypetalae (Petals free) - e.g., Magnoliaceae
     • Sub-class 2. Gamopetalae (Petals fused) - e.g., Solanaceae
     • Sub-class 3. Monochlamydeae (Perianth in one whorl or absent) - e.g., Euphorbiaceae
  2. Gymnospermae: (Ovules naked) - e.g., Cycas, Pinus
  3. Monocotyledonae: (One cotyledon, parallel venation, fibrous roots) - e.g., Poaceae, Liliaceae

Merits and Demerits

Merits:

• Very practical and easy to use for identification.
• Based on detailed, direct observations of specimens.
• Still used to organize herbaria in many parts of the world, including the Central National Herbarium (CAL) in India.

Demerits:

• It is not phylogenetic. It does not incorporate evolution.
• The placement of Gymnospermae between Dicots and Monocots is a major error.
• The group "Monochlamydeae" is highly artificial; it groups unrelated plants based on a *loss* of petals.

Engler and Prantl System

Published in Die Natürlichen Pflanzenfamilien (1887-1915), this was the first major phylogenetic system.

Outline:

• Based on the idea that the "simplest" flowers (no petals, wind-pollinated, e.g., grasses) are the most primitive, and complex flowers (with petals, insect-pollinated) are advanced.
• They placed Monocots *before* Dicots.

Merits and Demerits

Merits:

• It was the first system to be intentionally based on evolutionary principles.
• It treated families as the basic unit and provided keys and descriptions for all known plant genera.

Demerits:

• Its central evolutionary hypothesis is now considered incorrect. We now know that "simple" flowers (like in grasses) are often derived from more complex ancestors (a simplification), and that large, many-parted flowers (like Magnolia) are more primitive.

Takhtajan System

A modern phylogenetic system (1966, with later revisions) that incorporates a wide range of evidence, including morphology, anatomy, embryology, and phytochemistry.

Outline:

• Considers angiosperms to be monophyletic (having a single common ancestor).
• Considers plants with large, many-parted, spiral flowers (like Magnolia) to be the most primitive.
• Derives all other angiosperms from these ancestors through various evolutionary trends (e.g., fusion of parts, reduction in number of parts, zygomorphy).
• This system (and the similar Cronquist system) is the basis for most modern, pre-molecular classifications.

Numerical Taxonomy (Phenetics)

A system of classification that aims to be objective by using mathematical methods and computer algorithms. It is not based on evolution, but on overall similarity.

Process:

1. Select OTUs: Choose the "Operational Taxonomic Units" (e.g., the species you want to classify).
2. Select Characters: Choose as many characters as possible (e.g., 100+).
   • Characters: e.g., "Leaf shape", "Petal color", "Stamen number".
   • Character Weighting: In pure phenetics, all characters are given equal weight to be objective.
   • Coding: Convert the characters into a numerical form (e.g., 0 = absent, 1 = present; or 1=red, 2=blue, 3=yellow).
3. Cluster Analysis: A computer algorithm compares the coded data for all OTUs and calculates a "similarity coefficient" (a score of how similar each pair is).
4. Generate Phenogram: The computer produces a tree-like diagram called a phenogram, which clusters the OTUs based on their similarity score.

Cladistics (Phylogenetic Systematics)

This is the dominant method used today to reconstruct evolutionary relationships. It was developed by Willi Hennig.

Principle: Cladistics classifies organisms based on phylogeny (evolutionary descent), not just similarity. It groups organisms that share a recent common ancestor.

Key Concepts:

• Synapomorphy: A shared, derived character. This is the only type of character used to build a cladogram. It is an "evolutionary novelty" that a group inherited from their common ancestor (e.g., feathers in birds).
• Clade: A group that includes a common ancestor and all of its descendants (a monophyletic group).
• Paraphyletic Group: A group that includes an ancestor but not all of its descendants (e.g., "Reptiles" is paraphyletic because it doesn't include birds). Cladistics rejects these groups.
• Polyphyletic Group: A group derived from more than one common ancestor (e.g., "Algae"). Cladistics rejects these groups.

Cladograms vs. Phenograms