Integrative Biology 335
Taxonomic Hierarchy and Classification

Announcements:

If you haven't done so already, please hand in the Student Information sheet distributed during lecture 1.

Your first lecture quiz has been graded and will be returned to you in lab. Remember, there will be at least 13 surprize quizzes this term, with only the ten highest scores used to calculate a grade. So, if you miss a quiz or two or more, don't worry.

To get the most out of lectures, print off the appropriate class notes and bring them to lecture with you. Again, be aware that these web pages will be updated just prior to their use in class, so don't print them out too far ahead of time!

General Objectives:

After studying this material you should be able to:

  1. Know the approximate numbers of extant species and families of angiosperms

  2. Explain what is meant by classification, and especially a phylogenetic classification

  3. Explain what is meant by saying that a classification should be stable and predictive

  4. Define basic terms, such as phylogeny, grouping, and ranking, and know the correct usages of the terms species, genus (genera), and taxon (taxa)

  5. Know the ranked hierarchy used in flowering plant classification and its associated word endings

  6. Become familiar with some important non-Linnean categories, based on recent research of the Angiosperm Phylogeny Group

Text and Other Resources:

Judd et al., Plant Systematics, A Phylogenetic Approach, Third Edition, read relevant information in Appendix One, Botanical Nomenclature (pp. 543-551) and in Chapter 2, Methods and Principles of Biological Systematics, especially Constructing a Classification, pp. 32-35.

[Note: Again, be aware that you are responsible for material contained in these required readings. In order to address some of the objectives above, you will need to read the text.]

See pages 239-242 in your Class Notes for a classification and phylogenies of the plant families we will cover this semester. This information was summarized from Judd et al. (2008).

There is more information on the Taxonomic Hierarchy at Texas A&M University.

The Angiosperm Phylogeny Website, by Peter F. Stevens, for an excellent discussion on classifications.

The need for a taxonomic hierarchy

There is an incredible diversity of life on planet Earth. There are over 280,000 extant species of angiosperms, with the total number of angiosperm species reaching 400,000 by some counts. The number of angiosperm families is well over 400. No one person could possibly remember all of these. It is necessary to organize this diversity into a classification scheme to communicate with others. The classification system that has been traditionally most useful to biologists is one that groups related plants together into a series of hierarchical categories, so that very closely allied plants are placed together, plants somewhat related are grouped near each other, and plants that have very little in common with each other are placed far apart.

Classification:

The urge to classify is a fundamental human instinct; like the predisposition to sin, it accompanies us into the world at birth and stays with us to the end. (A.T. Hopwood 1959)

Great scientists are Peter Pans!

A classification of animals?

Linnaeus' sexual system of classification (rated R)

The characters which naturalists consider as showing true affinity between any two or more species are those which have been inherited from a common parent, all true classifications being genealogical. Charles Darwin 1859: 391.

CLASSIFICATION: the theory and practice of grouping and ranking organisms (or the system of internested groups resulting from this process).

Remember (Lecture 1), systematics (or taxonomy) is the theory and practice of grouping individuals into species, arranging those species into larger groups, and giving those groups names, thus producing a classification.

There are many ways to do this (and you will learn about some of these ways in our upcoming lectures), but the approach we and your textbook take is phylogenetic. There are two major steps in producing a phylogenetic classification:

A phylogeny-based classification attempts to arrange organisms into groups on the basis of their evolutionary relationships.

Because a classification is designed to aid communication, it should be STABLE and PREDICTIVE.

Some basic terms

PHYLOGENY: The evolutionary history of a group of organisms. A phylogenetic tree is a branching diagram representing phylogenetic relationships (the evolutionary history) of a group of organisms (taxa).

GROUPING: The delimination and naming of groups. Basically, the sorting of objects into groups that have some features in common.
[In a phylogenetic classification, only monophyletic groups are recognized and named, and this is the subject of a later lecture.]

RANKING: The placement of these groups into an appropriate category in a hierarchy of categories.

Botanical classification uses a system developed in the 18th century in which taxa are assigned to particular ranks, such as species, genus, family, order, class, phylum (or division), kingdom. These are called Linnaean ranks.

[Categories are ordered by inclusiveness. More inclusive categories have higher ranks (e.g., order and class) and less inclusive categories have lower ranks (e.g., genus and species). As we proceed up the hierarchy, the categories become more inclusive and of higher rank.]

[Here is an example of a hierarchical system - the way files are stored on a computer.]

Below is an example of a hierarchial system for the group of animals that includes humans.

[Now that you know what a “Great Ape” is, click here.]

Here is the same example, but with the names of the categories.

Homo sapiens subspecies sapiens (human being or human; Latin for "wise human" or "knowing human"; NOT Homo sapien [without the "s"]!!!)

[Note: As of 2008, humans are listed as a species of least concern for extinction by the International Union for Conservation of Nature]

Application of the classification hierarchy in plants:

Magnolia virginiana (bay magnolia)

[Note that the endings will always tell you what rank you are dealing with, even if you don't recognize the word. Also, note that in this example the higher names are based upon the name of an included genus. Generic names do not have standardized endings.]

Glycine max (soybean)

    Phylum or Division – Magnoliophyta – flowering plants
      Class – Magnoliopsida – ["dicotyledons" or "dicots"]
        Order – Fabales
          Family – Fabaceae
            Genus – Glycine
              Species – Glycine max

Zea mays (corn or maize)

    Phylum or Division – Magnoliophyta – flowering plants
      Class–Liliopsida – [monocotyledons or monocots]
        Order – Cyperales
          Family – Poaceae
            Genus – Zea
              Species – Zea mays

A sample quiz:

Which of the following groups of taxa are written in the correct DESCENDING order of the taxonomic hierarchy?

    1. Magnoliopsida, Magnoliophyta, Magnoliales, Magnoliaceae, Magnolia virginiana
    2. Magnoliophyta, Liliopsida, Poaceae, Cyperaceae, Zea mays
    3. Magnoliophyta, Magnoliopsida, Asteraceae, Asterales, Aster tenuifolius
    4. Magnoliopsida, Malvales, Sterculiaceae, Theobroma, Theobroma cacao

Some important miscellaneous things you should know:

  • The seven major ranks are Kingdom, Phylum (or Division), Class, Order, Family, Genus, and Species. Note the standard suffices for all ranks, except genus and species.

  • The plural of “genus” is “genera.”

  • “Species” is both singular and plural.

  • “Specie” means gold or silver rather than paper money. Do NOT use the word “specie” when referring to a single species of plant!

  • In a HIERARCHICAL SYSTEM of classification, names above the rank of GENUS are single words that are based upon the name of an included genus. Each rank has a distinctive ending (suffix) that is attached to the stem of the name of the genus.

  • The word TAXON is used to refer to a taxonomic group of any rank. This group is considered sufficiently distinct by systematists to be formally recognized and assigned to a definite category in a hierarchic classification. The plural of TAXON is TAXA.

    [Note: These are groups of real organisms, such as mammals, carnivores, vertebrates, flowering plants, bigleaf magnolia, etc.]

Classification of Angiosperms:

In this class, we will follow the system of classification outlined in your text (Judd et al., 2008, Table 9.1, pp. 230-231). This system of angiosperm classification is based on the Angiosperm Phylogeny Group (APG) system of 1998 and the APG II system of 2003.

The more inclusive categories of the taxonomic hierarchy that we will be studying this semester include:

  • Angiosperms
    • Magnoliid Complex (or magnoliids)
    • Monocots
    • Eudicots (or Tricolpates)
      • Basal Tricolpates
      • Core Eudicots
      • Rosid clade
      • Asterid clade

These names occur above the rank of order and are not based on the Linnaean hierarchical system (in other words, they are not linked to any particular Linnaean rank, nor do they have standardized and distinctive endings). Each of these groups, however, is monophyletic.

In addition, there are less inclusive groups, such as Eurosids I (or Fabids), Eurosids II (or Malvids), Euasterids I (or Lamiids), and Euasterids II (or Campanulids).

Note that the "dicots", as traditionally circumscribed (class Magnoliopsida), are not a monophyletic group.

Magnolia virginiana (bay magnolia)

    Magnoliophyta or Angiosperms – flowering plants
      Magnoliids (or Magnoliid Complex)
        Order – Magnoliales
          Family – Magnoliaceae
            Genus – Magnolia
              Species – Magnolia virginiana

[Lineage Eukaryota; Viridiplantae; Streptophyta; Embryophyta; Tracheophyta; Spermatophyta; Magnoliophyta; magnoliids; Magnoliales; Magnoliaceae; Magnolia; Magnolia virginiana]

Glycine max (soybean)

    Magnoliophyta or Angiosperms – flowering plants
      Eudicots (or Tricolpates)
        Rosid Clade
          Fabids (or Eurosids I) Clade
            Order – Fabales
              Family – Fabaceae
                Genus – Glycine
                  Species – Glycine max

[Lineage Eukaryota; Viridiplantae; Streptophyta; Embryophyta; Tracheophyta; Spermatophyta; Magnoliophyta; Eudicotyledons; Rosids; Fabids (or Eurosids I); Fabales; Fabaceae; Papiliionoideae (or Faboideae); Phaseoleae; Glycine; Glycine max]

The diversity of flowering plants is not evenly distributed:
  • Eudicots clade (75% of angiosperms)
  • Monocots clade (23% of angiosperms)
  • Magnoliid Complex (2% of angiosperms)
  • The remaining major clades contain a little over 250 species in total (less than 0.1% of flowering plant diversity)

Lastly, know that the categories (levels, ranks or classes) of the taxonomic hierarchy to which groups (i.e., taxa) are assigned are only human constructs, having relative but not absolute meaning. The group known as angiosperms, after all, has been treated as a Division, Subdivision, Class, etc. by various taxonomists over the years. Arguments against the use of ranks in classification have been proposed -- see you text for more information on an alternate system of nomenclature, known as the PhyloCode.

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