Phylogenetic Systematics: Evolutionary Patterns


The diversity of living things presents us with a seemingly infinite variety. The science of systematics is dedicted to identifying and ordering the diversity of living things.

Systematics: The ordering of this diversity. Since prehistory, systematists have employed taxonomic systems in which organisms are classified into groups or taxa (singular: taxon). Many different taxonomic systems are conceivable, but all have the following features:

For example, in our lives, we have all employed the taxonomic system in which animals are classified according to the organizational principle of their utility to humans. Generally, there is little ambiguity.

Livestock
 Pet
 Vermin


Green tree python, Morelia viridis
Problem: The criteria that we use to classify animals according to this system are arbitrary and subjective. For example, the green tree python Morelia viridis: A reptile enthusiast might classify this as a pet, where a person who was terrified of snakes would call it vermin, and an entrepeneur who raises reptiles for the pet trade would view it as livestock.


Carl Linnaeus on 100 Kronor note from World Banknotes and Coins.
The Linnaean System: The first attempt to organize the diversity of life in an explicit and non-arbitrary manner was made by Carl Linnaeus (a.k.a. Carolus Linnaeus and, after his ennoblement, Carl von Linné.) Linnaeus was a Swedish botanist who may be the first and last person in history to publish on every known organism. His Systema Naturae is the basis for the Linnaean system of taxonomy. Its first edition was in 1735, but the tenth is regarded as the most authoritative, published in 1758. Its major features:

A fantastic achievement, but late in his life, Linnaeus lamented that for all his work, he felt that he had never succeeded in identifying the true organising principle of the diversity he had described. In retrospect, that principle was the branching pattern of phylogeny produced by evolution, first proposed publicly a century after the publication of the 10th edition.


The Linnaean System in the age of Darwin: The recognition that Linnaean taxa were products of shared common ancestry provoked refinements of the system such that only groups that shared common ancestry were acknowledged. Thus, "Pachydermata" containing rhinos, elephants, and hippos, but not their common ancestor, was out. Beyond that, before the age of digital computers and electronic media, not much else was really practical. The result was an awkward century in which the Linnaean system was used and interpreted in an evolutionary sense, but where certain infelicities were tolerated.

The History of Phylogenetic Systematics (Cladistics)

Ideally, we would like to have some non-arbitrary, natural organizing principle for a taxonomic system that natural scientists can use. Indeed, Darwin, in the Origin noted that the Linnean system of taxonomy, based on general similarity, ought to be superceded by one based on closeness of common ancestry. Alas, on a practical level, such an undertaking was impossible until the invention of digital computers.

  • During the mid 20th century, two separate approaches developed seeking to use numerical algorithms to establish a rational basis for a system of taxonomy:

    By the mid 1970s, cladistics had eclipsed phenetics. By the 90s it was the dominant school of taxonomic thought. In North America, the 1980s were the heady era of taxonomic revolution in which cladistic revolutionaries in institutions such as the University of California at Berkeley and the American Museum of Natural History shaped the future of systematics. A revealing document from this era is Kevin DeQueiroz', 1988. Systematics and the Darwinian Revolution.

    DeQueiroz 1988's key concepts:

    Cladobabble: Terminology and Conventions of Phylogenetic Systematics

    The triumph of phylogenetic systematics sets the tone for contemporary academic discussions of phylogeny and systematics. This requires the well-informed reader to master graphic conventions and some rather clunky technical terms. You can do it.

    Cladograms: Throughout evolutionary history, lineages of interbreeding organisms have evolved through time and occasionally split into separate, reproductively isolated lineages. The result is an evolutionary "tree" with many branches. We represent this tree, or portions of it that we want to talk about, using stick-figure trees called cladograms.

    In this cladogram, the organisms A, B, and C at the ends of the branches are known as terminal taxa. The lines themselves represent evolving lineages. Branch points represent lineage splitting events. The point at the fork of each split is called a node, and represents the latest common ancestor of the descendants depicted above it. Time runs from oldest events at the bottom to youngest ones at the top. Thus, in this example, the last common ancestor of A, B, and C occurred earlier in time than the last common ancestor of B and C.

    Note that in a cladogram, it does not matter whether things appear on the left or right. What counts is the sequence of branching events (i.e. which ones appear on top or on the bottom). In the figure above, cladograms 1 and 2 depict exactly the same relationships, whereas cladogram 3 is different.

    The phylogenetic taxonomic system: Taxonomic groups can be named and defined based on their descent from a common ancestor. The cladogram below shows the real relationships between several major vertebrate groups.


    Working from this cladogram, systematists have named the following taxonomic groups:


    In this drawing, we have drawn circles around the groups that could be defined by the relationships shown on this cladogram, and indicated their names. Ordinarily, one would simply write the group names next to the node of the last common ancestor:

    Thus, the pattern of evolution provides:

    Presto! It's a proper taxonomic system.

    Definitions:

  • Phylogeny: The branching evolutionary pattern of ancestry and descent.
  • Phylogenetic systematics: The science of reconstructing phylogeny and developing a taxonomic system based upon it.

    Monophyletic groups: In phylogenetic systematics, taxonomic groups are defined strictly in terms of the non-arbitrary criterion of descent from a common ancestor. Such taxa are called monophyletic groups.

    Note carefully: Only monophyletic groups are based exclusively on natural, non-arbitrary criteria. When we define a paraphyletic group, we must arbitrarily decide which descendants to exclude. In the case of polyphyletic groups, we must decide which ancestors to leave out.

    Phylogeny reconstruction using parsimony

    • If God were to hand us the true phylogeny, and our only task were to read it and construct taxonomic system accordingly, our lives would be easy. Instead, we must somehow reconstruct phylogeny by making observations and testing hypotheses. This is where the "modification" side of "descent with modification" comes in. As lineages evolve, the characters of their members change. I.e. they go from ancestral to derived states.

    Synapomorphies allow us to identify monophyletic groups, because if a character is shared by two lineages, we assume that it was inherited from their most recent common ancestor


    Let's see how this works in a simple cladistic analysis of some imaginary beetles. We assume that they are related somehow, but we don't know if B shares a more recent common ancestor with C or A, or if C and D are more closely related to one another than to B.

  • What does this method yield:

    Of course, an alert observer would raise a serious question: Phylogenetic Systematics absolutely requires us to identify variation in homologous structures, genes, or behaviors. How do we know when character states are homologous? Cladists rely on three tests to falsify hypotheses of homology:

    BTW, I have listed these tests in order of increasing strength. We would, for instance, reject the homology of the angel arm and wing, no matter how similar they looked.

    Feeling vulnerable? For more review see:

    Additional reading:

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