Evolution as Pattern II

The Phylogenetic System of Taxonomy: The organizing principle of modern biology is evolution (descent with modification). Ultimately, evolution implies that all living things descend from single common ancestor. The history of these lineages is their phylogeny. (We already know how to draw it). This supplies the organizational principle used by modern systematists. It is hierarchical because groups that are descended from very recent common ancestors may be nested within groups descended from distant common ancestors.

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

Note: Just as we have disphonious cladobabble describing different types of taxonomic groups, we have it for characters, too:

Note that when we discuss types of characters, it is essential that we agree on the frame of reference. The opposable thumb is a synapomorphy of primates, but a plesiomorphy of hominids.

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:

Potatohead Exercise: The take home concept is that the hypothesis of phylogeny that our technique generates is falsifiable. We can falsify it by adding new information or changing basic assumptions like outgroup choice.

Feeling vulnerable? For more review see:

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 Indeed, Darwin, in the Origin noted that the Linnean system of taxonomy, based on general similarity, ought to be superceded by one based on closeness of common ancestry.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. Philosophy of Science, 55: 238-259.

    DeQueiroz 1988 key concepts: