Evolution as Pattern

The goal of systematics: 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.

But first, where does the diversity come from?



From PLOS blogs

Not from this!

The classic "ladder of evolution" - the iconic image of evolution (endlessly copied and parodied) is utterly misleading because:

Indeed, if we took this literally, the creationist argument that asks "If humans evolved from apes then why are apes still here?" might actually have validity.

Fortunately real life is more interesting.
Lineages: Just what is a lineage: = an interbreeding group of sexually reproducing organisms projected through time.



The first tree of evolution from a notebook of Charles Darwin from freerepublic.com

Speciation

Ironically, Darwin, in The Origin of Species, only scratched the surface of a real discussion of the origin of species, by simply proposing that it could happen through divergence from a common ancestor. Nevertheless doodles in his notebooks (right) showed that he grasped the concept perfectly.

But how can we use such diagrams rigorously to convey evolutionary history?

Graphically describing patterns caused by speciation - 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, we see the following: 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 anc C.

Note that in a cladogram, it does not matter whether things apear 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 on the right, cladograms 1 and 2 depict exactly the same relationships, whereas cladogram 3 is different.

Note: The taxa whose relationships are indicated above, A, B, and C may be individual species or they may, themselves, be taxonomic groups whose members' relationships could be shown with its own cladogram.


Typically, systematists employ a taxonomic system 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:

Problem: The criteria that we use to classify animals according to this system are arbitrary and subjective. For example:

A reptile enthusiast might classify a green tree python 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.

Ideally, we would like to have some non-arbitrary, natural organizing principle for a taxonomic system that natural scientists can use. Such a principle is provided by the pattern of evolution. In order to understand it, you must first understand the conventions for graphically displaying the pattern of evolution.

The phylogenetic taxonomic system: Taxonomic groups can be named and defined based on their descent from a common ancestor. The cladogram to the right 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.

    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.

    Character analysis

    One obvious utility of cladograms is that we can map evolutionary changes onto them. As lineages evolve, the characters of their members change. I.e. they go from ancestral to derived states.

    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.