How We Look at the Relationships of Taxa

How Relationships Happen.

Evolution has occurred

"There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved." -  The final sentence of "The Origin of Species" by Charles Darwin 1859.

Modern Systematics and Taxonomy are based on the knowledge that evolution has occurred.

The process of evolution is "selection" and its observable product is speciation.

The Basic Concepts and Terms

A  group of organisms that have begotten each other as ancestor and descendant (either as individuals or as a group) along a continuum of time are called a LINEAGE.
A lineage that is made up of an ancestor and ALL of its descendants is called a CLADE.
Clados is latin for branch.
When a new branch forms in PHYLOGENY the process is called CLADOGENESIS
A PHYLOGENY is the actual history of cladogenesis in a clade.
A CLADOGRAM is a schematic representation of the phylogeny.
 
 

Evolution operates at many scales.

Lineages can change over time or they can split.  Lineage splitting is cladogenesis, simple change over time without splitting is called ANAGENESIS.
The two processes are intertwined and often confused with each other.  When you look atthe phylogeny of a group, you are almost certainly observing both.
 

When Cladogenesis occurs at the appropriate scale, we call it SPECIATION.

Here is a Glossary that might help with definitions

When Cladogenesis occurs at the appropriate scale, we call it Speciation.  Anagenesis can result in speciation as well but the evidence is often much more subtle.

In Systematics we are concerned with the relationships within and among CLADES.
Given that a Clade is a monophyletic group by definition, the smallest clade must be the minimal monophyletic group.

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A SPECIES is the minimum diagnosable and defensible clade

That means that it is the minimum clade that you can find that you can diagnose or recognize by the presence some characteristic and that does not force another similar clade to be incomplete.

(this is essentially the definition of  Baum, D. A., and M. J. Donoghue. 1995. Choosing among alternative "phylogenetic" species concepts. Systematic Botany, 20(4):560-573)

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Species are real entities.  They represent the point at which CLADOGENESIS occurs.  This is the point at which the cohesiveness within a lineage (usually maintained through interbreeding) is broken and a branch takes on a new evolutionary direction independant of its previous connection with another species in the clade.
 

In order for a SPECIATION EVENT (or cladogenesis) to happen, some form of isolation must occur.  This can be as big as the movement of two continents splitting one species into two, or as subtle as a mutation occuring within an individual that prevents it from breeding but still allows it (or even helps it) to reproduce asexually (e.g. some polyploid events).

What was once represented as a single line on the cladogram, now splits into two.  Here is an example of a species that can't swim.  Its populations are divided by the raising of water levels over a large geographic area.

Of course, over time, what starts out as a species can diversify and become a genus which can diversify and become a family and so on.... or it can go extinct or create hybrids with other species.

GENE FLOW

HYBRIDS and RETICULATION

Generally interbreeding is the cohesive force that maintains a species.  It also sometimes complicates the picture after speciation.
 

EXTINCTION

Most species fail eventually.

Systematists: those of us who study the relationships of taxa use a specific methodology for determining those relationships.
As in any science, this methodology has evolved over the years and a few years ago we were in the midst of what Thomas Kuhn would have called "a paradigm shift".

The new paradigm is the cladistic method.  I will explain why we have adopted the new paradigm after you learn how the process works.

Previous Terms: CLADE, CLADOGRAM, PHYLOGENY

How do we do it?

Every clade must be defined by a unique characteristic.  This is called a SYNAPOMORPHY. (a liberal translation would be: syn = shared, apo = derived, morphy = characteristic).  This is exactly what a synapomorphy is - a characteristic that is shared among the members of the clade and which is evolutionarily derived.  For example you have wide leaves and narrow leaves in a group, "wide" and "narrow" are character states.

Which one is derived?
To figure this out, you need an OUTGROUP.
An outgroup is the clade most closely related to but not a part of the clade that you are analyzing.
The idea is that you have a group that you are analyzing and its closest relative.

Because the group that your clade shares a common ancestor with the outgroup, the character state in that ancestor must be the primitive state.  Thus, if there are two states in your clade and one of those is in the outgroup, that must be the primitive or basal state within your clade.

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