Terms in systematics and biogeography often have different even conflicting meanings. "Monophyly", "homology", "cladistics" and "evolution" are used in varying ways. The reason for this discrepancy is that terms, like people, change over time. A majority of concepts, like evolution, were in use before 1859 and have altered drastically since. The reason for this change is the ever increasing befuddlement between form and explanation, a symptom of 21st century systematics and biogeography.
A term such as "spoon" can be given many different definitions that attempt to represent meaning. The most fundamental definition being a "small concave dish that tapers into a stem posteriorly". The definition only describes its form and not its function. The reason why such a definition is "fundamental" is that it does not need a functional description in order to convey meaning. Someone who has never seen or heard of a spoon before may choose to use in an unconventional way. The spoon however used is still recognized as a spoon based on its fundamental definition. Whatever function the spoon has at one time (i.e., a tool for eating soup, a bowl scraper or medicine applicator) should not detract from its fundamental definition. The same is true for conceptual terms in systematics and biogeography.
Fundamentally, monophyly is defined as a "natural group or classification". What this means is that it is a "natural group or classification". That is all. We may choose to interpret monophyly in different ways. It could be "a group that includes a most recent common ancestor plus all and only all of its descendants" (Kitching et al. 1998: 210). It could be a group that may have a ".. general typical organization" (Agassiz, 2004: 182). In either case, the fundamental definition remains universal despite the explanation given.
The Sensible Way
The concept of a natural group stems back before Linnaeus and ultimately is a "pre-evolutionary" concept with a 20th century term, namely "monophyly". The explanations assigned to it, whether they are true or not, reflect the attitudes popular at the time, which we choose to term the "Sensible Way", referring to Felsenstein's recent commentary in a previous post.
The "Sensible Way" always refers to "Darwin and Wallace" in some elusive or nostalgic way as in "... since the time of Darwin and Wallace ..." (Waters, 2007: 871) or to a particular interpretation of a term (i.e., biogeography = population genetics etc.).
This usage often appears to indicate that no sensible ideas were espoused during the time between Aristotle and Darwin/Wallace. We call this the 1859-syndrome, which is synonymous with the phrase the "beginnings of Evolutionary Biology", another vague term. Everything prior to this is often thought of as "pre-evolutionary". It is only logical to conclude from this argument that the term "evolution" is pre-evolutionary.
Evolution means "change over time". This is a simple definition that can refer to any number of evolutionary mechanisms such as natural selection, Lamarckism and hologeneis. The term "evolution" does not exclusively relate to a particular explanation but to all. The "Sensible Way", however, dictates that evolution = natural selection. By associating a universal concept with a particular mechanism has been the cause of great debate in science and unfortunately has given fodder to anti-science (i.e. creationists).
The "Sensible Way", namely a misinterpretation between a thing or concept with a particular explanatory mechanism, is the bane of systematics and biogeography. So is the 1859-syndrome, which shows an ignorance of history and the literature. The majority of concepts in systematics and biogeography are pre-1859 and their definitions fundamental and their explanations tend to be post-1859, deriving in the late 19th century and during the Modern Synthesis. The term "homology", which is fundamentally defined as "manifestations of the same form", is constantly given an explanation in order to suit the "Sensible Way". At first it was a functional role (i.e. Carl Gegenbauer, Adolf Remane etc.) and later with the onset of molecular data, just "similarity between characters". The homology concept underpins systematics and biogeography and has been a topic of discussion over a 200 year period. Noteworthy 18th and 19th century naturalists including Vic D'Ayzr, Johann Wolfgang Goethe, Geoffroy Saint Hilaire and Richard Owen debated the relevance of homology. By 1858, all we know about homology had been discovered. Post-1859 "sensible" interpretations (i.e. explanatory mechanisms) were postulated and confused with its actual definition "manifestations of the same form". No further meaning had been added, only an endless variety of ad hoc ("sensible") explanations.
Explanations do not add greater meaning to form or to such fundamental definitions. They only confuse and cause senseless debates over which explanation has the most rational argument based on what is known at the time. These "sensible" explanatory mechanisms have taken terms and definitions beyond their intended role. Richard Owen's special homology and its relationships to an archetype have been contorted into mechanisms of transformation. Sclater's biogeographic regions transformed into evolutionary centres of origin and Goethe's urpflanze into an ancestor. Definitions such as archetype, homology, evolution and natural classification that originally had no explanatory mechanism, helped to establish systematics and biogeography. Why do we need them now?
Agassiz, L. 2004. Essay on Classification, with an introduction by Edward Lurie. Dover Press, Mineola, N.Y.
Kitching, I.J., P.L. Forey, C.J. Humphries, and D. Williams. 1998. Cladistics, 2nd ed., The Theory and Practice of Parsimony Analysis. Oxford University Press, New York, NY.
Waters, J.M. 2007. A review of: Biogeography in a changing world. Systematic Biology 871-873.