Today I dug into a chapter and a half out of the seminal piece Phylogenetic Systematics, THE piece of work that described and elaborated the foundation of modern phylogenetic thinking, and the use of the modern shared common ancestor driven heirarchies of biological systematics.
The full text is relatively easy to get your hands on through your local library, and will seem at first to be a rather dry read. But as someone who is already very familiar and has a lot of buy-in with phylogenetics, I was stunned at how articulate Hennig could be even with often overly technical/philosophical writing. He describes the various systematic methods "what we'd now refer to as phenetics", and piece by piece explores their deficits and subtly, almost sinisterly, brings phylogenetics to the fore as the only reasonable means to organize the collection of all living things. Not only does he do so beyond elegantly, but he utilizes the full suite of analogies, metaphors, and directly linear logic to examine all the conceptual possibilities.
"The idea expressed in all these utterances that phylogenetic systematics is based logically and/or historically on purely morphological or at least nonphylogenetic systems, and the view often derived from this idea that a pure (idealistic) morphological or at least nonphylogenetic system therefore merits precedence of the phylogenetic one because it stands closer to the natural facts and contains fewer hypothetical elements, is absolutely wrong" (p. 11)
He even highlights the basic logic that is contained within the idea of using genealogy, at the ontogenetic (development of an organism), tokogenetic (relation of organisms of the same species to one another), and phylogenetic levels (between species) are all natural extensions of one another.
"According to Arnold (1939), already in 1550 B.C., Eber's Egyptian papyrus recognized the development of the scarab from the egg, the fly from the maggot, and the frog from the tadpole. It would not have occurred to any of the ancient systematists to classify differently, because of morphological differences, the young and adult stages of animals, insofar as their genetic relationships were know to them."
Look at that sentence. It is a remark, in no lesser sense, that it makes no sense to treat the juvenile as a different organisms from the adult, and thusly it makes no sense to not organize organisms based on their origins, their descent from common ancestors, from a process akin, indeed identical to the elaborations of our own personal genealogies.
"The fact that genetic criteria were already used in the older systems does not mean, of course, that it was realized from the beginning that there are genetic relationships between all organisms and all semaphoronts. but the ontogenetic relationships of the semaphoronts, and the obvious genetic relationships between the individuals of a species, are in reality only partial phenomena of the hologenetic relationships that interconnect all semaphoronts. Therefore the inclusion of even only certain genetic criteria in biological systematics necessarily resulted in the realization that there are in fact genetic relationships between all organisms. The theory of descent, that is the perception that the existing diversity of life on the earth arose historically from an earlier simpler condition, and that the semaphoronts -- the elements of all systematic efforts in biology -- must be regarded among other thing as members of a community of descent, is thus derivable from biological systematics (not "morphological systematics"). This is. in fact, its most important result to date."
"It is doubtless evident from the observations that an "organisms" can change in a relatively short time form one form in which it resembles certain organisms into another form, dissimilar to its earlier condition that makes it more similar to other organisms, and from the further observation that the offspring of two different individuals may be dissimilar to their parents, to conclude that even greater changes in form may possibly take place in the course of periods of time so vast that they cannot be surveyed. Only a small, though decisive further step is needed to conclude that the graduated differences in form that are expresssed in the hierarchic system of species originated by similar processes of change lying farther back in time or requiring longer periods of time. Thus one has the most essential declaration of the theory of descent."
There it is in a nutshell. The theory of descent, the foundations of phylogeny, built up in a single pair of sentences. Hennig is, much to my admiration, master of the long sentence.
"If we could determine the genealogical relationships among all individuals over a long period of time, and present these relationships graphically, we would find gaps in the structure of the relationships. These gaps divide complexes of individuals, which we call species, from one another.Despite all the difficulties that the species concept poses in detail, it is an established fact that these species do in fact exist in nature, and that they are relatively stable, reproductively isolated, complexes. They are complexes of individuals interconnected by genealogical relationships like those show in Fig. 3, individuals capable of producing new individuals unrestrictedly only among each other."
There, in a nutshell, he portrays the elaborate tokogenetic relationships in regards to different adjacent species types. And he immediately recognizes the absence of this model's ability to explain the formation of species, that process is absent. And so in his very next figure, he clarifies.
He resoundingly states, utilizing my own personally favorite means of explanation, "The relationships of the species in a genus are most suitably expressed by the graphic presentation used in human genealogy, namely the family tree." (p. 21)
He clarifies, beautifully, the nature of the theory of descent as a scientific one, "'[...]The theory of descent is a hypothesis, and in any hypothesis are explained by an assumption, which was intself made on the basis of the facts.' (Wenzl 1938)." (p. 22)
He goes on, later (p. 85), to clarify one of my own personal challenges with examining phylogenetics, namely that of appreciating the methods of measurement of validity, saying, "Consequently it is almost impossible for a systematist with inadequate mathematical training to judge whether any progress has been made in measuring differences in form." How is one to gauge a phylogeny, unless they have the adequate training to understand the methods by which it was contituted.
Other topics are addressed, including the means by which the measure of similarity, that beloved by the pheneticists, would only be a functional means of exploring the relationships of communities of similarity if we had full access to the measures and metrics of all species ever known through time. By tracing changes in similarity in that context, we could draw meaningful phylogenetic and phenetic information simultaneously, but outside of such a case phylogenetic thinking is crucial to meaningful phylogenetic assembly of relationships.
He describes the uselessness of similarity (a community of similarity) as a measure of age of a branch (community of descent), citing Bigelow (1958): "Unequal rates of evolution will produce cases in which overall basic similarity does not correspond with recency of common ancestry. Slow evolution in two phyletic lines will tend to produce an appearance of recent common ancestry when actual common ancestry is remote in time."
Plesiomorphy and apomorphies, and by extension symplesiomorphies and synapomorphies are defined along with how these types of traits can and cannot be useful for phylogenetic construction and understanding.
The main challenges Hennig considers when it comes to considering the phylogenetic reconstruction of organisms are simple:
- Character phylogeny
Each of these essentially veers around ideas of how we determine what characters are derived from one another, the order in which they were derived, the possibility of a character changing to a previous form, the occurence of a single character independently from different characters in different contexts, and the occurence of identical shifts (morphoclines) occuring multiple times. Unfortunatey I lack the full content of the remainder of the chapter, and only was able to read some truly edifying workthroughs of the first two questions.
In time the ILL will provide me with a copy of the full book, and I will revisit that chapter's content. For now, I get to bask in the wonder of Hennig's thoughtful deliberations.