Evolutionary

More than a century after Linné coined the nomenclature associated to the established hierarchical nature of the organization of living organisms, Darwin provided the explanation to this structure: evolution. More precisely it is the process of transmission with modification that generates the hierarchy. Each time individuals replicate themselves, either by duplication or by sexuation, there is transmission, otherwise the next generation would not resemble the previous one, and modification, otherwise it would be a cloning. The transmission builds the evolutionary relationships and the modification creates the diversity.

An individual resembles his parents, but also his grandparents and so forth. This individual also resembles all his cousins, but the similarity decreases with the number of generations to the common ancestors. The growth of the discrepancy is a continuous process, and after a large number of generations, cousins are sufficiently different to decide that they belong to two distinct species.

William Hennig in 1950 realized that living organisms must thus be compared on the properties (characters) they have inherited from their common ancestors, rather than on their similarity. which can be fortuitous. In other words, they resemble each other because they have a common ancestor (transmission) but they diverged from it (modification). So Hennig invented cladistics, also called phylogenetic systematic.

Let us take an example with the same matrix:

c1 c2 c3
A 0 0 1
B 0 1 0
C 0 1 1

We have to determine which property can be transmitted (inherited) and how it is modified with time.

The first question is not easy to address, and many analyses have to be carried out. Indeed, this question might have no absolute answer. Nevertheless, cladistics, and all phylogenetic approaches, are also a means to investigate the intimate processes of diversification: which property is transmitted or modified, which one can serve as a tracer of evolution etc. This is a huge progress with respect to the traditional classification and the global similarity approach: in one case you take whatever you choose, in the other you take everything. But what about the relevance to the problem of classifying the objects and understanding their evolutionary relationships? Taking the few properties that suits you or all you can find are actually both subjective choices. Cladistics has clear requirements for the characters and is a tool to investigate the relevance of the characters to these requirements.

Let us assume that characters A, B, C are suited for a cladistic analysis. Then our knowledge or our guess or our hypothesis could say for instance that “0” is an ancestral value, i.e. the value that the ancestor had transmitted, and “1” did not characterize the ancestor, hence this value is an innovation, a derived property.

A and B have one derived character (respectively c3 and c2), while C has two such characters (c2 and c3). Both A and B are thus closer to the ancestor than C. The latter has the same ancestral value for c1 as A and B, so he has a common ancestor with both A and B. He shares the same derived c2 value with B and the same derived c3 with A. Clearly, C is the most derived object here, but we cannot tell whether it derived from A or from B. We would need more information, other characters, to decide.

This simple example should make it clear that the cladistics can be applied to any set of complex objects in evolution if their diversity is driven by a process of transmission with modification.

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DARWIN, C., 1859. The Origin of Species.  John Murray, London.
HENNIG, W., 1965.  Phylogenetic systematics.  Annual Review of Entomology  10, 97–116.

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