Astrocladistics

Classification

Classification reflects our vision of the diversity. It aims at putting the objects into some boxes that ease their designation and comparison. For instance, it is more convenient to talk about “elliptical galaxies” than to give the list of all such objects. As Adanson nicely wrote in 1763, classification eases memory and helps understand the relationships between the objects.

It could seem that galaxies are much simpler objects that living organisms. But is that really true? Not sure since the complexity of an entity strongly depends on the details and the number of descriptors experimentally available to us. In reality, galaxies appear simple if we restrict ourselves to a small number of observables. Naturally, this number increases with technology as it has tremendously done in the past few tens of years in astronomy. Nowadays it should be clear to everyone that galaxies are complex objects in evolution. It thus seems legitimate to better understand how the biologists have tackled the huge diversity of the living organisms.

The first thing we can discover is that the concept of classification is not exactly the same as physicists are used to. There is indeed a science of classification, especially in the context of evolution, called the systematics. Astronomers have this chance to talk to people who have more than two thousands years of experience dealing with complexity. It would be sad not to use this opportunity.

In this topic, I describe the parallel between the evolutions of the knowledges in biology and in astrophysics. It is absolutely not to seek an analogy between the types of objects, but rather to understand the intellectual paths accomplished in both disciplines as the exploration of our World goes on.

This is exactly this conceptual convergence that lead the beginning of the astrocladistics and contributed enormously to its formalization.

Systematics

Systematics is the Science of Classification. We have learned that there are three ways to compare two objects:

1. the traditional approach based on a few apparent properties
2. global similarity based on all available observables
3. cladistics based on the common history of objects

We describe the differences between these approaches in three different pages. To formalize a little bit, we use the same table below showing three objects, A, B and C, described by three parameters, c1, c2 and c3, that can each take only two values, 0 or 1.

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ADANSON, M., 1763.  Famille Des Plantes.  volume Num. BNF de l’éd. de, Paris : INALF, 1961- (Frantext ; R263Reprod. de l’éd. de, Paris : Vincent).