Beyond galaxies

Cladistics is based on the phylogenetic concept of transmission with modification. This is easily understood in the frame of replication of living organisms, be it sexuation or duplication.

Astrocladistics was initially attempted in the case of galaxies because their transformation (“formation and evolution”) is a transmission with modification process (see the explanation of Galactogenesis).

I found that we can generalize this idea to many astrophysical objects. I give two examples here.

Dark Matter Halos

At recombination, just after the phase called Big Bang, when the Universe became transparent, some 13.7 Giga year ago, the gravitational matter (dark mainly and baryonic) was distributed very uniformly, with some tiny density fluctuations. These latter grew up by attracting matter around them. These overdensities are called halos, that merged together to form more massive halos in which the baryonic matter is essentially trapped.

This process is usually represented by a tree that shows the genealogy of a single massive and current Dark Matter halo. One such representation issued from a numerical simulations by STEWART et al (2008) is shown below on the left. This representation has too principal drawbacks: i) it gives the feeling that small halos all disappear, and it does not give the true diversity of halo masses that ever existed.

A simpler representation is given on the right side of the Figure, using a cladogram which is easy to built since there is only one character: the halo mass (FRAIX-BURNET 2009). Each node corresponds to a merging event of two halos yielding a more massive one.

The huge difference with phylogeny is that we have here a merging process, not a replication. In other words, the number of species increases but the number of individuals decreases.

But what is wrong with doing cladistics since the merging of two halos is obviously a transmission with modification process?

Globular Clusters

Globular clusters a big ensemble of stars that are devoid of gas, thus making them fit into our definition of a galaxy. Since they have no gas, there is no star formation and since they are rather dense, they do not interact much with their environment or with other clusters. Hence, to a first approximation, their evolution is due only to star evolution with some evaporation (escape of stars). Their observed properties closely reflect the state in which they formed.

All stars in a globular cluster are formed together following the collapse of a molecular cloud. Simply speaking, this cloud is part of a huge reservoir of gas that is at the origin of galaxies. It is believed that when this gas fall onto a galaxy, turbulences create heterogeneities, i.e. clouds, that could end up into forming stars and clusters.

So, the properties of globular clusters entirely depends on the state of its progenitor cloud of gas, that is of the bigger reservoir. All clusters forming from clouds of the same reservoir will have the same properties. If the gas of the reservoir evolves, through gravitational torques or through metal enrichment by supernovae explosions, then the new globular clusters will be different.

It should be then easy to understand that the evolutionary link, the transmission with modification process, between globular clusters comes from the gas reservoir. Globular clusters can be seen as tracers of this gas, like an “expression” of the fundamental “genotype” of the reservoir. It is the not the clusters that evolve, but the gas from which they formed.

Indeed we have found that the globular clusters of our Galaxy can be placed into three groups, each one corresponding to a clear phase of our Galaxy assembly. This is described in FRAIX-BURNET et al (2009).

And then…

Then, it seems that most objects in the Universe could be analyzed with cladistics, more or less like any other multivariate statistical method. I am working in this direction, but even if robust cladograms can be obtained, one has to understand them to show that it can be useful.


FRAIX-BURNET 2009 :  Galaxies and Cladistics, In Evolutionary Biology: Concept, Modeling and Application Pontarotti, Pierre (Ed.), Springer, pp 363-378. (
FRAIX-BURNET, DAVOUST, CHARBONNEL 2009,: The environment of formation as a second parameter for globular cluster classification,  Monthly Notices of the Royal Astronomical Society, 398, 1706-1714. (
STEWART et al 2008, The Astrophysical Journal, Volume 683, Issue 2, pp. 597-610. (ApJ Homepage,

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