January 18, 2007 | David F. Coppedge

No Evolutionary Tree for Galaxies

Edwin Hubble was famous for many important discoveries, including the confirmation of external galaxies and the expansion of the universe (no, he did not build the Hubble Space Telescope; he died in 1953).  One of his theories, though, a kind of evolutionary story of galaxies, has not fared so well.  Sidney van den Bergh discussed this subject in Nature this week.1  He commented, “Galaxies are like people: the better you get to know them, the more peculiar they often seem.”
    Hubble had classified galaxies with his famous “tuning-fork diagram” showing ellipticals evolving into normal spirals on one fork and barred spirals on the other.  Each branch supposedly evolved into more open forms.  It was never clear, however, whether the evolution proceeded from left to right or from right to left.  The situation has not become more clear over time.  In fact, Hubble’s classification does not mesh with more recent plots of galaxy color vs. brightness (luminosity), on so-called Hertzsprung-Russell (H-R) diagrams.  Other complications include differences as a function of mass or of lookback time (i.e., distance).  “It is not clear,” he said, “how the apparent dichotomy of galaxy characteristics, seen in the distribution of galaxies over the colour-luminosity diagram, can be reconciled with the continuous change of galaxy characteristics along the Hubble classification sequence.”
    After surveying the evidence, van den Bergh concluded that no simple diagram explains the observations: “A grand unifying scheme that incorporates both the continuity of the Hubble diagram and the dichotomy in the galaxian colour-magnitude diagram does not yet seem to be in sight.” 


1Sidney van den Bergh, “Concept: Galaxy Morphology: Out of order,” Nature 445, 265 (18 January 2007) | doi:10.1038/445265a.

Van den Burgh raised another interesting point in passing.  Is the evolutionary tuning-fork diagram an artifact of human psychology?  Maybe what’s evolving is not the phenomenon under observation, but our science:

Albert Einstein and many others have commented on the effectiveness of mathematics for formulation of the laws of nature.  As a result, science sometimes evolves in those directions in which mathematics can be applied.  However, several areas, including friction, turbulence and morphological classification, remain largely in the mathematical wilderness.  Progress in galaxy morphology has mainly resulted from the remarkable human capacity to recognize patterns.

Van den Bergh here speaks as a believer; he is not criticizing mathematics or pattern recognition.  He speaks of “progress” instead of “random walk” in our science.  The question deserves consideration, though, to what extent humans impose their psychological predilections on the observations instead of truly understanding what is “out there” in nature.  Could there be a selection effect that biases our outlook?  Because of our skill at pattern recognition, are we seeing mainly the phenomena that can be classified by simple mathematical laws, and overlooking the more difficult phenomena in the “mathematical wilderness”?  If so, how well do we really understand the universe?
    Hubble’s evolutionary diagram – a tree with only two branches – appears simplistic.  It discords with the observations.  How much more the imposition of an tree pattern onto the bewildering diversity in the living world?  As we have seen, evolutionary biologists strive to impose “tree-thinking” onto their psyches before the observations speak (11/14/2005).  What if the data are discontinuous at a fundamental level?  Human psychological needs do not justify imposing continuity on a discontinuous data set that refuses to be so classified, any more than you can have pi or e integers, evolutionize the prime numbers, or rationalize irrational numbers (or people).
    Hubble’s tuning fork can still be salvaged.  Instead of making it an evolutionary symbol, make it do what tuning forks are designed to do.  You’re not supposed to look at it for meaning as if it’s some kind of divining rod.  You strike it and listen to a very precise, pure tone.  Compare that to the observations, and you hear a very strong concordance (11/27/2006, 08/11/2006).
    Science has strayed so far off pitch from its original program (online book) that it has become, like a John Cage concert, a bewildering cacophony of conflicting and nihilistic discords, with blaring trumpets and clanging cymbals going nowhere without a score or conductor.  Time out for a tune up.  Following the right pitch can lead to harmonious and pleasing insights into the world (see Kepler). 

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