Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications (Darwin, 1859, p. 413f).

Tuesday, 20 January 2009

The Absence of Evolution (Homology)

ResearchBlogging.orgWe almost labeled the paper entitled Bringing Homologies Into Focus by Anastasia Thanukos as 'Paraphyly Watch'. Here is why.

The paper is part of a new journal called Evolution: Education and Outreach - aimed at science school teachers rather than academia in general. The paper is a rather typical guide to homology - for instance:
    "Homologies are traits present in two or more organisms that were inherited from the common ancestor of those organisms. The human five-fingered hand and the five-toed foot of a lizard, for example, were both inherited from our common ancestor that lived more than 300 Mya" (p. 498).
Almost right. Homologies, as a concept, existed in the literature before 'ancestors' were accepted in an evolutionary context - so it would be technically incorrect to associate them with ancestors per se - in the same way Alexander von Humboldt wasn't a biogeographer (the concept may have existed but not as we understand it today; and the term was coined much later to refer to something else). Is this a case of whiggish history? Possibly. Homologies are the only evidence we have of a common history, that is evolution. We may be splitting hairs but explanations as to how things originate does rather detract from the meaning (as we will show later). A clearer definition is "Homologies are relationships and occur when the same structure exists in two organisms but as different manifestations (in this case forearm and wing)". What this implies (and how it got there) is another matter that detracts from the homologies themselves. And this is exactly what happens with the rest of the paper.

Why would a paper titled 'Bringing Homologies Into Focus' give more space to explaining analogies and homoplasies? Is the absence of homology really more interesting? If so, we would assume non-evolution is of greater interest. This is a typical trend in evolutionary biology - attempting to explain why evolution is not present by invoking other assumed 'evolutionary' mechanisms. This is contradictory and send out the wrong message. What is important is when evolution is present - namely homologies. When it is not present it should really be of little or no interest. Then why probe into the absence of evolution?

There is a misconception in science that everything needs to be explained. This is the underlying premise of paraphyly 'enthusiasts'. When a group turns out to be non-monophyletic, that is non-evolutionary, people insist that evolution has gone on anyway. Apart from flying in the face of empiricism, explaining the absence of evolution by using other explanatory 'evolution' mechanisms is meaningless. Convergent evolution is not evolutionary. It does not result in homologies, only in analogies, that is non-homologies. Why this is even taught as 'evolution' mystifies us. We wonder if this happens in other fields? When volcanic rocks are absent from an area, do geologists explain it through volcanism? They could, but it would be very silly indeed.

Anastasia Thanukos (2008). Bringing Homologies Into Focus. Evolution: Education and Outreach, 1 (4), 498-504 DOI: 10.1007/s12052-008-0080-5


Nick (Matzke) said...

Could you please explain what you mean by "evolution"? I think evolution in the most important sense means "descent with modification." Under that definition, convergent evolution is evolution, and evolution has occurred whether a group is monophyletic or para/polyphyletic...the only problem in the latter cases is that the "group" has been incompletely/incorrectly described and so the inferred evolutionary changes, their polarity, etc., may also be somewhat mistakenly described. But unless everything is the same, i.e. no modification, you've still got evolution. (Or you could deny descent, but presumably you agree that everything shares common ancestry at some level, even though the detailed pattern of relationships is sometimes difficult to work out.)

Pre-Darwinian Homology « Archetype said...

[...] In a recent post Malte C. Ebach and David M. Williams criticize a paper by Anastasia Thanukos for bringing up the concept of common ancestry into the definition of homology. [...]

RPM said...

I agree with Nick. It seems as if you have a very narrow definition of what evolution is. Convergent and parallel evolution are both examples of evolution and they are interesting. They are not examples of shared characters based on common ancestry. But I don't see how you can argue that they don't represent evolution.