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).

Friday, 25 July 2008

The Dispersal Myth

Organisms disperse. We do it, birds do it, plants do it. Dispersal is a fact of life. As an unobserved mechanism that explains distribution and evolution, dispersal fails. The evidence that 'supports' hypotheses of dispersal is the same 'evidence' that discredits the same mechanism. Like any explanatory mechanism, evidence that apparently 'supports' it is the same that also supports a competing and conflicting hypothesis and so forth. Dispersal in its descriptive form (i.e., iguanas disperse within their area of distribution) can be shown empirically. Dispersal as an explanatory mechanism (i.e., iguanas dispersed to an oceanic island and subsequently evolved into a new species) is merely hearsay.

Despite its non-empirical and highly subjective nature, many still feel that dispersal is the best or unifying theory for organic distribution and subsequent evolution. However, the less evidence there is it seems, the greater validity the hypothesis gains. In the case of pan-Pacific distributions, for instance, there is little or no geological evidence to suggest what the Pacific Plate looked like 200 million years ago. Without such geological evidence, dispersalism gains greater validity as being the only possible mechanism to explain distribution. The option of not knowing how things got to where they are, or using phylogenetic patterns to retrodict former area relationships, are rarely considered. For the most part a trivial answer will suffice - How else do things get to where they are?

The introduction of the molecular clock has only increased our obsession with dispersal (see de Querioz, 2005). These 'clocks' are based on geological (i.e., palaeontological) data (i.e., the age of fossils) in order to measure divergence times. Since we will never know if we have the oldest fossil, the chance that a single fossil will tell us any more is highly unlikely. But yet proponents of the molecular clock believe it does. This reinforces what I herein term the Dispersal Myth, namely, the less data there is, the more likely things are to disperse (Figure 1).

But what if we lack data altogether, that is, we have no information regarding the organism and its distribution? Do we still speculate in a highly subjective and non-empirical manner, or do we give up this flight of fancy altogether?

This week in Astrophysics and Space Science: An International Journal of Astronomy, Astrophysics and Space Science Wickramasinghe & Wickramasinghe (2008) push the limits of the Dispersal Myth. In short they propose the hypothesis that life may have dispersed from Venus to Earth.

The thinking behind this incredulous theory (first proposed by Barber in 1963) links three ideas together:

1. The discovery of complex "molecular structures" in Martian meteorite ALH84001.
2. Possible "transfer pathways" between Venus and Earth via solar winds and;
3. The possibility that extremophilic "Venusian microbes" may form within the atmosphere of Venus.

So why resurrect something endemic to the psychedelic 60s? Namely "at the time a mechanism for aerosol transfer between the planets was not identified ...", that is, there was no satisfactory explanatory mechanism proposed at the time. In other words, the mechanism makes the theory appear real, even if evidence, in this case extremophilic "Venusian microbes, are completely missing or unknown. What is more, the theory gains acceptance because the explanatory power of the mechanism now makes the eventuality of life on Venus more likely or predictable.

The hypothesis proposed by Wickramasinghe & Wickramasinghe (2008) is extremely subjective. It puts the cart in front of the horse. It proposes a mechanism to explain data before the data even exists. Even if life was discovered on Venus, it does not make the explanatory mechanism more likely. In fact, the reverse is true. If we, for arguments sake, we investigated Venusian microbes, we would know more about their physiological limits to disperse via solar winds than if they did not exist. Therefore the possibility of microbial dispersal from one planet to another is far more likely if the evidence was non-existent.

The message in figure 1 rings loud and clear - the less data there is, more "relevant" or "likely" dispersal appears. Dispersal, as an explanatory mechanism, is highly subjective, non-empirical and is nothing more than a myth.

Barber, D. (1963) Perspective 5: 201–208.
de Queiroz A. (2005). The resurrection of oceanic dispersal in historical biogeography. TREE 20: 68-73.
Wickramasinghe, N.C. & Wickramasinghe, J.T. (2008) On the possibility of microbiota transfer from Venus to Earth. Astrophys Space Sci. DOI 10.1007/s10509-008-9851-2

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