Woese concludes the following:
The types of phenotypic changes that accompanied the
formation of the three primary kingdoms are of a special
nature. General differences in cell architecture among the
three groups are remarkable, as are their differences in
intermediary metabolism, and each kingdom seems to have
its own unique version of every fundamental cellular function:
translation, transcription, genome replication and control,
and so on. The kind of variation that subsequently
occurred within each of the kingdoms is minor by comparison.
Thus the mode of evolution accompanying the transition
from the universal ancestor is unusual; far more novelty
arose during formation of the primary kingdoms than during
the subsequent evolutionary course in any one of them.
It is hard to avoid concluding that the universal ancestor
was a very different entity than its descendants. If it were a
more rudimentary sort of organism, then the tempo of its
evolution would have been high and the mode of its evolution
highly varied, greatly expanded.
Were the actual root of the universal tree (Fig. 4) located
in the vicinity of the deepest branchings in any one of the
three primary kingdoms, the above argument concerning
sequence distances would not apply to that kingdom, which
makes it conceivable that the universal ancestor had the
basic phenotype of that group. (This argument is particularly
attractive as regards the archaebacteria, for the group sits
relatively close to the intersection of the three primary
lineages; see Fig. 4.) However, this would still leave the
problem of deriving the other two phenotypes from a third
comparably complex one, which entails drastic changes at
the molecular level in most functions in the cell. In my
opinion the changes in overall cell structure, organization,
etc., required to change one of the three phenotypes into
either of the others are too drastic and disruptive to have
actually occurred.
Accepting all this, the only solution to the problem is for
the universal ancestor to have been a progenote.
pg 264 (pg 44 in the link)
http://www.pubmedcentral.gov/picrender.fcgi?artid=373105&...
This interests me for a couple of reasons. First, Woese acknowledges that the data is such that:
In my opinion the changes in overall cell structure, organization, etc., required to change one of the three phenotypes into either of the others are too drastic and disruptive to have
actually occurred.
In other words, he cannot see anyway that is feasible or reasonable for a common ancestor to evolve into what he classifies as the 3 primary kingdoms, at least if their shape and characteristics are handed down by their genes and so mutations are selected via natural selection and so the creatures' features evolve via gradual change into different creatures. This is important. He says the changes "are too drastic and disruptive to have
actually occurred."
He goes on to posit a hypothetical creature, a progenote, as the "only" solution which he describes thus:
The progenote is a theoretical construct, an entity that, by definition, has a rudimentary, imprecise linkage between its genootype and phenotype (251, 256). (Extant organisms, which have precise, accurate links between genotype and phenotype, are then genotes.) The certainty that progenotes existed at some early stage in evolution follows from the nature of the translation apparatus.
page 263 (pg 43 in the link)
So Woese recognizes that there is a serious problem claiming that the "three kingdoms" as having evolved by observable processes that we know of today. He posits a non-observed theoritical construct as a solution.
But there are problems. If there is "imprecise linkage between phenotype and genotype", then how is natural selection suppossed to work? If an organism has a beneficial trait, that trait won't necessarily be passed on and so the fact that organism survives does not mean it's progeny is more likely to.
Furthermore, isn't the claim that natural selection can work with the precursor, the hypothetical (mythical?) RNA-based duplicators, of the progenote (another imagined construct).
Maybe the truth is simply that the 3 primary kingdoms Woese defines did not evolve from a common ancestor at all?
Edited by randman, : No reason given.
™ Version 4.2