davoid

We have then two competing views of the origin of the protistan eukaryotes. Both views agree that the protists arose from the prokaryotes; both can account for the observed molecular homologies; and both can explain the morphological gap that separates the prokaryotes and eukaryotes. Also, both suggest that there was a multiple or polyphyletic origin of the Protista. (They both disagree with Pascher.) In other words, both can explain the same set of phylogenetic data. To determine which view is the correct one, other predictions from the two hypotheses will have to be made and tested. This has not yet been done and therefore we are left with both views as options. A summary of protistan phylogeny. Note that other views on protistan evolution differ from this one. Five problem areas can be mentioned: (1) Origin of the protists. This could be by endosymbiosis or by transformation. (2) Diversification of the protophyta. This could be the result of a polyphyletic origin of the algae from the Monera. (1) Origins of the protozoa. There is convincing evidence for a polyphyletic appearance of animal protists (protozoa) from certain colorless algae. (4) Pseupodial evolution. Many different evolutionary experiments among ameboid forms. (5) Kinetidal evolution. Polyphyletic zooflagellates were presumably replaced as free-swimming forms by ciliated cells.

In closing, one more complication should be noted and that pertains to the validity of macromolecular homologies. Usually the argument against homology is stated in terms of random events producing comparable arrays of amino acids or nucleotides, and the argument for homology is stated as due to simi-larity from a common ancestry. That is not correct. It is really convergence versus homology, not random chance versus homology. And we really do not yet know how precise macromolecular convergence can be. The fact that hemoglobin appears in such diverse animals as flatworms, annelids, certain molluscs and insects, some echinoderms, and vertebrates has been widely attributed to convergence. Unfortunately, only the sequence of the vertebrate hemoglobins has been studied; therefore we cannot determine the nature of molecular convergence between different phyla precisely. But it is conceivable that there are only a limited number of ways to build an oxygen-transport molecule. Perhaps only a rather specific sequence of amino acids will be functional, and hence, similar sequences will appear due to similar selection pressures, a process quite different from chance. The same might also apply to similar RNA molecules--their role in ribosomal function might well be so precisely defined as to select for highly similar nucleotide sequences. Evolution produces remarkably similar, complex organs of sight in the cephalopod molluscs (octopus and squids) and vertebrates, which are convergent, not homologous. Perhaps less complicated structures, such as certain functionally identical molecules, will also be precisely convergent. It will be of great theoretical interest to see if the Remanian criteria for homology can be used to detect convergence of molecules.