Why and How Would a Welladapted Aquatic...

gogol — Tue, 05 Aug 2008 14:46:29 +0200

The answer seems to lie in the realization that for freshwater algae--the green algae are the most common freshwater forms--their habitat, when it consists of shallow ponds or streams, often, and even annually, dries out. Adaptations for surviving desiccation would be advantageous, and such adaptations would be preadaptations for living on land. When we consider the evolution of green algae in such terms, it is not improbable that terrestrial forms evolved. But we have no fossil evidence or other data to document that evolutionary breakthrough. There is a gap. It could well be another case of tachytelic evolution, wherein a form adapted to one adaptive zone invades a new zone, evolutionary changes are rapid, and no fossils are found. Furthermore the intermediates are not really successful aquatic plants nor are they successful land plants. They lose out in competition to both. Hence, no intermediates survive. But we cannot, from present information, document Lignier's hypothesis.

This again illustrates the frustrations of phylogenetic research: the concept of evolution encourages us to look for phyletic series, but the action of natural selection tells us we must both expect and accept gaps. Perhaps the most disconcerting aspect of such a gap is our lack of insight into how the primitive transport tissue--the stele--of Rhynia arose. It is disconcerting for two reasons. First, despite the relative simplicity of the rhyniophyte stele (it is a thin strand of long, slender cells in the middle of the stem), it makes a rather sud-den or genuinely neosemic appearance. It cannot be homologized with any green algae cells. Such apparently sudden and discontinuous changes, as we have emphasized, are inconsistent with the known process of evolutionary change. We can only hope that new data from living or fossil plants that represent a useful missing fink here will become available. The second reason is the origin of the nonvascular plants.

The Origin of Mosses and Liverworts

gogol — Tue, 05 Aug 2008 14:46:48 +0200

Phylogenetically, these known nonvascular plants are off on an evolutionary side branch, quite distinct from the other metaphytan species. The reasons for this are their lack of vascular tissue and the dependence of their sporophyte generation on their gametophytic one. Thus, although there are useful homologies between the green algae and these bryophytes as regards pigments, photosynthetic products, and cell walls, the course of evolution from the ancestral algae was probably independent of that taken by Rhynia and other vascular plants. This could even mean--and it is not unlikely--that land habitats were successfully invaded at least twice by descendents of the green algae. And these two invasions were based on two different kinds of adaptations, notably descendents with and without vascular tissues and their associated patterns of alternation of generations. But again there is a gap. The transitional forms between the green algae and ancestral mosses and liverworts are not in evidence today any more than are those forms between the green algae and vascular plants.

It may be that these gaps can be narrowed, if not closed, by demonstrating conservative or plesiosemic molecular characters. Molecular evolution might reveal changes that indicate important homologies between multicellular green algae and plesiomorphic vascular and nonvascular plants. Of course, a fossil plant like Rhynia will have few proteins in which amino acids can be sequenced, but the lycopods may turn out to be quite informative.

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Why and How Would a Welladapted Aquatic...

The Origin of Mosses and Liverworts