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 Flowering Plants

gogol — Tue, 05 Aug 2008 14:46:08 +0200

The emergence of the flowering plants is still something of a mystery. Darwin, in 1879, wrote his botanist friend Hooker that "the rapid development so far as we can judge of all the higher plants within recent geological time is an abominable mystery. I would like to see the whole problem solved." Seeds emerged as a further development of sporangia, for example, appearing within the cones of the conifers. Then seeds appeared within the flower. But where did the flowers come from? A possible candidate for the plesiomorph of flowering plants is a magnolia such as Magnolia soulangeana. As with all plesiomorphs, it shows a series of characters that are conservative and therefore representative of the ancestral condition, although a magnolia plant as a whole is not consistently a conservative plant. The actual ancestor of the flowering plant has not been found in the fossil record, and efforts are still being made to reconstruct it. In terms of the flower, which is the critical feature here, we see a structure still bearing parts highly suggestive of evolution from a twig.

A long fascinating story can be summarized by looking at some of the trends in floral evolution. This story tells us that flowers underwent an adaptive radiation and in some cases, reverted to pollination by wind, which probably occurred in the ancestral plants from which they somehow evolved. We say somehow because Darwin's "abominable mystery" is still with us. Statistical analyses, such as that by Kenneth Spome, of the University of Cambridge, England, can tellus that a plausible ancestor for the flowering plants is the curious group of plants known as the cycads. They are not conifers, but members of a group that seems to have had a parallel, but somewhat independent evolution. But that still begs the question of how they arose. That must be answered in selectionist terms. Something about the flowering plants gave them a survival advantage over other plants. A recent approach to this question is that of Philip Regal, of the University of Minnesota. He starts at an obvious point, i.e., the flower. This feature of these plants is obviously unique and neosernic, and it would not exist unless it had a selective advantage. Other angiosperm features can be found to one degree or another in other seed plants. These include broad leaves, substances that repel predators, and vessels instead of theA recent approach to this question is that of Philip Regal, of the University of Minnesota. He starts at an obvious point, i.e., the flower. This feature of these plants is obviously unique and neosernic, and it would not exist unless it had a selective advantage. Other angiosperm features can be found to one degree or another in other seed plants. These include broad leaves, substances that repel predators, and vessels instead of theregarding the evolution of the vascular plants. Quite clearly, Lignier is looking for aposemic changes connecting an aquatic green alga to a plant that is adaptive to the land.

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Why and How Would a Welladapted Aquatic...

The Flowering Plants