The tree of life

Does this tree look primitive to you?

DSC_0281.resizedIt’s a ginkgo tree in the cemetery, one of two. In the autumn they have beautiful yellow leaves.

The ginkgo is very unusual. It has its own phylum. For comparison, we mammals share the phylum Chordata with swallows, snakes, frogs, guppies, sting rays, Amphioxus and sea squirts. In botany there is a phylum for all flowering plants, one for mosses, another for liverworts. A phylum is a very high level category. Ginkgos, which seem to be native to China, were once found worldwide. Fossils go back 270 million years, well before the dinosaurs.

So, does it look primitive? The technical reason for its classification is to do with how the seed is fertilised (by a moving sperm), but the plant itself has some strange features. The branches above are studded with regularly spaced stumpy side branches that remind me of sea anemones. The fan shaped leaves are very simple in design. Two veins divide repeatedly and regularly; there is no network of vessels, no asymmetry, no fitting of the vascular tissue into the outline of the leaf – the leaf gets its shape from the dividing veins. The ginkgo seems to owe its looks to the relentless application of a few very simple algorithms.

The orthodoxy in evolution forbids any talk of progression or ascent. Even the organic idea of the tree of life is suspect. Trees grow up. You can tell the top from the bottom. Evolution is more like a bush, branching equally in all directions and allowing selection to prune it. Perhaps a branch that separated from all the others long ago will prove successful for millennia, like the ginkgo, or perhaps a family of newcomers will take over the world, as the monocotyledons have been doing in more recent times: they are the grasses and cereals.

Orthodoxy says that mutation creates variety within all species and that selection will either favour some new varieties over others, leading to change in the species, or if it is already very well adapted to its environment, all adaptations will be disadvantageous and selection will work to keep the species the same.

I don’t believe this, though. The coelacanth, known from Devonian to Cretaceous fossils (390 – 66 million years ago), was discovered unchanged in 1938, a ‘Lazarus taxon’ (!). How come it hadn’t changed? Surely the environment, even in the deep Indian Ocean, isn’t perfectly stable. Since the Cretaceous the climate has changed, and other creatures and bacteria have all been busy evolving, including the prey and predators of the coelacanth. Why has it not evolved?

I can’t help suspecting that it has failed to evolve because it can’t. And I suspect the ginkgo is the same. Probably for genetic and ontogenic reasons, the species has become an evolutionary dead end, and can persist as long as its niche lasts, but can no longer adapt. I think it has become genetically stuck.

Conversely, there are examples of clusters or swarms of closely related species where an ancestor species has been able to diversify and fill a large number of environmental niches. These suggest to me, not only a particularly suitable body type, but a genetic kit that is good at producing variation.

But what do I know? I’m no biologist. I would really welcome the thoughts of anyone out there who is.

Meanwhile, blackcaps have arrived and are singing in the cemetery. Not a very long or especially loud song, but rhapsodic and so good to hear.

 

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