Sunday, March 7, 2010
A Classification Conundrum
Jerry Coyne, during much of the first chapter, cites examples such as the Tiktaalik roseae and Archeopteryx as "transitory" species in the fossil record that show the change from one specific classification of organism to another distinct classification. For example, the Tiktaalik roseae links the change from reptile into fish by possessing hinged limbs and a neck, distinctly reptilian features, yet also has gills and other fishlike features! However, this presents a problem: the present means of classifying animals does not provide a place for such organisms to "fit in." Are organisms like the Tiktaalik roseae and Archeopteryx able to be considered both animal and reptile? Neither? More or less one than the other? Can you find a way to determine how they can fit into a presently available classification? Cite examples from the text to support your claim, and it may be helpful to include an explanation of what a "mosiac" is in terms of transitory organisms as evidence.
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According to dictionary.com, mosaic is defined as an organism that exhibits a condition in which all of it or part of it is composed of two or more genetically distinct tissues owing to experimental manipulation or to faulty distribution of genetic material during mitosis. This describes the situation with the Tiktaalik because although it possesses gills observed in all species of fish today, it also had a set of sturdy ribs that helped the animal pump air into its lungs and move oxygen from its gills. In my opinion, an organism like Tiktaalik roseae was the transitional form between land-dwelling organisms and fishes, but was more of a fish that had the capability of coming up to shore and supporting itself upright with wrist bones. In evolutionary history the Titaalik roseae provides the "missing link" between Eusthenopteron foordi (early lobe-finned fish; 385 m.y.a) and the Acanthostega gunnari (land-dwelling tetrapod; 365 m.y.a). The thing that is difficult about classifying this organism is that fish don't have necks, yet the Tiktaalik possessed one along with many other amphibianlike features such as a flattened head and eyes and nostrils on the top of the head. It is believed that although it had the capability to breathe air, it was a fish that lived in shallow waters because it possessed gills, scales, fins, and was clearly not ready for full-time life ashore since it had not yet evolved a limb that would allow it to walk. Despite the fact the Tiktaalik had evolved many amphibianlike qualities, I would still classify it as a fish because it wasn't until later years when its descendants finally to be "Bold enought to venture out of the water on their sturdy fin-limb, perhaps to make their way to another stream, to avoid predators, or perhaps find food among the many giant insects that had already evolved" (38). This is similar to hippos and their on-going evolution to become fully-aquatic like their closest ancestors: the modern whale. The transition from being fully aquatic from a terrestrial species (or vice versa), there has to come a point where it is no longer an advantage to have to inhabit the old habitiat any longer. For example, hippos stay in the water most of the day and usually come out at night because the risk of sunburn. In fact, if they have adequate amounts of food in the water, they don't leave the water because there is no point and the action of intruding land would just be extra energy expenditure. This links back about the descendants of the Tiktaalik because as they foraged on land, they had to have evolved to the point where it was simply unnecessary to inhabit the water because all their food, reproduction, and development occurred on land.
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