วันพฤหัสบดีที่ 24 กุมภาพันธ์ พ.ศ. 2554

Two Different Types of Evolution

When people ask why something evolved, usually they mean what use that feature served the organism in its environment. These are called adaptive changes. An example of an animal that shows amazing functional adaptations to its environment is the Maned Three Toed Sloth. The maned three-toed sloth is so extremely well adapted for its life hanging upside down out of trees that its fur grows in the opposite direction to every other mammal. Extra vertebrae in its neck give it greater flexibility for swiveling upwards while it is upside down. The sloth also cultivates algal blooms in its fur to provide excellent camouflage against the greenish grey bark.

However a few scientists, most notably Stephen Jay Gould (1941-2002), believe that rather it is nonadaptive changes that are the main driving force behind evolution. It was argued by Gould that the flexibility of human consciousness comes precisely because of its nonadaptive functions. Natural selection may have selected for larger brains but once they had reached a certain size all the characteristics we recognize as human consciousness could have come 'for free'. A characteristic may have many more uses than the function it was originally adapted for. Human brains are very flexible precisely because they are not adapted to any single particular use. Our hands are good for typing with, but that is not what they were originally evolved for! The mountain tapir's long prehensile truck is used for grasping leaves while feeding, but it also can be used as a snorkel allowing the animal to hide underwater from hunters and predators. These are all examples of nonadaptive uses of a feature.

Scientists have discovered a way to measure whether a genetic change is adaptive or not. Genetic mutations have been discovered to occur in a predictable manner, much like nuclear decay. This is called the molecular clock of evolution. According to the neutral theory of evolution most of these mutations have no effect on the organism at all. Natural selection acts to keep highly functional genes as they are, while nonfunctional genes mutate at random. Mole rats are totally blind but still have a lens buried under skin and fur. It is thought they use this lens for adapting to the changing seasons by some unknown mechanism. A protein in this lens has undergone 4 times more mutations over 100 million years than in sighted vertebrates, but only once fifth of the mutations expected had the lens had no function at all. This suggests the lens is still functional, but that its function is less specified than in animals with vision.



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