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September 25, 2008 11:44A closer look at evolution.How does evolution really work? How do changes in the physical environment cause living creatures to literally transform their physical and sometimes mental abilities? |
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Dear hblackwell,
Your question about how evolution works and the role of the environment in driving evolution is excellent and is at the heart of many research questions in evolutionary biology.
Living organisms evolve by accumulating changes in their DNA (mutations) that can alter the way their genes function, and new gene functions can give rise to new physical or mental abilities. These new abilities can be:
1)beneficial and increase the fitness of the organism
2)harmful and decrease the organism's chances of survival and reproducing
or
3)neutral and have no effect on the fitness of the organism
The environment defines whether a new physical or mental ability is beneficial, harmful, or neutral to the survival and reproduction of the organism. One example of how the environment has played a role in selecting particular physical features can be seen in desert plants.
Desert plants have evolved many adaptations that allow them to conserve water in dry environments. These adaptations include having thick water storing stems (succulent), reduced leaves, and specialized photosynthetic mechanisms that reduce water loss. While these adaptations allow desert plants to grow and survive in extremely dry environments, they come at a cost. Most desert plants grow extremely slowly and would not be able to compete and survive in other environments. The physical adaptations that are selected for as beneficial in a desert environment would probably be harmful in a tropical environment where plants grow and reproduce very quickly.
So you can see that the environment in which these plants are growing plays a central role in determining which physical features are beneficial and will be passed down to the future generations through the increased survival and reproduction of the parent plant and which physical features are harmful and will not be passed down because the parent plant is unlikely to survive and reproduce.
There is a very cool experiment that has been happening for more than 20 years, tracking the evolution of a fast growing bacteria called E. coli. Here's a review of the experiment: http://scienceblogs.com/loom/2008/06/02/a_new_step_in_evolution.php
The researchers started out with 12 separate flasks of genetically identical bacteria. They fed the bacteria glucose (simple sugar) every morning and watched as DNA mutations that accumulated over tens of thousands of generations gave rise to new physical abilities that allowed the bacteria to be more successful in the lab environment. Two examples of how the lab has shaped the physical abilities of the bacteria are: (1) the current lab bacteria can grow 75% faster when living off of glucose than the initial cells that started in the flasks 20 years ago, and (2) one of the flasks of bacteria evolved the ability to eat a new carbon source called citrate.
This experiment is really exciting because it is a demonstration of evolution in action but there are still a lot of questions to be answered. Only about 1% of our DNA codes for genes, and we don't really know how mutations in the other 99% of our DNA change our physical/mental abilities. Also, what kinds of mutations have led to the evolution of really complex structures like the brain and the heart?