MadSci Network: Evolution |
People often get the idea that every organism alive today is "optimally fit" and that every feature of every organism has been rigorously selected for. On top of this, some people go further to think that each feature has a well thought out plan, and a purpose. In reality, as far as we know, organisms are not necessarily "optimally fit" today. They are fit enough to survive, and that is enough. An "optimally fit" human today might have bullet-proof skin, retractable wings, and thousands of other cool features. Therefore, an existed feature may not have a strong underlying "purpose". I like to think of "elimination of the weakest" as much as "survival of the fittest". But both of these are cliches that only tell a very small part of the total theory of evolution. The larger picture includes things like mutation rates, population sizes, selfish DNA, predator- prey relationships and so on. The better question about human eyes might be "Why are our irises so small." Nocturnal animals need to have a huge pupil to let in as much light as possible at night. Because of this, they might have been selected for large pupils and large irises to go with them. Humans spend most of their time awake during the day and have no need for great night vision. Whether we were really under enough selective pressure over this feature to actively change pupil and iris size over time, or whether our genes just happened to limit us to a certain iris size and we then adapted to that by choosing to be awake during the day, is an open question. I suspect that both cases contributed. Brian Foley
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