|MadSci Network: Evolution|
This question really comprises two distinct questions: (1) How can evolution produce something complex, like an eye or a heart, by a succession of random mutations? and (2) Why do these random changes put the lens (for example) in the right place, instead of some wrong place?
The first question is addressed below, but the short answer is that something as complex as an eye, or even a lens, arises not from one big change, but from a long succession of small changes.
The second question is relatively easy to answer: as the questioner seems to suspect, a random mutation is more likely to do harm than good. However, a random mutation that does something harmful results in an organism that reproduces less successfully than its mutation-free relatives (to a Darwinian, this is the definition of "harmful"), with the consequence that the mutated gene fails to spread and eventually dies out. Once in a great while, a random mutation happens to do something beneficial, and, by improving reproductive success, spreads copies of itself throughout the population, thereby improving the base on which subsequent random mutations may build.
If you doubt that a mutated gene could really spread throughout the population if it conferred only a small benefit on its organism, try the following simple exercise. Assume that some gene in some individual organism has changed in a way that increases by just one percent the average number of viable offspring produced by an organism that has the changed gene. Assume that just one organism in a population of one million has this changed gene. Compute the fraction of the population that will have the changed gene 10,000 generations later. (The answer will show that the old gene has become exceedingly rare.)
Now, back to the first question: how can something as complex as an eye result from this process? The answer summarized above is that it must happen as a succession of small changes. This puts an interesting constraint on the evolutionary process, since each of those small changes must by itself improve the success of the organism.
Can you build an eye using only small changes? We're so in love with our good vision that we tend to think an imperfect (say, lensless) eye would be useless; but even people with bad vision usually feel that bad vision is better than worse vision --- or no vision at all. So we must be careful not to underestimate the value of imperfect eyes. In fact, in nature one finds eyes of widely varying engineering finesse, ranging from, say, the lensless, light-sensitive eyespots of the flatworm, to the intricate apparatus of the vertebrates or the octopus.
The particular example of the evolution of the eye is treated at great length in Richard Dawkins's "The Blind Watchmaker", and some discussion on exactly this subject appears on a web page about Dawkins's recent book, "Climbing Mount Improbable".
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