Evolution does not work towards fitness alone… bacteria are often as fit (if not more fit) than human beings. Evolution seems to work towards optimal fitness within increasingly complex designs. If evolution was concerned only with fitness, there would be as many examples of complex creatures devolving to simpler forms (which happen to be more "fit") as there are of simpler creatures evolving towards more complex forms. But I'm not aware of any such examples, leading me to conclude that either: 1. There is always an evolutionary advantage to greater complexity. --or-- 2. Evolution simply runs towards the more complex.I'm choosing option number two. If there were an evolutionary advantage to greater complexity, I would expect to see no bacteria left in the environment – they would be out-competed by the more complex life forms. Of course, bacteria are still everywhere. So I have to think about option number two. Phil makes an important point about de-evolution – it doesn't happen. Complexity is a one-way ratchet. I think the easiest answer to this is that nature doesn't have a good eraser. If the environment changes, all surviving life forms will become more complex in an effort to adapt, never less complex. It's like a Champaign pyramid where the top glass represents the least complex form of life. Before levels of increasing complexity are filled, the simpler level above must be filled. The simpler levels are not abandoned as the lower more complex levels fill. Simple gives birth to complex, but complex can never give birth to simple. DNA would no sooner abandon its precious store of information than Champaign would flow uphill. The only eraser is extinction. If a species grows too complex for it's environment, perhaps by being too highly adapted to a narrow biological niche, then environmental change might wipe the species out. But extinction is not directly recorded into the DNA of anything alive today. Nothing alive today descends from an evolutionary dead-end. And so DNA is the ultimate spaghetti-code. Patched-on "it'll do" solutions have been cobbled together sometimes to solve problems we no longer have. People in industrialized nations, for example, are struggling with obesity because our metabolism is so thrifty - an unneeded gift from our starving ancestors. Posted by Stephen Gordon at March 2, 2004 02:51 PM | TrackBack
Over at the Loom Carl Zimmer talked about this recently. Unfortunately, I can't find the exact entry but he gave several examples of cases where evolution did reduce the complexity of an organism. Some examples I can think of:
Mitochondria. The theory is that mitochondria (sub-cellular organisms inside most cells that produce chemical energy for use by the cell) started out as full fledged bacteria that first became symbionts with other organisms, then eventually completely merged with them. In the process they lost the vast majority of their DNA and functionality.
Many types of parasites. Tapeworms, for example have extremely simple bodies because their host does most of the work for them.
Virsues might also be an example, since none of them are able to live without their hosts. However, I'm not sure how viruses evolved, so I don't know if they started out as more complex organisms. There is at least some evidence, though, that viruses actually started out as a segment of DNA from a much larger organism. Especially in plants, but even in humans there are some stretches of DNA that are prone to cut themselves out of their host genome and wander freely.
However, I don't think it's fair to call this 'de-evolution'. In all cases there must have been some reason why reduced complexity was more favorable than increased complexity.
Posted by: Andrew Salamon at March 2, 2004 03:40 PMFirst, a disclaimer: I strongly subscribe to Wolfram's work on cellular automata. That said, simplicity is possible, but only in outward appearance. It would be more likely that expressions which are not useful or detrimental under certain conditions are suppressed, not eliminated. Even the simplest of CA tend toward the appearance of complete randomness (complexity) when run for millions of iterations; they don't tend to run towards predictability. Run a few rounds of "Game of Life", try it out for yourself.
Posted by: rayne at March 2, 2004 05:30 PMHey, I just realized...snakes!
Posted by: Phil at March 2, 2004 10:13 PMSnakes and blind cave fish are good examples of animals that have simplified structurally. This doesn't mean that the DNA coding for those lost limbs and organs just disappeared.
On the other hand, DNA coding that is unused or is of no benefit tends to evolve quickly. There is no loss of fitness if an organism has a mutation in unused coding.
Posted by: Stephen Gordon at March 2, 2004 10:39 PMColonize Mars. Take any ten species you like.
Over the next million years, assuming that the planet is closed to further colonization or contamination, tell me how the complexity of humanity changes.
I bet your descendants will fill niches that are "less complex".
Might it be that the force driving increase in complexity is simply that ecological niches of "lower complexity" are all filled?
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