|MadSci Network: Evolution|
Hi -- There have been many studies of how long it would take for some particular evolutionary change to be achieved, but these normally are focussed on the development of particular organs or the time it takes to adapt to new environments. Dawkins in several of his books discusses calculations for how long (specifically how many generations) may be needed to convert a crude eye into a more sophisticated imaging device, or other examples. To calculate how long it would take to go from a single cell to a human being, you would have to know ALL the genetic changes which took place over history. This is not possible yet, since we have not mapped out the human genome fully, nor do we understand the interactions between genes (There is rarely one gene "for" individual traits. Genes act in concert, and we are just now understanding some of the complexities of these interactions). Even in the restrictive case of trying to compute (say) the development of the eye, there are many surprises. These calculations use "toy" or "model" genes, which may or may not correspond to real genes, but the hope is that these calculations give about the right time scales. The calculations show that evolution can proceed surprisingly fast. For example, if a gene arises which produces a 1% improvement in visual acuity, so that (say) there is a 1% differential reproductive success, then the animals with the better eye will completely dominate the population in only 1000 generations. Since many species reproduce annually or faster, these 1000 generations can occur in a very brief span of geological time. If you have 10,000 or 100,000 generations, then there can be a tremendous improvement in the eye (or any other function) in a very short period of time. When you are faced with the question "how could purely random mutations create such complex beings?" you need to remember that the random mutations are only the first step. The environment imposes a superstrong filter on these mutations, so that only those which improve the chances of surviving to reproduction are those which actually make it. The filtering by the environment is decidedly NONRANDOM: true there are occasionally accidents, but most of the time survival is determined by an animal's (or plant's) success in finding food, keeping enemies away, and reproducing.
Try the links in the MadSci Library for more information on Evolution.