University of Utah biologist Jon Seger helps us make sense of the randomness (and nonrandomness) of evolution.
Looking back through the history of a species' genome, mutations do indeed appear to be attracted to certain genomic locations (and likewise repelled by others). But appearances can be deceiving, and selection is a great illusionist. Mutations that initially occur at random may end up seeming to be "directed" in highly nonrandom patterns since most mutations that occur are quickly lost from the population, often in just one generation. The relatively few mutations that are not lost are the ones that contribute to evolutionary change.
Within a population, each individual mutation is extremely rare when it first occurs; often there is just one copy of it in the gene pool of an entire species. But huge numbers of mutations may occur every generation in the species as a whole. At more than six billion individuals, the human species is now so large that every single base pair of the three billion in the genome is mutated several times, somewhere in the population, every generation. Some of these mutations are so harmful that they're eliminated before their carriers are even born. But the great majority of mutations are harmless (or at least tolerable), and a very few are actually helpful. These enter the population as exceedingly rare alternative versions of the genes in which they occur.
Most new mutations are going to be lost just because they are rare (even if they are beneficial); however, very small effects on survival and reproduction may greatly affect the long-term rates at which different mutations accumulate in particular genes and at particular sites within genes. The result is a pattern of evolutionary change that looks nonrandom and in fact really is nonrandom: some sites almost never change, some change occasionally and others change relatively often.
But this does not mean that the mutations themselves occurred nonrandomly. In retrospect, it's as if they occurred where needed. But in fact they just accumulated where needed—first one, then another, and another, over very many generations. Getting two or more helpful mutations together in the same genome may take a while, but if they are not lost from the population, then this will eventually happen in a sexual species.
Sometimes, looking back, biologists can infer that an eye or some other complex adaptation was assembled in a particular way (through a particular sequence of evolutionary changes). This leads naturally to the thought that this adaptation had to be assembled in that particular way, following exactly that sequence of mutations. But a great deal of evidence and theory shows that this is almost never true.
A crude and relatively ineffective light-sensing organ may be much better than none at all, and there may be thousands of different mutations that would slightly improve its functioning in different ways. When one of them occurs and is lucky enough not to be immediately lost and then rises in frequency within the population, it sets the stage for others. But there's no way to predict which mutation will be the next to succeed.
Some recent human adaptations with known genetic histories nicely illustrate this principle. For example, the widespread but not universal ability to digest the milk sugar lactose in adulthood (lactose tolerance) has recently been shown to arise from any of several different mutations in and near the lactase gene. These occur in geographically isolated populations descended from early pastoralists who lived in different parts of Africa and Eurasia. In this case as in others, there appears to have been much randomness in the process that determined which of many possible mutations would be the one that ended up answering the call at a given time and place.
Perhaps it was predictable that adaptation to a novel food resource (the milk of domesticated cows and goats) would occur, but apparently it was not predictable, even in principle, exactly how it would occur.
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4 Comments
Add CommentEven if we theoretically agree that a string of mutations in a particular sequence is required for development of a complex organ as eye, to the extent that the components for a"developing eye" would not confer an evolutionary advantage to a mutant, until all "eye components" are assembled, how would such a mutant have a reproductive advantage in a population to allow ordering of the remaining eye parts?
Reply | Report Abuse | Link to thisIn "The Movies in Our Eyes" (Scientific American, April 2007) Frank Werblin and Botond Roska indicate that, "We have found that specialized nerve cells, or neurons, deep within the retina project what can be thought of as a dozen movie tracks- distinct abstractions of the visual world. Each track embodies a primitive representation of one aspect of the scene that the retina continuously updates and streams to the brain."
Reply | Report Abuse | Link to thisI would like someone to read the above article and to genuinely ask themselves if they believe, as Seger asserts, that the "eye can easily evolve through the same ongoing interaction between mutation and selection that drives the evolution of other adaptations."
The belief that nerve cells can organize themselves and coordinate their various activities to engage in behavior that has purpose and function, and the belief that they can develop the tremendously sophisticated technical capacity to produce, for example, twelve movie tracks that the eye sees as one...such a belief leaves one in disbelief.
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Edited by Borenson at 03/31/2008 1:59 AM
You don't get an eye in one or two mutations. You start VERY simply, with a cell that can detect whether or not light is hitting it. The first "eye" was nothing but a light/dark detector. Then, over MANY, many, many "improvements, we ended up with an eye.
Reply | Report Abuse | Link to thisHow else CAN you explain it? If God made everything, why did he begin with such primitive "eyes" as we see in nature? Why not just give everything the "finished" eye?
Was God learning as he went? Then God IS evolution.
Nobody really knows for sure. But I'm sure of one thing, there isn't a "God" out there that knows whatever I and everyone else does/thinks and judges us on it. Maybe there IS a God who designed and created life...but he SURELY is NOT the God of the Old Testament! Personally, I have to go along with simple evolution because IF there is a "great designer God", then HOW did he come about? There must be other designers that designed him. And going down that path goes nowhere fast...
Evolution, and the keeping of mutations that enhance chances for reproduction seem the best way to go. It's the easiest answer. It has problems and a lot of questions, but it's the best explanation we have...
There is not just one possible string of mutations that can lead to the structure of the eye, some of the mutations could have happened in a different order, and with different species, different mutations led to different eyes. The developing eye didn't have to wait until all the eye components were assembled for the mutant to gain an advantage. The only requirement of the particular sequence of mutations is that each mutation made the eye work just a little bit better, and made the owner of the eye more likely to survive and pass those mutant genes on to its offspring. And I sit here squinting at this screen, I realize that the developing eye has definitely not finished developing.
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