The eye could not have evolved gradually, because intermediate eyes would have been useless.
There are gradations of functional eyes in the natural world from simple patches of light-senstive cells all the way to the camera eyes we see in humans. Modeling shows that transitions across this entire range could take as little as a few hundred thousand years, as far as the structures of these eyes are concerned.
I discuss elsewhere the general idea that certain things could not have evolved because they supposedly could not have had functional intermediates. Here, I focus specifically on the eye (no pun intended).
I. Could eyes have evolved gradually?
Naturally, because of the timescale involved, no one has observed the evolution of the eye in a laboratory; however, the gradations in the present world offer many clues. Nature displays a whole progression of visual structures, from simple to complex (Ecker 1990:65-66), such that one can see from them how the eye could have evolved gradually. A video from the National Center for Science Education goes through all of it simply:
For much more detail, and multiple lines of evidence, consult Lamb et al. (2007), P. Z. Myers's on-line review of that article, and the special issue of Evolution: Education and Outreach (Volume 1, Number 4) devoted to the evolution of eyes.
Paul R. Gross explains how a model by Dan Nilsson and Susanne Pelger has bolstered the case for the natural evolution of the eye by small steps:
In a 1994 theoretical paper, Nilsson and Pelger modeled one possible evolutionary pathway to the geometry of a fish-like eye from a patch of photo-responsive cells. There were already such cells—among the oldest organisms on Earth—a billion years before there were eyes. Nilsson and Pelger used pessimistic estimates of the relevant parameters (such as the intensity of selection) for their number-crunching. The point was to determine how many plausible, populational micro-steps of variation would be needed, under minimal assumptions, for very weak selection to yield a fish-like eye—and then under reasonable assumptions to convert micro-steps into generations and years. The order of magnitude answer was 350,000—a geological blink of the eye. (Gross 2003)
There has been a strange controversy about the fact that some popular writings by Richard Dawkins (e.g. 1996: 161-165) describe Nilsson's and Pelger's work as a computer simulation, when it is in fact a mathematical model. Creationists got very worked up about this at one point, but as Nilsson himself explains:
I have not considered this to be very serious, because a simulation would be a mere automation of the logic in our paper. A complete simulation is thus of moderate scientific interest, although it would be useful from an educational point of view.
The Nilsson and Pelger (1994) paper remains scientifically sound, and has not been challenged in any scientific journal with a peer review system (Nilsson 2003).
We may regret Dawkins's lack of absolute precision, but the point is in the end a mere quibble, affecting none of the substance or importance of Nilsson's and Pelger's work.
Dawkins R. 1996. Climbing Mount Improbable. New York: W. W. Norton & Co.
Ecker EL. 1990. Dictionary of Science and Creationism. Buffalo: Prometheus.
Futuyma DJ. 1983. Science on Trial: The Case for Evolution. New York: Pantheon.
Gross PR. 2003. A scientific scandal. www.talkreason.org/articles/blurred.cfm#scandal.
Lamb TD, Collin SP, and Pugh EN Jr. 2007. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nat Rev Neurosci 8(12):960-76.
Nilsson D-E. 2003. Beware of Pseudo-science: a response to David Berlinski's attack on my calculation of how long it takes for an eye to evolve. www.talkreason.org/articles/blurred.cfm#lund.
Last updated: 8 Mar 2017
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