If mutations occur at random over a species' entire genome, how can an organ as complex as an eye evolve? —V. Rautenbach, London

University of Utah biology professor Jon Seger explains:

Although it is highly unlikely that such an intricate and useful organ would arise spontaneously from random hereditary accidents, an eye can easily evolve through the same ongoing interaction between mutation and selection that drives the evolution of other adaptations.

Within a population, each individual mutation is extremely rare when it first occurs; often just one copy of it exists in the gene pool of an entire species. But huge numbers of mutations may occur every generation in the species as a whole. Some of these mutations are so harmful that they are eliminated before their carriers are even born. But the vast majority of mutations are harmless or at least tolerable, and a very few are actually helpful. These surviving mutations enter the population as exceedingly rare alternative versions of the genes in which they occur.

Most are then lost just because they are rare; however, very small effects on survival and reproduction may significantly affect the long-term rates at which different mutations accumulate in particular genes and at particular sites within genes. The resulting pattern of evolutionary change looks nonrandom and, in fact, really is nonrandom: some sites almost never change, some change occasionally and others change relatively often.

Nevertheless, that does not mean that the mutations themselves occurred nonrandomly. In retrospect, it is as if the mutations occurred where needed, but appearances can be deceiving, and selection is a great illusionist. In actuality they just accumulated where needed—first one, then another and another, over very many generations. Although getting two or more new “cooperating” mutations together in the same genome may take time, they will eventually find one another in a sexual species, assuming they are not lost from the population.

Visually oriented vertebrates such as humans have stunningly intricate eyes, but there is great variation in most aspects of the organ even within vertebrates, and several fundamentally different designs have emerged in animals as a whole. This diversity shows with living examples how simple, nonfocusing light sensors could gradually be elaborated and refined to become the complex, sophisticated imaging eyes of the kinds we know today.

To an organism that needs information about fast-changing aspects of its environment, a crude light-sensing organ may be much better than none at all. Given such a primitive eye, there may be thousands of different mutations that would slightly improve its functioning in various ways. When one such mutation occurs, is lucky enough not to be immediately lost and then rises in frequency under the force of natural selection, it sets the stage for others. Given enough time and continued selection, this process will readily improve the functioning of the eye, often by making it more complex.

Why do parrots have the ability to mimic? —H. Messing, Westwood, Mass.

Michael Schindlinger, an avian biologist at Lesley University who maintains the Web site www.freeparrots.net, replies:

Parrots most likely imitate for reasons that seem to vary from species to species. In some cases, the regional dialects that arise from this mimicry may help males and females from similar areas find—or perhaps avoid—one another. Similarly, song learning in some species allows territorial neighbors to know one another and helps to distinguish drifters. And one study on small Australian parrots known as budgerigars reports that they seem to use call similarity in judging mates.

Imitative vocal learning is also a reliable display of neural functions that may be under consideration by a potential mate or ally, including hearing, memory and muscle control for sound production.

Playback studies of geographic dialects from wild parrot populations have shown that birds react more strongly to their local tongue. This phenomenon hints at another important benefit of imitation: to better command the attention of a potential listener by producing sounds for which the listener already has a memory, or a so-called neural template. The existence of this preformed perceptual template makes another parrot's vocalizations easier to perceive in a noisy environment.

HAVE A QUESTION?... Send it to experts@SciAm.com or go to www.SciAm.com/asktheexperts