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Can eagles have sharper eyes? Or cheetahs run even faster? Or must the evolution of any adapted trait in nature reach a limit?
James V. Stone
Buxton, Derbyshire, UK
Yes, the evolution of any adapted trait in nature must reach a limit. In principle, this limit is defined by the laws of physics, but, in practice, it may also be defined by the constraints of biology.
Physics dictates there must be a fundamental limit to the resolution of any eye. The eye’s lens forms an image on the retina, which is densely packed with photoreceptors. These act like optic fibres, guiding light to light-sensitive photopigments. But a photoreceptor with a diameter less than half the wavelength of visible light (0.5 micrometres) becomes increasingly “leaky”, so can’t guide light efficiently. A diameter of 1.5 μm is required for reasonably efficient transmission. This sets a limit on the smallest useful diameter of photoreceptors, which constrains the maximum useful photoreceptor packing density (photoreceptors per square millimetre).
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Given the constraints on eye design, it seems likely that evolution has crafted the eye of an eagle for optimum performance
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In practice, the smallest diameter of a photoreceptor is limited by the size of an essential component of all cells: the mitochondrion, which supplies cells with energy in the form of adenosine triphosphate. A mitochondrion is an ellipsoidal structure with a width of about 1 μm. By contrast, the diameter of each photoreceptor is 1.6 μm in an eagle, so this is probably the lower limit dictated by the width of a mitochondrion (a photoreceptor must contain many mitochondria).
The lens of a wedge-tailed eagle (Aquila audax) eye has a focal length of 22 millimetres, so if the retinal image “matches” the packing density implied by a photoreceptor diameter of 1.6 μm, then this predicts a lens diameter of 8.4 mm (assuming a visible light wavelength of 0.5 μm). In fact, the lens diameter is 10.5 mm, so the retinal image resolution is higher than required by the photoreceptor packing density.
However, because the brightness of an object’s image decreases inversely with the object’s distance squared, photoreceptors are starved of light from distant objects, especially in dim conditions. This is particularly relevant for eagles, which search for distant prey under variable light conditions.
Given the demands and various constraints on eye design, it seems likely that evolution has crafted the eye of an eagle for optimum performance in the real-world conditions of a predator.
John Davies
Lancaster, UK
Of course, but at what cost? Evolution is all about survival of the fittest. An eagle has retinal cells at five times the density of a human, plus two foveas, the retinal area where the cells are concentrated, and their eyes are much larger in proportion to their skulls than ours are. More cells and foveas would necessitate even bigger eyes. But would those make the super-eagle even more successful? If so, then fine. If not, it is down among the dinosaurs!
Averell Kingston
Banbury, Oxfordshire, UK
Why would cheetahs need to run faster? If they can run fast enough to catch adequate amounts of food to enable them to breed and have babies, that is fast enough. However, if their prey evolved to run faster than at present, then those cheetahs that could run fastest would be the most successful at hunting, so would leave the most offspring in the next generation. This could lead to faster cheetahs. In order for evolutionary change (faster cheetahs) to happen, there needs to be selection pressure, in this case faster prey.
As for eagles’ eyesight, a possible selection pressure could be something that reduces visibility, for example mist, increased vegetation on the ground or, in the case of sea eagles, cloudier water.
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