We’ll get back to weightier matters before long, but the demands of the workplace press heavily just now, and there will be scant time over the next few days for serious posts.
Tonight, though, I have an interesting little item for you natural-history buffs.
Evolutionary theorists often talk about “Design Space”; by this phrase they refer to the ungraspably enormous set of all the possible ways to construct a living organism. The actual path of evolution — that is, the ever-branching tree of life, past, present, and future — occupies only a vanishingly small subset of this colossal imaginary space.
Evolution is constrained in various ways. Obviously it cannot make what cannot be made — animals that can swim through solid rock, or that expend more energy than they consume — but there are other, less obvious limitations as well. Many of the mechanical devices that we have invented for our own advantage are not, though they seem simple enough, suitable for use in living organisms. For example, there are no multicellular animals with wheels; the problem of connecting nerves, blood vessels and so forth to a freely rotating body part puts such a design out of reach.
Another constraint is that the tree of life is connected. Descent with modification means that evolution must work, when putting together the child, with the inventory of parts found in the parent. There are, to be sure, startlingly large modifications that can be made by small mutations in the genome, but the continuity of the tree means that radical jumps in Design Space are out. It often happens as well that even plausible and attractive nearby places are off limits to a growing branch of the tree, simply because to get to them means passing through intermediate designs whose fitness is sharply reduced.
Sometimes different branches may converge on the same area, though; if a particular feature confers an enormous fitness boost — the ability to fly, for example — various lineages may implement it in different ways. One of the best-known examples of this sort of convergent evolution is vision. The ability to react in useful ways to the frequencies of light at which the Earth’s atmosphere is transparent is so very helpful, in so many ways, that natural selection has generated an impressive range and variety of eyes — from the rudimentary pigmented spot of the rotifer, to the compound eyes of insects, to the all-seeing eye of the eagle. We humans have gone even further, devising an assortment of prosthetic enhancements: from simple optical tools such as spyglasses, magnifying lenses, and spectacles all the way to electron microscopes and the Hubble Space Telescope.
To make a sensitive and sophisticated optical device, one needs to gather light, and bend it so as to concentrate it upon some sort of detector. One way to do this is by refraction: light passing at a non-perpendicular angle through a boundary between air and some other transparent medium will bend. By using a rigid medium, such as glass, and shaping this boundary in just the right way, it is possible to construct a lens that will project a well-focused image. This is how our eyes, and indeed all vertebrate eyes, work.
Lenses aren’t perfect, though. One drawback is that when passing through a refracting lens, different wavelengths of light will be deflected to different degrees (the familiar principle behind the optical prism, and the rainbow). In a refracting optical device, such as a telescope, this means that the focal length will be different for different colors, and the image will be blurred: a problem known as chromatic aberration. Another problem with refracting lenses is that it’s hard to make big ones: the larger and thicker they get, the more perfectly transparent they have to be, and the heavier and more unwieldy they become.
There’s another, elegant solution, however: mirrors. Light reflected by a mirror bounces off at the same angle regardless of wavelength, so chromatic aberration is eliminated at a stroke — and because, with a mirror, all that matters is the reflective surface, it is possible to make far larger optically accurate mirrors than lenses. It was the incomparable genius Isaac Newton who first realized a telescope could be made in this way, and nowadays all the world’s largest telescopes are reflectors, not refractors.
But despite the advantages of reflection over refraction, there has never, it seemed, been a living eye that occupied that particular corner of Design Space. Until now, that is. Meet the spookfish.