Читать книгу Your Body - The Fish That Evolved - Dr. Keith Harrison - Страница 14

T he body shape of any one species at any one time is a collection of features its history has handed down to it. If a species is to survive a change in its environment or to exploit a new way of life, natural selection can only act on the body that exists. It cannot always address a species’ problems in the most efficient way because there may not be the genes for this. If the species is to survive, natural selection must work with the tools history has given it. It’s time for another analogy.

If your ship sank without warning and you were washed alone onto a desert island, you would have to survive with whatever you had in your pockets at the time. The best survival aids for the situation might be a 36-piece set of woodworking tools and a copy of Island Survival (4th edn) by R. Crusoe, but you don’t have these. You would have to use what you did have as best you could and adapt or die.

In your pocket you might have a coin. You sharpen the edge on a stone and use it to whittle points onto arrows for hunting and fishing. A knife would have been better but you don’t have one. The coin will do the job and, in fact, the raised lettering that indicates the denomination helps your grip when cutting. Later you find a better way of sharpening your arrows. You no longer need the coin as a cutting tool but you do need a weight for your fishing line. The ideal solution would be a split lead shot but you don’t have any lead. Instead, you bend the coin double and clamp it over the line. Now the raised lettering aids the grip of the coin on the line. You have found two important uses for something that was initially for a completely different purpose; a purpose of no relevance whatever to your current situation. The coin was not ideal for either job but it worked. You catch fish and you survive. Meanwhile, the coin bears the scars of its past history. It still has the lettering and a mainly curved edge from its life as a coin, and the sharp edge from its life as a cutting tool. The lettering is now useful for a reason never intended when it was first developed; the curved and sharpened edges serve no purpose in a weight, but do no harm.

What we do by craft, evolution does as an automatic consequence of natural selection, but the results can be similar. Like the coin and its lettering, the bodies of many animals include structures now employed for purposes that were not their original use. The teeth of sharks are modified skin scales; the wings of birds are modified arms; the membranes of bats’ wings evolved as skin to cover the body but later extended to provide a large aerodynamic surface. Also, like the curved and sharp edges of the fishing weight, animals’ bodies contain structures which serve no purpose today but which were evolved by ancestors who did need them. These have passed down through the generations ever since. You are probably sitting on the remnants of a tail, a tail evolved and used by your ancestors but now just a line of bones inside your body at the bottom of your spine. Another example would be the dewclaw of dogs. This is the small redundant toe which is seen above the foot and doesn’t touch the ground. The ancestors of dogs had five full toes but, as the dog evolved to move faster, one toe shrank and moved up the leg away from the others (reduction in toe number is common in animals which have evolved to move quickly over the ground – see p127). Dogs now have only four functional toes. Dewclaws serve no purpose today and dog breeders often have them surgically removed. If the dog’s ancestors had remained subject to the pressures of natural selection, rather than forming a relationship with humans and allowing us to control the evolution of their bodies, it is possible the dew claw would eventually have disappeared completely, or at least become wholly internal, like our tails.