I spent some time reading up about woodpeckers yesterday morning. There is a great spotted woodpecker which visits the bird feeder in our garden most days. Never ones to avoid clichés, we have nicknamed him Woody. On Friday, I managed to get this photograph of him.
Woodpeckers' toes are a familiar example of an evolutionary adaptation. Instead of the usual (in birds, at least) three toes pointing forward and one toe back, most woodpeckers have two toes pointing forward and two back. This adaptation makes it easier for them to cling to tree trunks.
Yesterday, I learnt that the scientific adjective to describe this toes-in-pairs phenomenon is zygodactyl. Apparently, several different lineages of birds (including parrots and treecreepers) have independently evolved zygodactyl toes for clinging to tree trunks—demonstrating that Nature isn't afraid of reinventing the wheel.
As we sometimes see individuals of a species following habits widely different from those both of their own species and of the other species of the same genus, we might expect, on my theory, that such individuals would occasionally have given rise to new species, having anomalous habits, and with their structure either slightly or considerably modified from that of their proper type. And such instances do occur in nature. Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing insects in the chinks of the bark? Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing; and on the plains of La Plata, where not a tree grows, there is a woodpecker, which in every essential part of its organisation, even in its colouring, in the harsh tone of its voice, and undulatory flight, told me plainly of its close blood-relationship to our common species; yet it is a woodpecker which never climbs a tree!
As usual, Darwin hits the nail on the head: although evolution through Natural Selection provides organisms with adaptations ideal for certain environments, so might a benevolent creator. But to see an animal, such as a woodpecker, with adaptations better suited for one environment, living in a very different environment is the best sort of proof that something has changed (for which, read evolved). A few paragraphs later, Darwin continues:
He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not at all in agreement. What can be plainer than that the webbed feet of ducks and geese are formed for swimming; yet there are upland geese with webbed feet which rarely or never go near the water…
He who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one of another type; but this seems to me only restating the fact in dignified language. He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may be from its own place.
No matter how well Nature might hone certain species to certain environments, it can only work with the material (species) already available to it—so species' ancestral heritages often show through in their current designs. Stephen Jay Gould referred to this phenomenon as the Panda's Thumb, but I like to think of it as Nature's Kludges.
See also: Can Red Lions Evolve?