Working Out Adaptation

How to Assess Adaptation

The morphology of an organism is controlled by at least three major factors.
  1. Adaptation. But how do we tell what characters are associated with adaptation? By recognizing and allowing for two others:
  2. Phylogeny: the characters that an organism has inherited from its distant and near ancestors. For example, humans have four limbs, a character that we have inherited from a remote Devonian ancestor that was still a fish. What we currently do with those limbs may be different, but we nevertheless have them (and five fingers on the end of each, which is inherited from a Carboniferous reptile ancestor of ours.)
  3. Ontogeny, growth. Typically, organisms change shape as they grow, and usually get bigger. The classical examples of these are bivalves and brachiopods, animals that live inside shells. The shells grow by adding more material at the edges, yet the shell that housed the baby clam is still present as a part of the central section of the shell. The bivalve cannot change the shape of the juvenile part of its shell, even though it might benefit by doing so: it can grow only by adding to it. This could conceivably become a constraint on the range of shell shapes that brachiopods and bivalves can grow.

    [ASIDE: I pointed out that bivalves can do more with their shells than brachiopods can because they have an extra fold of mantle tissue at the edge of the shell.]

    There may be clever ways to get around the constraints of simple shell growth. For example, brachiopods have little pores in their shells, which seem to act as deterrents to creatures boring holes in a brachiopod. For efficiency, the brachiopod should insert these pores uniformly, in hexagonal patterns, even though it is growing its shell non-uniformly, in growth lines that are wider at the front than at the sides. So I think that it accomplishes that by setting in new pores at points where the chemistry of the pores reaches a low point, resulting in a uniform pattern of pores on a non-uniform shell.

    Many structures in nature are arranged in hexagons, in fact.

    Plates on a sea-urchin.
    Sea-urchins have shells made up of little plates, and the pattern of plates is standard within any species. Why does a sea-urchin lay out its plates in set patterns? This question was posed by David Raup, who did experiments on soap-bubbles laid out along wires. If the wires were a certain distance apart, the resulting pattern of soap-bubbles roughly resembled that in a sea-urchin. Since soap-bubbles take on a pattern of least energy, perhaps the sea-urchin too lays out its plates in a least-energy arrangement. So we'd take that as our starting assumption for a growth pattern, and any deviations would then be related to factors other than growth.

    Individuals in a colony of bryozoans.
    Bryozoans form colonies of little animals that bud off new individuals. Those individuals are clones of one another, genetically identical. Yet they grow in different positions as the colony grows, with feeding individuals sucking water downward in feeding currents on to the top of the colony. Eventually the colony reaches a size where adding more down-current is not going to be worthwhile, so growth stops and the colony reproduces by setting free larvae instead of growing any more itself. The larvae go off to begin new colonies.

    So now, knowing that we can account for growth and phylogeny, we will look at adaptation and try to assess its significance.

    The three major ways of assessing adaptation are by using

    1. Homology
    2. Analogy
    3. Engineering Principles

    Homology is the study of organs or organ systems that have a shared ancestry. Thus all tetrapods share in their ancestry the possession of four legs. They may have been evolved to do different things: for example, a baseball pitcher's arm, the front leg of a brontosaur, carrying 12 tons on each foot, and a bat's wing are all homologous to the front flippers of a whale.

    Analogy is the study of organs or organ systems that do the same job, no matter whether they share the same ancestry. (Structures can be analogs and homologs at the same time.)

    Analysis of a fossil by analogy is very useful if we don;t have a living representative of a group we need to study. So one might compare a Triceratops with a rhinoceros. They are both big, horned, 5-ton herbivores, even though one is a dinosaur and the other is a mammal.

    Where organ systems in two organisms are mostly analogous, we can speak of convergent evolution. Thus placental moles and marsupial moles have evolved to be almost mimics of one another: their limbs, body shapes, diet, and burrowing ability are uncannily alike, even though they are only related very distantly to one another. Dolphins and the extinct reptiles called ichthyosaurs are another example.

    In another comparison, creatures have evolved different (analogous) ways of achieving the same diet. Thus the beak and tongue of a woodpecker, used for prying up bark and eating insects underneath it, are analogous to the teeth and fingers of the little lemur called Daubentonia from Madagascar, which does exactly the same thing with different "tools". The woodpecker analogy is matched also by marsupials in Papua New Guinea, by the cactus finch of the Galapagos, and by an extinct bird from New Zealand.

    Brachiopods and bivalves both have to open and close their shells to make a living. But brachiopods use two sets of opposing muscles, one to open and one to shut the shell. Bivalves use closing muscles that work against a rubbery pad along the hinge-line of the shell.

    Engineering Principles. One might ask how a worm would move to crawl through mud and eat it efficiently. We simply ask an engineer to design us an efficient system. One might be a system in which a worm starts at a point, and simply winds round and round in an increasing spiral, meeting new mud at every point. (Going back on its tracks would involve eating already-digested mud, not a pretty thought.) And indeed, although early trace fossils of worm tracks from the Cambrian show that worms were not very good at this 540 m.y. ago, the fossil record shows them quickly getting better at it, evolving more efficient feeding very quickly.

    Suspension feeders, who pump water and filter prey from it, would be poor engineers if they filtered the same water twice. So bivalves and brachiopods, for example, typically pump water in from one direction and pump it out in a different direction, getting rid of the filtered water.

    Stephen J. Gould has made a fuss about "Just So" stories of adaptation in fossils, which can be made up and told even if we have no way of testing them . I argue that they can be tested, and here is an example.

    Most trilobites have eyes that give a set of black and white dots that make up a picture in the same way as a newspaper photo does: many dots, from many lenses, give one picture for each eye. But some trilobites have special eyes, in which the many lenses have REAL lenses on them: each lens makes a picture, so that there may be dozens of pictures going into each eye.

    What's more, the lenses in each eye are arranged in lines, which happen to be vertical lines when the trilobite is in its life position. Why?

    A student of mine once suggested that he had seen the same thing: a set of overlapping air photos, laid side by side, can be examined to give a 3-D image. Perhaps, we thought, these special trilobites were able to SEE IN STEREO IN ONE EYE! So we wrote it up. But how could we test the idea?

    If any one of these tests had failed, the story would not have worked. But it passed all the tests. So it's not just something we made up, it also happens to be true (at least, it still survives tests.) ASIDE: you would predict that to see color, these trilobites would all have had to live in very shallow water. As far as I know, they all did.]

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