Full House +xtras

  In other words, progress as a purely incidental consequence (and limited to a small right tail) just won't do as a validation for our traditional hopes about intrinsic human importance—the spin-doctoring that prevents the completion of Darwin's revolution in Freud's crucial sense of pedestal smashing (see chapter 2). I think that virtually every evolutionist who has ever considered the issue in the terms of this hook (that is, as a history of variation in all life—the full house—rather than as a tale told by abstracted means or extreme values only) has come to the conclusion that the appearance of progress as an expanding right tail must arise as an incidental consequence, not as a main result.

  The traditional hope for intrinsic progress as an explicit result must therefore rest upon a fallback position—not nearly so grand as the original formulation, but a source of some potential solace nonetheless. Even if we must admit that an expanding right tail arises as an incidental consequence of origin at a left wall with subsequent proliferation, could we not also hold that other forces operate as well on life's bell curve—and that some of these other forces do include an intrinsic and predictable drive to progress?

  As stated in point 6 of my epitome (see page 173). such an argument could be true, would take the following form, and can be tested empirically: life as a whole begins at the left wall and is therefore free to expand in only one direction. Therefore we cannot use life-as-a-whole to test for drives to progress—because upward movement of the mean must, in part, reflect the left wall's constraint, not any potential drive. But if we could study the history of smaller lineages with founding members far from the wall—and therefore free to vary in either direction—then we could devise a clear test for general progress. Do such "free" lineages show a tendency for increases in complexity to be more frequent, or greater in effect, than decreases? It most free lineages show a trend to increasing complexity, then we could assert a general principle of progress as a main result for its own sake. The full phenomenon of life's expanding right tail would then arise by two separate and reinforcing processes; an incidental consequence based on constraints of origin at the left wall, and a direct result of intrinsic bias to greater complexity in lineages free to vary in both directions.

  This conjecture is logically sound but, by all evidence so far in hand, empirically wrong. I would raise two arguments against intrinsic progress, the first briefly and subjectively, the second at greater length and based upon some compelling recent evidence.

  First, if I were a betting man, I would wager a decent sum (but not the whole farm) on a small natural preference for _decreasing_ complexity within lineages, and not for the traditional increase, if any general bias exists at all. I make this surprising claim because natural selection, in its purest form, only yields adaptation to changing local environments. These changes should be effectively random (with respect to "progress"), for fluctuations in climate show no temporal trend. A bias for or against increasing complexity therefore requires a general advantage for one direction as life plays its Darwinian game. I can think of a reason why a bias for decreasing complexity might exist, but I cannot defend any corresponding preference for increases. Hence I would bet that a slight overall bias for decreasing complexity might well prevail in the aggregate of all lineages.

  I have long been entirely underwhelmed by the standard arguments for general advantages of increasing complexity in the Darwinian game—adaptive benefit of more elaborate bodily form in competition for limited resources, for example. Why should more complex conformations generally prevail? I can imagine such an argument for mammalian brains—if complexity translates to rising flexibility and computing power. But I can envisage just as many situations where more elaborate forms might he a hindrance—more parts to fail, less flexibility because all parts must interact with precision.

  But one common mode of Darwinian success (local adaptation) does entail an apparent preference for substantial decreases in complexity—namely, the lifestyle of parasites. We are not speaking here of an organic rarity, but of a mode of life evolved by probably hundreds of thousands of species—a substantial percentage of all living forms. Not all parasites gain adaptive benefit through simplification, but one large group of species certainly does—those that live deep within the bodies of their hosts, permanently attached and receiving all their nutrition by commandeering the blood supply, or some of the food already digested by the host. Such species require neither organs of locomotion nor digestion, and natural selection favors their loss. One or a few novel organs might evolve for special needs—hooks for attaching to the host, or suction devices to drain off food, for example—but these elaborations are more than offset by a far greater number of lost organs.

  Often these immobile parasites become little more than bags or tubes of reproductive tissue—simple machines for propagation attached to the internal organs of their host. _Sacculina_, the famous barnacle parasite of crabs and other crustaceans, consists of a formless sac (acting as a brood pouch) attached to the crab's abdomen, with a stalk protruding inside to a system of roots that drain food from the crab's blood spaces. A twenty-foot-long tapeworm in a human intestine may contain of hundreds of sections (strobilae), each little more than a simple sac containing members of the next generation. The entire phylum Pentastomida, parasites of the respiratory tract of vertebrates, builds an elaborate organ for sucking blood, but no internal parts for locomotion, respiration, circulation, or excretion.

  Thus, if "standard" natural selection on free-living creatures produces no bias in either direction, and if parasites tend to become simplified while no countervailing bias toward greater complexity exists, then a small overall tendency toward decreasing complexity may characterize the history of most lineages (as their parasitic species simplify, while their free-living species show no trend). Please note that the right tail for the full bell curve of life will still expand through time—even if a bias toward decreasing complexity operates in most lineages. For species moving left to less complexity enter a domain already inhabited, while rarer species moving right may enter a previously unoccupied realm of complexity. The drunkard will end up in the road even if, for some reason, he moves more often toward the wall than toward the gutter—for he bounces off the wall but falls prostrate (and permanently) in the gutter. An entire system can extend its extreme in one direction even if individual lineages have a bias for excursions in the other direction.

  But I can also think of an argument against my own claim for parasites. Adult forms do indeed tend to evolve toward greater simplicity, but when we confine our attention to adults, we fall into another conventional bias (not as general or pervasive, no doubt, as our preferences for progress, but a seriously distorting limitation nonetheless). A human being is not defined by the nongrowing form of adult years; kids are people too. Evolution shapes a full life cycle, not only an adult body. The immobile blood-sucking or food-draining adult parasite may have evolved toward greater simplicity compared with free-living ancestors, but full parasitic life cycles often change in the other direction toward great elaboration, sometimes with adaptation to two or three different hosts in the course of a full ontogeny.

  The adult _Sacculina_ may be an external blob attached to some internal roots, but the larval life cycle is astonishingly complex (see Gould, 1996)—several free-living planktonic forms, followed by a settling phase that cements to the crab, grows a dart that pierces the crab's body, and then injects the few cells that eventually grow into the adult blob and roots. Similarly, pentastome larvae first bore through the gut of an initial host. When a vertebrate eats its first home, the matured pentastome moves to the respiratory tract either by crawling from the vertebrate's stomach to the esophagus and then boring through, or by tunneling through the intestinal wall and in to the bloodstream. The pentastome then attaches to its final site by means of complex hooks surrounding the mouth.

  I therefore have little confidence that we can specify a clear bias one way or the other on general principles. But we
do have a wealth of empirical data available for study. After all, the founding species of most multicellular lineages docs not begin at a wall—and subsequent evolution remains free to produce either more or less complex species. If we can agree on a measure of complexity, and document enough lineages, we may be able to extract a general conclusion. This subject has just begun to interest paleontologists In the past few years. We have not yet compiled nearly enough cases for any confident general solution. But the initial studies offer great promise, for we have at least made this vital subject tractable and testable. And the first few cases all point in the same radical direction—no bias toward increasing complexity has yet been measured.

  This line of research has been pioneered by Dan McShea of the University of Michigan, now at Santa Fe's Institute for the Study of Complexity. Much of the technical literature must focus on providing an unambiguous and quantifiable definition for a very fuzzy vernacular term with a wide variety of meanings, some contradictory—namely, complexity itself. What do we mean when we say that a thing is more complex than something else? Several criteria fit our vernacular sense, depending upon the context. Complexity has morphological, developmental, and functional aspects. A junk heap (to use an example favored by McShea and Thomas) may be morphologically very complex (in consisting of so many highly varied and independent parts) but functionally quite simple (just glop for a landfill). On the other hand, what is functionally simple for us might be quite complex to other users—in this case, to the seagull who must distinguish all the little bits while searching for morsels of food.

  I do nor wish to address this technical subject at length in a book for general readers (but see McShea, 1992, 1993, 1994, and Thomas, 1993, for interesting discussion), though the importance and nature of the problem must be recorded. I do not think that any general solution can be found—because "complexity" is a vernacular term with several legitimately different meanings, and we may well be interested in all of them. For science as "the art of the soluble" (to use P. B. Medawar's felicitous phrase)—an enterprise dedicated to posing answerable questions—we must only resolve that we will choose a rigorously quantifiable definition of complexity and be very clear about which aspects of vernacular meaning will be thus addressed, and which omitted. (Someone else, or you yourself in a subsequent study, may then measure other aspects of complexity.) The literature has been admirable on this account, and therefore happily free of the muddiness that accompanies so much science.

  McShea has favored a morphological definition—not because he views this meaning as closer to some vernacular norm, but because it permits well-defined measurement and rigorous testing. He writes (1996): "The point is to rescue the study of biological complexity from a swamp of impressionistic evaluations, biased samples, and theoretical speculations, and to try to place it on more solid empirical ground." McShea employs the following conceptualization to construct his quantifications:

 
  The complexity of a system is generally acknowledged to be some function of the number of different parts it has, and of the irregularity of their arrangement. Thus, heterogonous, messy, or irregularly configured systems are complex, such as organisms, automobiles, compost heaps, and junk yards. Order is the opposite of complexity. Ordered systems are homogeneous, redundant, or regular, like picket fences and brick walls (1993, page 731).

  >

  In his major study of the vertebrate spinal column, for example, McShea (1993) operationalizes this definition by measuring complexity as the degree to which individual vertebrae differ among themselves. (In the less complex backbone of a fish, forty or more vertebrae may be effectively alike as simple discs of similar size; the more complex mammalian spine has fewer vertebrae differentiated into the varied forms and sizes of neck bones, back vertebrae, and sacral discs that support the pelvis.) In practice (see Figure 32). McShea measures six variables (five linear dimensions and an angle) on each vertebra and then calculates the difference among vertebrae. He uses three assessments of complexity as variation among vertebrae: (1) the maximal difference between any two vertebrae of the same spinal column; (2) the average difference between each vertebra and the mean for all vertebrae; and (3) the average difference between each pair of adjacent vertebrae.

  McShea's framework for testing harmonizes perfectly with the perspective of this book. He holds that trends come in two basic modes with strikingly different fundamental causes. He names these categories _driven_ and _passive_, and argues that they represent natural "kinds," not just conceptual conveniences for human understanding. He writes (1994, page 1762): "These results do raise the possibility that the passive and driven mechanisms may be natural categories and that they may correspond to distinct and well-defined causes of large-scale trends."

  Driven trends correspond to the traditional view of an overall movement achieved because each element evolves with a bias for change in this direction. A driven trend to complexity would arise because evolution generally favors more complex creatures—and each species of a lineage therefore tends to change in this manner, (in other words, natural selection acts as a driver, conveying each vehicle in a favored direction.) Passive trends (see Figure 33) conform to the unfamiliar model, championed for complexity in this book, of overall results arising as incidental consequences, with no favored direction for individual species, (McShea calls such a trend passive because no driver conducts any species along a preferred pathway. The general trend will arise even when the evolution of each individual species confirms to a "drunkard's walk" of random motion.) For passive trends in complexity, McShea proposes the same set of constraints that I have advocated throughout this book: ancestral beginnings at a left wall of minimal complexity, with only one direction open to novelty in subsequent evolution.

  McShea proposes three tests for distinguishing driven from passive trends:

  1. THE TEST OF THE MINIMUM. In passive systems, minimum values of complexity should be preserved by some species throughout the expanding history of a lineage because no general evolutionary preference for complexity exists, and some species should therefore do best by remaining simple. In driven systems, both minimum and maximum complexity should increase through time because higher complexity confers such general advantages that evolution of all species should be biased in this direction. (The preservation and continuing enhancement of life's bacterial mode strongly points to the passive mode for life as a whole.)

  This test, although indicative, does not fully distinguish passive from driven trends because even a driven trend might permit a few species to retain minimal values. (In a driven trend, the minimum might not disappear, but these lowest values should at least become less frequent over time.)

  2. THE TEST OF ANCESTOR-DESCENDANT PAIRINGS. This powerful and obvious test identifies an ancestral species for an expanding lineage and then simply tabulates all descendants to judge whether most become more complex, simpler, or stay the same. In principle this is the most decisive test of all. But in practice we cannot always use it because the fossil record is so imperfect. We often do not know the ancestral species, or we do not have enough descendants to make a proper randomized test of subsequent directions.

  3. THE TEST OF SKEWING. For life-as-a-whole, both the passive and driven mechanism can produce the same overall result of a right-skewed distribution with an expanding tail at maximal complexity. McShea argues that we might distinguish passive from driven modes by studying the skewness of component lineages that begin far from the wall and can therefore vary in either direction (see Figure 34). In driven systems, the component lineages should also tend to be right skewed because all species experience the bias at progress as a favored direction and should therefore contain more species moving along this preferred pathway, thereby stretching the en tire distribution toward the right. But in passive systems, component lineages should develop no skew because increases and decreases in individual species should be equally common—that is, as many species should move leftward to less complexi
ty as rightward to more elaboration.

  In his major study, McShea (1993, 1994) has applied these tests to evolution of the vertebral column. A general trend obviously exists for vertebrates as a whole because the first vertebrates were fishes with a backbone built of essentially identical elements, while later mammals evolved considerable variation among vertebrae along the spinal column. But is this trend passive or driven? (Tradition says driven, but one fact certainly leaves maximal "room" for passivity. Much like the initial living thing at the left wall of minimal complexity, or the founding foraminiferal species at the absolute left wall of minimal sieve size [see pages 157—158], vertebrates begin at a theoretically minimal value of complexity by McShea's measurements. Since the founding fishes tend to have vertebral columns made of identical elements, their measured complexity will be close to flat zero [McShea measures complexity as differences among vertebrae]. There really is no place to go from this initial point but up!)