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HOME :: CHAPTER 23 :: WHY ARE THERE NO NEW ANIMAL PHYLA? :: WHY SO FEW PHYLA? |
Why So Few Phyla?
Only about three dozen animal body plans are currently being used on this planet (Margulis and Schwartz 1988; Brusca and Brusca 1990). These constitute the animal phyla. This is not to say that these body plans are the only possible ones. The Burgess Shale, a repository of early Cambrian soft-body fossils, is interpreted as containing representatives of 20 more phyla that never evolved descendants in the upper strata. In addition, this small band of sediment, about the size of a city block, contains about a dozen previously unknown classes of arthropods. These animals are not "primitive" members of existing phyla or classes, but are specialized examples of their own groups (Whittington 1985; Gould 1989). There are also two specimens in the Burgess Shale that may be related to ancestral forms of existing phyla. One is a peripatus-like animal that may be close to the ancestral form of insects; the other appears to be a well-preserved chordate called Pikaia gracilens that may be related to the ancestral chordates. This latter fossil has several features that recommend its being classified in our phylum: it appears to have a notochord, and the zigzag bands along its sides look very much like the somite-derived musculature found in amphioxus (Conway Morris and Whittington 1979). Thus, all the known metazoan phyla (and many heretofore unknown ones) appear to have been formed by the Cambrian radiation about 540 million years ago (Bowring et al. 1993; Wray et al. 1996).
How is it that no new phylum has emerged in the past half-billion years? Kauffman (1993) proposes a mathematical model that predicts that any evolving system (whether it be a phylum, species, automobile, or religion) displays this pattern of divergence followed by the locking in on a particular subset of the original diversity. Kauffman uses the metaphor of a rugged fitness landscape wherein there are peaks and valleys of fitness, and all organisms start out with the same average fitness value (in the middle of a peak). If they take large jumps, they have a 50% chance of becoming fitter organisms. Eventually, the chance of finding a fitter body plan decreases if an organism takes a jump far from where it is situated. Long jumps become risky, and the chance that these higher peaks are already occupied increases. Instead, small jumps (on the same peak) may make the organism somewhat fitter than the surrounding population. Then what we see is a diversification around a few successful models. In general, the duration between successful long jumps doubles with each attempt.
During the early Cambrian, it is possible that the genome had not become stabilized into the sets of interactions that we see today. Moreover, in many invertebrate groups, there is an alternation of generations, wherein a sexual form generates an asexual form (zooid, polyp, bud) that then gives rise to the sexual form again. In such cases, somatic mutations in the asexual form can enter into the body of the sexual form and be propagated very rapidly (Figure 1; Buss 1987).
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| Figure 1 Rapid appearance of new variants in invertebrates with alternation of generations. Here, a somatic mutation occurs in the cells of a hydroid colony. Some of these mutant cells become part of the reproductive polyp, thereby giving rise to medusae (jellyfish) containing the mutant alleles. These medusae reproduce to form a new colony that can be made of the mutant cells. |
Another point of view looks at the constraints upon embryos. Hall (1992) has summarized these arguments. First, there are structural constraints such as diffusion, mechanical load, and the tensile strength of the skeletons that constrain how embryos can be organized. Second, related to this, are the metabolic constraints of oxygen utilization, amino acid use, and so forth. Third, there are genetic constraints such as the rates of mutations and the interrelationships between genes and their products (such as gene activity in several tissues). It is difficult to rearrange what has already been established. Fourth, there are developmental constraints involving interactions between parts of the embryo. If the formation of the heart is necessary for the induction of gut tissue, then it would be difficult for descendents of such an organism to build their gut apart from their heart. Fifth, there are functional constraints. To have a functional jaw, the skeletal, muscular, and nervous systems must be integrated. Interdependent systems are difficult to disassociate. Therefore, once a Baupläne is generated, it is difficult to reassort its elements.
How, then, can one modify one Bauplan to create another Bauplan? The first way would be to modify the earliest stages of development. According to von Baer (see Chapter 1), animals of different species but of the same genus diverge very late in development. The more divergent the species are from one another, the earlier one can distinguish their embryos. Thus, embryos of the snow goose are indistinguishable from those of the blue goose until the very last stages. However, snow goose development diverges from chicken development a bit earlier, and goose embryos can be distinguished from lizard embryos at even earlier stages. It appears, then, that mutations that create a new Bauplan could do so by altering the earliest stages of development.
These early developmental changes can be effected by changing the localization of cytoplasmic determinants, changing the rate of cell division of one cell or group of cells relative to the others, or changing the positions of the cells as they divide. In Chapter 8, we saw that a modification of molluscan cleavage can give the bulk of cytoplasm to the ectodermal cells that form the larval shell. This is due to changing the manner in which the blastomeres divide and apportion cytoplasm. In annelid worms, the differences between polychaetes and oligochaetes stem from differences in the cytoplasmic localization of morphogens within the egg (Figure 2). Although they both undergo spiral cleavage, they apportion their morphogens into different cells. Polychaetes undergo a relatively standard spiral cleavage to give rise to the trochophore larva. Oligochaetes, however, put most of their cytoplasm into those cells destined to form adult, rather than larval, structures. This group then skips the larval stage. If a mutation were to place a certain cytoplasmic morphogen into one region of the egg instead of another, or if a mutation caused a change in the axis of cell division so that different sets of cells acquired these determinants, then a radically different phenotype could be produced. As E. G. Conklin wrote in 1915, "We are vertebrates because our mothers were vertebrates and produced eggs of the vertebrate pattern."
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| Figure 2 Comparison of the development of two classes of annelid worms, (A) the polychaete Podarke and (B) the oligochaete Tubifex. Their cleaving embryos, blastula fate maps, and products of gastrulation are seen. In Podarke, gastrulation leads to the formation of a trochophore larva. In Tubifex, there is no larval stage, and the embryo develops directly into a segmented body. (After Anderson 1973.) |
Another way of evolving new phyla may involve modifying the larva. Darwin and others thought that similarities of larval form signified common descent. However, this can be reinterpreted to mean that changes that give rise to different phyla may occur in larvae. Snails, echiuroids, and polychaetes have very similar patterns of division and form trochophore larvae (Figure 3). In fact, the placement of the newly discovered phylum Vestimentifera (the bright red, gutless invertebrates found in the deep ocean trenches) near the annelids was made in part on the basis of vestimentiferans having trochophore larvae (Jones and Gardiner 1989; Young et al. 1996).
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| Figure 3 Divergence of development after the trochophore larval stage. (A–C) Metamorphosis of the polychaete annelid Polygordius from its free-swimming trochophore larva shows the formation of a segmented trunk. Eventually, the larval structures shrink at the anterior end as the head forms. (D–E) Metamorphosis of the prosobranch (clam) mollusc Patella. After the trochophore stage, it develops a molluscan foot, shell gland, and visceral hump. (F) Scanning electron micrograph of the trochophore larva of a vestimentiferan. (A–E after Grant 1978; F from Jones and Gardiner 1989, courtesy of the authors.) |
Thus, one of the principal mechanisms for establishing new phyla and classes may be the rearrangement of development during the larval stage so that metamorphosis brings about new types of organization. Garstang (1928) showed how the veliger larva of certain snails could have arisen by mutation and then been selected because the new arrangement of the head and shell allowed the head to retract beneath the shell for safety. He also framed the hypothesis that chordates arose from ancestral tunicate larvae that had become neotenic. Unfortunately, soft-bodied larvae rarely fossilize, so we know very little about the mechanisms by which chordates and other phyla may have arisen from early Cambrian larvae.
Larval forms often bridge the gap between the different adult forms (see Hall and Wake 1999). The larval form is seen either as being ancestral to two groups or as "breaking away" by neoteny and forming a different type of organism. This has often been hypothesized as the mechanism by which chordates emerged from invertebrates and vertebrates arose from chordates. The tornaria larva of hemichordates is formed in a deuterostome manner similar to that of echinoderm larvae and looks enough like echinoderm larvae to have been originally mistaken for them. This would link echinoderms and chordates. Garstang (1928) and Berrill (1955) hypothesized that the larvae of certain tunicates could have evolved into chordates such as amphioxus by neotenic development. In this way, the tunicates would keep the notochord, larval musculature, and feeding apparatus of the larval tunicate while becoming sexually mature. There are, in fact, neotenic free-swimming tunicates (such as Larvacea). Modifications of this view (using a different protochordate stock) have been suggested by Jefferies (1986). The origin of chordates remains a difficult problem.
The phylogenetic relationship of the animal kingdom can be viewed on the Tree of Life.
The origin of animal body plans is discussed in a well-illustrated article by Douglas Erwin, James Valentine, and David Jablonski.
Literature Cited
Berrill, N. J. 1955. The Origins of the Vertebrates. Oxford University Press, New York.
Bowring, S. A., Grotzinger, J. P. Isachsen, C. E., Knoll, A. H., Pelechaty, S. M. and Kolosov, P. 1993. Calibrating rates of early Cambrian evolution. Science 261: 1293-1298.
Brusca, R. C. and Brusca, G. J. 1990. Invertebrates. Sinauer Associates, Inc., Sunderland, MA.
Buss, L. W. 1987. The Evolution of Individuality. Princeton University Press, Princeton, NJ.
Conklin, E. G. 1915. Heredity and Environment in the Development of Men. Princeton University Press, Princeton, NJ
Conway Morris, S. and Whittington, H. B. 1979. The animals of the Burgess Shale. Sci. Am. 240(1): 122-133.
Garstang, W. 1928. Presidential address to the British Association for the Advancement of Science, Section D. Republished in Larval Forms and Other Zoological Verses, 1985.
Gould, S. J. 1989. Wonderful Life. Norton, New York.
Hall, B. K. 1992. Evolutionary Developmental Biology. Chapman and Hall, New York.
Hall, B. K. and Wake, M. H. (ed.) 1999. The Origin and Evolution of Larval Forms. Academic Press, San Diego.
Jefferies, R. P. S. 1986. The Ancestry of the Vertebrates. British Museum of Natural History, London.
Jones, M. L. and Gardiner, S. L. 1989. On the early development of the vestimentiferan tube worm Ridgeia sp. and observations on the nervous system and trophosome of Ridgeia sp. and Riftia pachyptila. Biol. Bull. 177: 254-276.
Kauffman, S. A. 1993. Origins of Order. Oxford University Press, NY.
Margulis, L. and Schwartz, K. V. 1988. The Five Kingdoms, 2nd ed. W. H. Freeman, San Francisco.
Whittington, H. B. 1985. The Burgess Shale. Yale University Press, New Haven.
Wray, G. A., Levinton, J. S. and Shapiro, L. H. 1996. Molecular evidence for deep precambrian divergence among metazoan phyla. Science 274: 568-573.
Young, C. M., Vázquez, E., Metaxas, A. and Tyler, P. A. 1996. Embryology of vestimentaran tube worms from deep-sea methane/sulfide seeps. Nature 381: 514-516.
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HOME :: CHAPTER 23 :: WHY ARE THERE NO NEW ANIMAL PHYLA? :: WHY SO FEW PHYLA? |