TABLE OF CONTENTS ABSTRACT………………………………………………………………………………..………2 INTRODUCTION …………………………………………………………………………………. 3 WHAT IS COMPLEXITY? ………………………………………………………………... 3 HIERARCHY ……………………………………………………………………………... 5 DIVISION OF LABOR & PARTS …………………………………………………………...7 COMPLEXITY VERSUS REDUNDANCY…………………………………………………… 8 REFINED DEFINITION OF COMPLEXITY ………………………………………………… 9 WHY FOCUS ON WHALES?..... ………………………………….......…………………..…9 METHODS………………………………………………………………………………..………14 APPLICATIONS …………………………………………………………………………. 20 SOURCES OF DATA ……………………………………………………………………... 20 RESULTS ………………………………………………………………………………..………..20 DISCUSSION….………………………………………………………………………………..… 28 TREND TOWARDS SIMPLICITY ...………………………………………………………...28 FEEDING BEHAVIOR OF EXTINCT CETACEANS ……………………………………..….. 30 ROLE OF HABITAT IN DENTITION SIMPLIFICATION ……………………………………. 31 CONCLUSIONS …………………………………………………………………………………...35 ACKNOWLEDGEMENTS ………………………………………………………………..………. .36 APPENDIX A……………………………………………………………………………..……… 37 APPENDIX B……………………………………………………………………………..……… 39 APPENDIX C……………………………………………………………………………..……… 40 REFERENCES……………………………………………………………………………………. 42 [1] ABSTRACT Over billions of years, evolution has given rise to organisms that have increased dramatically in complexity, from microbes alone to communities that include modern humans and the great whales. But change in individual lineages, occurring by chance and in adaptation to unpredictable circumstances, does not necessarily involve increasing complexity. Complexity may increase or decrease depending on the immediate situation. Metrics of change based on a single unit, such as numbers of genes or cells, are inadequate to quantify such shifts in complexity, because the structures of organisms and communities are hierarchical. Teeth and jaws of cetaceans provide an opportunity to assess changes in complexity simultaneously at different levels of organization. A set of 17 standardized variables has been established to characterize the form of each tooth in the cetacean jaw. These include aspects of shape, curvature, carinae, serrations, cusps, and other attributes that vary according to the degree of morphological differentiation within the jaw and among taxa. Measures of complexity for each tooth in the jaw are derived from these variables. These indices of the complexity of individual teeth are integrated to derive simple information functions that quantify the complexity of the dentition as a whole. Our preliminary data show that the earliest cetaceans, derived from terrestrial, hoofed mammals, had fairly simple teeth, with a modest degree of differentiation in the jaw. By Oligocene time, taxa with more complex teeth had emerged, but dentitions did not become significantly more differentiated. Subsequently, the trend changed. Tooth forms in most lineages became simpler and much less differentiated, most dramatically among the mysticetes, where teeth were lost and replaced by baleen. These paths of change at two levels of organization represent a common pattern of evolution. A novel structure emerges, it is replicated to constitute a series of elements, and these evolve more or less independently to take on varied functions. Finally, the entire structure is refined as a single, highly integrated functional system, or it is lost as further novelty emerges, typically at a higher level of organization. [2] INTRODUCTION The transformation of cetaceans from scavenging terrestrial carnivores to streamlined oceanic hunters and suspension feeders within about 30 million years provides an excellent opportunity to clarify our understanding of patterns of evolutionary change in complexity. In spite of extensive discussion of the evolution of complexity, there is no one metric by means of which to assess such change. This is a result of difficulty in defining parts that remain comparable in complex structures with multiple hierarchical components. Here we define metrics that can be used to assess change in complexity at two hierarchical levels, in individual teeth and in the set of teeth that constitutes a cetacean jaw. Our data enables us to assess change in complexity simultaneously at two different levels of structural organization, thereby enhancing our understanding of the major patterns of cetacean evolution. WHAT IS COMPLEXITY? Complexity is widely acknowledged as an important general property of natural and artificial systems, but it has no single definition. The idea itself is so widely used in everyday expression that people tend to take it for granted, assuming that structural complexity manifests itself simply as something with many intricate parts. We recognize systems with many parts, many different kinds of parts, and a great variety of connections amongst those parts as being complex. Groups of parts with a common structure or function that can consistently be recognized among the organisms under study constitute levels of organization at which complexity can be compared. Further, in systems like living organisms and human activities, a distinction can be drawn between structure and dynamics, which is to say between complexity of form and complexity of interaction (McShea 1996). There is also a subtle relationship between complexity and order that needs to be clearly understood. In some systems, for some purposes, it is appropriate to identify complexity with disorder, which allows for more possible states. In living systems, however, maintenance of complex structure and function depends upon the highly ordered integration of similar and different components. Given a wide array of definitions based on various aspects of complexity, it is increasingly evident that the concept itself is very hard to pin down. Most people have a solid grasp on what complexity is. Scratch below the surface, however, and one finds that different observers have strikingly different comprehensions of the concept. [3] All too often, the preconceived notion that evolutionary change goes hand-in-hand with increased morphological complexity is taken for granted, as such a trend seems “too obvious to question” (McShea 1996). In the long run, as a net effect, this holds true. If one traces the evolution of life on Earth over its entire span, organisms are seen to have been transformed from primitive microbes into creatures such as sperm whales, marking an obvious transition from simple to complex anatomy. This has been expressed in more abstract terms, with the suggestion that “In evolution, it is clear that the hierarchical maximum—the degree of nestedness of the hierarchically deepest organism in existence—has increased over time.” (Marcot & McShea 2007). Moreover, an irreversible relationship between morphological complexity and evolutionary change over time is evident. Complexity is likely to increase, on the average, as certain evolutionary advances can be compared to bridges that, once crossed, cannot be retraced. This is one aspect of Dollo‟s Law, which states that the same species or organic structure cannot appear twice in evolutionary history, and thus evolution is not reversible. For example, “No solitary bacterium has ever arisen from a eukaryotic cell” (Marcot & McShea 2007). More empirical arguments suggest that evolutionary complexity is driven in large part by natural selection. According to Bonner (1988), selection favors increasing complexity because complex organisms are composed of more specialized components, and thus their internal division of labor is much greater. As expected, with increased division of labor comes improved effectiveness of the working parts, enabling the organism to adapt to novel situations (Bonner 1988, McShea 1993). Further, Bonner argued that the emergence of higher-level individuals is evolutionarily favored for their large size. Size increase allows division of labor and thus differentiation among lower-level entities. Logic dictates that selection should favor collaborations among groups of lower-level entities, leading to increased interaction and complexity (McShea 2001b). If a larger organism has more working parts, does it necessarily mean that bigger is better? Are larger organisms more complex than smaller ones solely on account of increased size and division of labor? Would a more specialized animal lacking the varied skill set of its generalist peers be considered simpler? Not necessarily. We will see here that certain organisms have become adapted to circumstances where smaller and more specialized (thus simpler as [4] defined above) components are most effective. Larger organisms tend to be more complex simply by virtue of being larger. Structurally simple organisms may evolve to large sizes, especially in the case of colonies, but generally scaling factors require that they be more complex. While there is general acknowledgement that evolutionary complexity has progressively increased, it is not the case that this necessarily occurs in any given clade, at the level of evolution of particular genera, families or even higher taxa. HIERARCHY While it may be relatively easy to define complexity in broad terms, it is very difficult to quantify complexity for an entire organism. Body size and genome size have been employed as proxies of complexity as reviewed by Bonner (1988), but these standing alone have a relatively low correlation to overall complexity. At lower taxonomic levels, most previous studies of morphological complexity employ metrics that quantify it at a single hierarchical level of structure. In the simplest terms, structural components on the same hierarchical