Lower-level Entities Research Articles

Testing for bias in the evolution of coloniality: a demonstration in cyclostome bryozoans

Colonial organisms vary in the degree to which they are individuated at the colony level, i.e., in the degree to which the colony constitutes a unified whole, as opposed to a group of independent lower-level entities. Various arguments have been offered suggesting that evolutionary change along this continuum may be biased, that increases may be more probable than decreases. However, counterarguments can be devised, and the existing evidence is meager and inconclusive. In this paper, we demonstrate how the question can be addressed empirically by conducting a test for bias in a group of stenolaemate bryozoans, the cyclostomes. More specifically, we suggest three criteria for colony individuation: degree of connectedness among lower-level entities (in this case, zooids), degree of differentiation among lower-level entities, and number of intermediate-level parts. And we show how these criteria can be used together with a phylogeny and ancestral-state reconstruction methods to test for bias. In this case, results do not unambiguously support any single interpretation but are somewhat supportive of a null hypothesis of no bias in favor of increase.As part of the demonstration, we also show how results can be transformed into a quantitative estimate of an upper limit on bias. Finally, we place the question of bias in a larger context, arguing that the same criteria and methods we employ here can be used to test for bias in other colonial taxa, and also at other hierarchical levels, for example, in the transitions from free-living eukaryotic cells to multicellular organisms.

The hierarchical structure of organisms: a scale and documentation of a trend in the maximum

The degree of hierarchical structure of organisms—the number of levels of nesting of lower-level entities within higher-level individuals—has apparently increased a number of times in the history of life, notably in the origin of the eukaryotic cell from an association of prokaryotic cells, of multicellular organisms from clones of eukaryotic cells, and of integrated colonies from aggregates of multicellular individuals. Arranged in order of first occurrence, these three transitions suggest a trend, in particular a trend in the maximum, or an increase in the degree of hierarchical structure present in the hierarchically deepest organism on Earth. However, no rigorous documentation of such a trend—based on operational and consistent criteria for hierarchical levels—has been attempted. Also, the trajectory of increase has not been examined in any detail. One limitation is that no hierarchy scale has been developed with sufficient resolution to document more than these three major increases. Here, a higher-resolution scale is proposed in which hierarchical structure is decomposed into levels and sublevels, with levels reflecting number of layers of nestedness, and sublevels reflecting degree of individuation at the highest level. The scale is then used, together with the body-fossil record, to plot the trajectory of the maximum. Two alternative interpretations of the record are considered, and both reveal a long-term trend extending from the Archean through the early Phanerozoic. In one, the pattern of increase was incremental, with almost all sublevels arising precisely in order. The data also raise the possibility that waiting times for transitions between sublevels may have decreased with increasing hierarchical level (and with time). These last two findings—incremental increase in level and decreasing waiting times—are tentative, pending a study of possible biases in the fossil record.

Individuality and the existence of species through time.

The individuality of species provides the basis for linking practical taxonomy with evolutionary and ecological theory. An individual is here defined as a collection of parts (lower-level entities) that are mutually connected. Different types of species individual exist, based on different types of connection between organisms. An interbreeding species is a group of organisms connected by the potential to share common descendants, whereas a genealogical species is integrated by the sharing of common ancestors. Such species definitions serve to set the limits of species at a moment of time and these slices connect through time to form time-extended lineages. This perspective on the nature of individuality has implications that conflict with traditional views of species and lineages: (1) Several types of connections among organisms may serve to individuate species in parallel (species pluralism); (2) each kind of species corresponds to a distinct kind of lineage; (3) although lineage branching is the most obvious criterion to break lineages into diachronic species, it cannot be justified simply by reference to species individuality; (4) species (like other individuals) have fuzzy boundaries; (5) if we wish to retain a species rank, we should focus on either the most- or least-inclusive individual in a nested series; (6) not all organisms will be in any species; and (7) named species taxa are best interpreted as hypotheses of real species. Although species individuality requires significant changes to systematic practice and challenges some preconceptions we may have about the ontology of species, it provides the only sound basis for asserting that species exist independently of human perception.

Evolutionary Transitions and Artificial Life

A major challenge for artificial life is to synthesize the evolutionary transitions that have repeatedly formed differentiated higher-level entities from cooperative organizations of lower-level entities, producing the nested hierarchical structure of living processes. This article identifies the key elements and relationships that must be incorporated or synthesized in an artificial life system if these transitions are to emerge. The processes currently included in artificial life systems are unable to provide an adequate basis for the emergence of the complex cooperative organization that is essential to the transitions. A new theory of the evolution of cooperative organization is developed that points to the additional processes that must be included in artificial life systems to underpin the emergence of the transitions.

Evolutionary Transitions and Artificial Life

A major challenge for artificial life is to synthesize the evolutionary transitions that have repeatedly formed differentiated higher-level entities from cooperative organizations of lower-level entities, producing the nested hierarchical structure of living processes. This article identifies the key elements and relationships that must be incorporated or synthesized in an artificial life system if these transitions are to emerge. The processes currently included in artificial life systems are unable to provide an adequate basis for the emergence of the complex cooperative organization that is essential to the transitions. A new theory of the evolution of cooperative organization is developed that points to the additional processes that must be included in artificial life systems to underpin the emergence of the transitions.

Frpm artificial individuals to global patterns

Artificial Life is a model of biological systems that describes lives archived by computer simulation, chemical substrates or any other non-biological substrates. Artificial Life simulation adopts a bottom-up approach in which behavior of lower-level entities (e.g. molecules, cells and individuals) is all that is programed; global patterns (e.g. evolutionary patterns observed at the level of the population and the community) can emerge as a result of interaction among lower-level entities. Artificial Life simulations will be used not only to test ecological and evolutionary hypotheses explaining real organisms but also to show the validity of general theories, processes and concepts such as natural selection, theories of complexity, hierarchical relations and self-organization.

Feature Extraction for Casting Core Development

Feature extraction techniques are presented for the generation of casting core patterns from a boundary representation (B-Rep) solid model. Techniques are presented which would allow for automatic extraction of three classes of core features (internal voids, single and multi-surface holes, and boundary perturbations). The task of extracting casting cores from solid models involves recognizing a collection of entities (i.e., slots, bosses, undercut surfaces, local and global concavities, etc.) from the set of lower level entities (i.e., the B-Rep structure). To this end, a combination of solid modeling B-Rep and graph structures and their associated methods will be used for casting core development. Appropriate local features are identified and extracted from the original object, and are grouped into one or more new object(s) (termed a core-object). If the core-object is multiply connected (i.e., composed of multiple objects), it is graph separated into global feature objects. Each of these global feature objects represents a core in the final pattern. Lastly, the geometry of the original part is augmented to add core prints where core geometries were extracted. The core print, as currently developed, combines the extracted core geometry and its convex hull.

Top-down geometric modelling of buildings on network database

To integrate existing cad capabilities more closely into structural-design practice, the paper presents a top-down approach to the geometric modelling of building structures. A network database with newly developed data abstraction is utilized to organize the geometric and topological information obtained, so that it can provide a central communication area between applications in structural engineering. To support the top-down structural-design process, facilities for intersecting higher-level primitives and splitting them into lower-level entities, and for evolving simple wireframe models of entire buildings to complex solid models of their components, were developed. The system is a geometric modeller for the integrated structural design of buildings, providing the designers with easy construction of the geometric model, and effective management of the model data of buildings.

Production planning for mixed assembly/arborescent systems

AbstractIn the automotive industry, components are fabricated from raw materials; modules from components; and subassemblies from modules. Final assembly of the end item, the vehicle, then follows. Such a process would be called an “assembly system” if each lower‐level entity (raw material, component. module) were used in only one immediate successor entity (component, module, subassembly).It is common to represent the bill of materials as a directed network. The nodes of this network are entities in the bill of materials; arcs in the network point from lower level entities in the direction of higher level entities and toward the end product. For example, if components 1 and 2 were combined to produce module a, there would be an arc from node 1 to node a. Similarly, there would be an arrow pointing from node 2 to node a.A pure assembly system may thus be described by saying that each node has at most one direct successor, although it may have several immediate predecessors. Similarly, a pure “arborescent network” is the prototype for a distribution system, whereby each node (say a warehouse) may have at most one immediate predecessor (distribution center), but possibly many immediate successors (retailers or smaller warehouses).The focus of this paper is that a complex manufacturing system often has arborescent or non‐assembly portions in its network. In automotive assembly, a given component frequently appears in more than one subassembly in the manufactured product. Certain components are used in each of four wheels; left‐ and right‐side body subassemblies contain common modules. The result is a mixed assembly/arborescent structure. Mixed assembly/arborescent structures also merit consideration as the prototype for combined production/distribution systems.In this article, we consider such mixed structures by building on properties of pure assembly and pure arborescent systems. First, for a wide class of parameters and various pure assembly structures, we study several single‐level lot‐sizing algorithms in conjunction with the cost‐parameter‐revision method of Blackburn and Millen (1982b). Cost‐parameter revision is a way to account for the impact at other stages, of decisions made at a given stage, in a pure assembly system. We also show it is better not to revise these cost parameters for a pure arborescent system.Our approach to a mixed assembly/arborescent system is thus based on identifying its “largest, independent pure assembly sub‐graph.” This is a significant subsystem of the original graph or network, the largest portion that is pure assembly in nature. Modification of the cost parameters there, followed by stage‐by‐stage application throughout the whole network of the best single‐stage lot‐sizing method, gives lower total costs than when no cost revision is applied in the mixed system. Suggestions are then made for further research.

The Ecological Concept of Disturbance and Its Expression at Various Hierarchical Levels

Current definitions of disturbance are intuitive, narrow, and only implicitly based on system structure. This is because the concepts are based on experience at particular levels of organization or on systems whose structure is well known. The definitions are thus inadequate for the development of a general theory of ecological disturbance. A universally applicable definition would 1) identify the object disturbed; 2) distinguish between change in the object that is disturbance versus change that is not; and 3) distinguish between direct and indirect consequences of disturbance. To meet these requirements, we formally link the hierarchical organization of ecological objects and the concept of disturbance. Any persistent ecological object will have a minimal structure, or system of lower level entities that permit its persistence. Disturbance is a change in the minimal structure of an object caused by a factor external to the level of interest. Using these definitions, disturbance can be unequivocally identified and associated with various specific ecological levels of organization. Because of the dependence of the concept of disturbance on recognizing the minimal structure of ecological systems, application of the concept will advance as refined models of the hierarchical structure of ecological systems are elaborated.

Diversity-dependent species dynamics: incorporating the effects of population-level processes on species dynamics

A general model of species dynamics must incorporate the effects of species number on the processes of speciation and extinction. Previous models make specific assumptions about these effects, but do not consider the effects of dynamics of lower level entities on speciation and extinction rates. A hierarchical model is developed which explicitly describes the effects of energy use by species on speciation and extinction rates. The effects of energy use are represented by parameters that characterize the average effects of energy use by each species in the biota on speciation and extinction rates. The dynamics of the model describe a sigmoidal increase in species number over time, as does the logistic model of species dynamics. However, the mechanisms of those dynamics are assumed to be different in the two models. Empirical analysis of a data set on the diversification of fossil Miocene horses suggests that the logistic and hierarchical models have similar descriptive power. The hierarchical model incorporates insights from recent considerations of the nature of a hierarchical theory of biology. Further progress in developing such a theory will depend on the success with which relationships among levels in the biological hierarchy are able to be defined.

TERRITORIAL-PRODUCTION COMPLEXES

In light of the renewed stress on the creation of territorial-production complexes during the current five-year plan (1976–80), a prominent economic geographer reviews the Soviet approach to such complexes, with particular emphasis on lower-level intraregional entities, combining a series of interrelated productive and service activities as well as population (as the source of labor) into an integrated whole. The cost benefits that can be derived from common engineering communications, transport lines and support industries are discussed in detail. A simple classification distinguishes industrial complexes, combined agricultural-industrial and industrial-agricultural complexes (depending on the predominant activity), combined industrial-transport complexes, and service complexes (in which research and development, education, and other service functions predominate). [The author died in February 1976 at the age of 72.]