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doi: 10.2307/sysbio/14.4.249pmid: N/A
AbstractContemporary population and developmental genetics sees no problem in finding mechanisms adequate to effect a change from one level of adaptive organization to another. Natural populations contain sufficient genetic variability to respond to directed selection for a change in the mean of almost any character. Such responses usually come by the accumulation of small increments and can be attributed to shifts in the frequencies of different alleles in polygenic systems. The extent of the response to persistent selection depends on whether the remainder of the genome adjusts to include the new mean in an integrated whole. The greater the change and the more fundamental the reorganization, the lower the probability of occurrence. Hence the prediction of such events with any degree of precision presents difficulties close to insuperable.
doi: 10.2307/sysbio/14.4.259pmid: N/A
AbstractThe relationship of morphogenesis to evolution of higher levels of organization is discussed, with three embryological examples: the origin of the cerebral inductor complex, the probable origin of paired fins and of variation in the skull bones of fishes, and the reduction of the fibula and digits in tetrapods. Minor changes in morphogenesis may lead to major alterations in adult structure, usually affect the entire organism, and must be balanced with respect to the morphology and function of the organism.
doi: 10.2307/sysbio/14.4.272pmid: N/A
AbstractOne of the most controversial questions in the study of major evolutionary changes is whether or not all macroevolutionary steps are fully adaptive and completely understandable in terms of the known principles of mieroevolutionary change. The answer appears to lie not in the search for new evolutionary mechanisms, but in the correct description and analysis of the events involved in the origin of new higher levels of organization, as has been stressed by several workers who are dissatisfied with the synthetic theory of evolution.Evolutionary adaptation is the relationship of a species to a specific set of environmental conditions; it connotes both the process and the state of being. An adaptation is a form–function complex (= faculty) that has two components: it has a definite biological role that interacts with a selection force; and it has a relative degree of efficiency. The degree of adaptation, as a state of being, is defined as the ability of the feature to execute successfully its biological role with the minimal expenditure of energy. As a process, adaptation is defined as any evolutionary change that reduces this amount of energy. Preadaptation occurs when a feature, by virtue of its present form and functions, acquires a new biological role and thus gains a new adaptation. Key innovations are those adaptations focal to the exploitation of a new habitat.The model suggested for the sequence of events in the origin of new groups is an evolutionary change that: (a) involves a series of many steps, each representing an intermediate stage between the ancestral and descendent groups; (b) occurs in an intermediate adaptive zone; and (c) involves a number of phyletic lines. Each step involves preadaptations, key innovations and a postadaptational period with an adaptive radiation of forms. This radiation, in turn, may result in a new preadaptation leading to another step, and so on. if the evolving group in the transitional zone reaches a new major adaptive zone, a larger adaptive radiation may occur, marking the primary flowering of the new group. However, no single point can be cited as the origin of the group. The series of modifications involved in the evolution of a new higher level of organization is exactly the same as the continuous evolutionary change within the old adaptive zone, except for the steady change in niche relationships in the transitional zone. The adaptive mechanisms involved are identical for both series of changes; a search for new mechanisms is not needed.
doi: 10.2307/sysbio/14.4.288pmid: N/A
AbstractEvolution is neither an autonomous sequence of spontaneous organic changes nor a predictable chain of unalterable reactions of organisms to their environment and its changes; hence ecological factors are neither just the background for evolution nor do they determine it directly and unequivocally. The contribution of ecological factors to the timing and the direction of evolution depends upon the role they assume, or are allotted, in the life history of the species in question. The adaptation of species to momentary relationships with their particular environments may preadapt them to the establishment of new ecological relationships, Preadaptation is achieved when new or even existing components of the organism's environment acquire additional or totally novel roles in the ecological niches of these organisms, i.e., when the organism interacts with parts of its environment in new and different ways. The exploitation of the evolutionary opportunities emerging from the interaction between organic and environmental diversity involves both chance and choice. The central role of behavior permits the element of choice; the importance of choice in evolution increases with increasing complexity of animals and their behavior. The role of ecological factors in evolution must be investigated by clarifying the multiple feedback relationships between structure, habit, and habitat.
doi: 10.2307/sysbio/14.4.301pmid: N/A
AbstractThe process of natural selection operates at the population and species levels, but its effects are seen at all levels of organization. Directional selection is the primary mechanism for changes that result in the evolution of higher levels. The effectiveness of natural selection can be measured by three types of evolutionary rates: taxonomic, morphological, and biological (species origin). The most useful rate is that of species origin, which can be compared with rates obtained from experimental or mathematical models. The rates computed from the fossil record of many Pleistocene mammals are higher than those derived from models. The history of several well-known vertebrate lineages clearly demonstrates that higher levels of organization evolve by means of microevolutionary processes rather than by special genetic and selective mechanisms. Phenotypic variation observed in phyletic lines does not give a direct index of the genetic variability underlying the phenotype.
doi: 10.2307/sysbio/14.4.318pmid: N/A
AbstractThe transition from one higher level of organization to another always involves some form of biological improvement for the same or a new way of life. Except in rare cases when a single lineage attains a new level, the transition is expressed in terms of similar adaptations (broad adaptations) that evolve more or less in parallel in lineages of common ancestry. Within the limitations imposed by ancestry, canalization and directional selection, experimentation with various combinations of primitive, intermediate, and advanced characters will lead to both niche adaptation and to broad adaptation. Broad adaptations may evolve at different rates and times in related lineages, but they are, in effect, responsible for the recognition of a higher level. The partly opportunistic nature of this process implies that these lineages will approach or enter a new level with varying degrees of success. A particular higher level of organization may therefore have a multiple origin even though it has a monophyletic origin at a particular lower level.
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