|McKinney M. L. & McNamara K. J.
Heterochrony: The Evolution of Ontegeny: Bibliographical Excerpts
|Alberch et al. (1979) showed that between ancestor and descendant, development can either be reduced (resulting in paedomorphosis) or increased (resulting in what they termed peramorphosis). Each could be produced by three processes, involving: developmental rate change, change in onset time of development, or change in its offset time. In the case of paedomorphosis, reduced rate is neoteny; delayed onset time is postdisplacement; and earlier offset is progenesis. For the opposing case of peramorphosis, increased rate of acceleration; earlier onset predisplacement; and delayed offset hypermorphoses. These six processes could therefore describe all heterochronic processes. (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 11)
"Many cases of environmentally induced phenotypic variation show that it is the organism's developmental program that is affected, inducing heterochronies. The balance of effects of intrinsic and extrinsic agents determines the evolutionary potential of heterochrony. When extrinsic agents (e.g., temperature perturbations) affect the developmental program of an organism, the heterochronic changes producing phenotypic variation may not often have great potential for being the springboard for the evolution of a new species. However, it is this very lability, the power of the organism to be externally "manipulated," that may the important target of selection. By responding morphologically or behaviorally to extrinsic agents in a "positive" manner, the species thrives and prospers; it moves with the external changes; it sways in the environmental breeze. If its developmental system was not able to respond in such a way, then the species would snap off the evolutionary tree, as readily as a rigid branch in a gale. As Tomlinson (1987) has observed in plants, "plasticity of organization rather than initial architecture may be the more significant adaptive mechanism." However, from an evolutionary viewpoint, internally generated phenotypic variation, with its basis in heterochrony, is far more potent in the generation of evolutionary novelties." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 102) [use this quote to summarize the novelty of human polymorphism]
"However, other environmental factors can produce dramatic phenotypic effects. Bernays (1986) demonstrated how differences n diet in the grass-feeding caterpillar Pseudoaletia unipuncta can have a pronounced effect on head size, changes in diet inducing changes in head allometry. Individuals reared on hard grass developed heads with twice the mass of those fed on soft, artificial diet, even though body masses were the same. Individuals fed on an intermediate diet (soft wheat seedlings) had intermediate head masses (Fig. 4-2). Bernays attributed these allometric differences to an increase in muscular development, which resulted in a significant morphogenetic effect on head size. Size differences, with correlated differences in mandibular strength, directly affect the insect's ability to cope with foods of different hardnesses; those with large heads are adaptively more suited to dealing with hard grasses. " (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 105)
"Consequently, by virtue of earlier maturation induced by higher water temperatures, successive paedomorphs may have been restricted to progressively more oxygenated, shallower waters. Thus, the effect of temperature was not only to induce variable timing of maturation, but also, as a by-product, effective niche partitioning. In the Early Cambrian seas, vacant niche space is likely to have been common, enabling effective ecological isolation, restriction in gene flow, and subsequent allopatric speciation. Extrinsic perturbing factors, particularly temperature, therefore were particularly crucial in initiating heterochronic changes early in the Phanerozoic." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 118)
"Here we encounter the truly crucial distinction between neoteny and hypermorphosis: the former is a process of paedomorphosis such that the descendant adult (us) never attains behaviors possessed by the ancestor. In stark contrast, hypermorphosis is a process of peramorphosis such that the descendant adult goes beyond behaviors of the ancestor." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 309)
"Probably the most familiar metamorphic heterochronies are those of salamanders, especially the axolotl. In one of the simplest changes, (indefinitely) delayed secretion of the thyroid hormone thyroxine will result in delayed metamorphosis. The result is that larval somatic traits are never lost although sexual maturation and large size will be attained (see Raff and Kaufman, 1983, for review). Similar delays are common in frogs (e.g. Emerson, 1986) and insects (Matsuda, 1987). (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 63)