Developmental plasticity

It is advantageous for organisms if they are able to modify their ontogenesis in dependence on the local conditions in which they find themselves. This ability is especially advantageous for immobile organisms that are bound all their lives to the place where they were born and grew up.  It is thus encountered primarily in plants (Pigliucci 1998) and to a lesser degree in other organisms (Gotthard & Nylin 1995; Brönmark & Miner 1992). In a great many species of plants, the phenotype of individuals growing at relatively dry sites differs from the phenotype of individuals at damp sites; however, frequently even the phenotype of separate parts of a single individual differs in dependence on the local conditions (Fig. XII.12). The phenotype of water plants differs according to the speed of water flow at the particular sites. In a great many species of plants, preference is given to the phase formed by the vegetative or by the reproductive organs, in dependence on the amount of resources available to the particular individual. Development of the organism is thus programmed so that it occurs in dependence on the external conditions and so that it results in an individual with the phenotype that is best adapted to these local conditions. The ability to purposefully modify ontogenesis in dependence on the external conditions is called developmental plasticity. From the viewpoint of the individual, it is advantageous if its phenotype corresponds best to the local conditions of the environment in which it finds itself. However, from the standpoint of the existence of the species, adaptive developmental plasticity reduces the ability of the species to submit to the action of selection pressure in the environment, and thus to evolutionarily adapt to changing conditions. If the environment undergoes cyclic changes, i.e. oscillates between several different states at regular or irregular intervals, this evolutionary stability of the species can be advantageous, as it reduces the substitution burden to which the species would otherwise be exposed. In contrast, if the changes are acyclic and irreversible or cyclic, but with a periodicity that is comparable with the usual length of existence of the species, developmental plasticity and the related reduced evolutionary plasticity will decrease the chance of long-term survival of the particular species. While species without developmental plasticity have a chance to gradually adapt to the particular changes in the environment as a consequence of the action of selection, a developmentally plastic species only avoids the relevant selection pressure temporarily and frequently imperfectly within the limits of its developmental plasticity. Whether and how much developmental plasticity actually reduces the evolutionary potential of a species is still a subject of discussion. The fact that, on a drastic change in the conditions in the external environment, it partly protects the species against the action of selection, simultaneously provides the species with time to accumulate mutations that gradually lead to the formation of adaptive adjustment to the new conditions (see XVI.3.1). See also developmental canalization.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more