Somatic Adaptation and Evolution

The lesson from Baldwin, Schmalhausen, and Waddington is that the organism has a great deal of latent novelty within its own somatic adaptability. As West-Eberhard has extended the lesson, all phenotypic novelties are reorganizations of preexisting phenotypes. In effect, the organism can express many alternative phenotypes—phenotypes that are stable like the different insect castes and the male-female alternatives of sex determination, or phenotypes that are readily reversible like the alternative oxygen-loading and oxygen-unloading forms of hemoglobin. Because they have already been tested in evolution, these phe-notypes are necessarily viable and adaptive to the ambient conditions. They are a special set of phenotypes and anything but random.

Some of these somatic adaptations are simple whereas others, like the various forms of developmental plasticity, incorporate an entire suite ofanatomies, physiologies, and behaviors. These alternative paths of development can be easily stabilized and modified, in part because they normally require signals to proceed from one state to another. Omitting or inhibiting signals is readily achieved by simple mutations, an example of gene-environment interchangeability, which has been documented on the morphological level in both natural and laboratory studies.25

To counter the argument that stabilization of the somatic adaptations is an important means of facilitating evolutionary adaptation, a skeptic might argue that not much new has been achieved. The modifications might be characterized as modest, sometimes just a simplification of the complexity that was built up in evolution. To choose one of the alternative phenotypes of the honeybee for further evolutionary elaboration is not to invent that phenotype in the first place. The requirement that abundant novelty must already exist within the organism could be a real limitation on these ideas, or at worst a prescription that evolution always proceeds from the complex to the simple.

Yet the examples described here are not simplifications. Even in the mechanistically obvious case of hemoglobin, the development of a placental physiology dependent on a new form of hemoglobin physiology is not an elaboration of what is already there. It is an exploitation of the capacity of the hemoglobin molecule to modify its physiological range by mutation or genetic reassortment from existing genetic variation in the population. The capacity of hemoglobin to be modifiable was not selected for ease of future evolutionary genetic modification, but for reversible modification by environmental conditions, which has value in each generation.

Although much of biology may seem ad hoc—polyphenism in bees, sex determination in reptiles, hemoglobin physiology—under-neath these processes are some very general and ancient mechanisms. As we shall see, hemoglobin is but one of many examples of highly poised proteins that, unlike hemoglobin, function in all the cells of our body. Adaptability is a key characteristic of many of the conserved core processes of eukaryotes. Polyphenism and environmental sex determination make use of highly modifiable transcriptional mechanisms (another set of core processes), which also play a large role in embryonic development.

It may seem counterintuitive that mechanisms such as oxygen regulation, which function to maintain the existing phenotype by buffering the effects of variation in the environment, should simultaneously serve as vehicles for creating variation in evolution. This pseudoparadox of stability versus change stands juxtaposed to another of conservation and diversity. How do highly conserved processes like oxygen transport in hemoglobin or determination of sex in mammals lower the barrier for the generation of diversity?

The answer to both apparent paradoxes is quite obvious when one examines them at the molecular level. The organism is not robust because it has been built in such a rigid manner that it does not buckle under stress. Its robustness stems from a physiology that is adaptive. It stays the same, not because it cannot change but because it com pensates for change around it. The secret of the stability of the phe-notype is dynamic restoration. Mutations or genetic reassortments that target these dynamic restorative systems can reset their optima and generate a class of significant phenotypes with reduced lethality. Evolution can achieve new forms of somatic adaptation so readily because the system, at all levels, is built to vary.

+1 0

Post a comment