Morphological Stasis: an Energetic and Ecological Perspective Incorporating Stress

Conventional interpretations of morphological stasis are assessed in terms of energetic costs to organisms in their stressful environments. Assuming that the target of selection of such stress is at the level of energy carriers, this provides a reductionist model to derive the conditions under which morphological stasis is expected. In natural habitats, stress can be widely fluctuating or stable: under extreme conditions these are, respectively, the habitats of "living fossils" and of relict species, which both show morphological stasis. When the intensity of stress is relaxed, and, especially, when there is energy from additional resources, the likelihood of diversification increases, since extreme variants are then more likely to survive. Conversely, morphologically complex species would be most at risk during extinction events, so that the diversity of surviving phenotypes would be reduced, giving a tendency towards stasis.