Classes of Adaptational Patterns

Every organism is impinged upon by a variety of physical factors: temperature, oxygen, ions, water, nutrients, light, and mechanical and electrical forces. Every organism is also influenced by biotic factors: food, predators, prey, and conspecific individuals including mates and offspring.

Biochemical, anatomical, and physiological adaptations to specific environmental factors have been described at cellular and molecular levels; interactions among environmental factors add complexity to the analyses.

Adaptations at cellular levels are integrated by nervous and hormonal actions that provide for adaptations of whole organisms. More is known about cellular adaptations to single factors than to multiple factors, and more is known about cellular effects than about the integrated responses of whole organisms.

Two general classes of adaptations are (1) сараcity adaptations, which provide for maintenance of normal function in response to stresses of an altered environment, and (2) resistance adaptations, which provide for function, survival, and reproduction at environmental extremes. The two patterns merge according to environmental change, but the mechanisms of capacity and resistance adaptation are generally different.

Capacity adaptations include (a) internal states such as body temperature, osmotic and ionic balance, blood concentrations of sugar, and other metabolites, the so-called milieu interior of Claude Bernard, and (b) rate functions such as heart rate, respiration rate, rate of energy production, etc.

Two patterns of internal state as a function of external state are conformity and regulation (Fig. 1). In conformers, the internal state comes to be similar to the external state; in regulators, the internal state is maintained at relative constancy. For conformers, the prefix poikilo- applies, as in poiki- lothermic and poikilo-osmotic. In conformers, the internal state rises or falls in parallel with the environment, but the internal state may be separated from the external state by a constant amount. As shown in Figure 1, the range of normal function can be shifted by acclimation toward higher or lower levels. For temperature, body temperature rises or falls with ambient temperature; for oxygen, the measure of conformity is the level of metabolism that rises or falls with Р_O2.

FIGURE 1. Internal state (I) as a function of the same parameter outside (O) and inside the organism. Patterns for conformers (a) and regulators (b). A! and A: refer to two states of acclimation. Solid lines are ranges for normal function; dotted lines are ranges for unstable or limiting states. (Reproduced, with permission, from Comparative Animal Physiology, Vol. Ill, 1973.)

In regulators, the internal state is maintained constant over a range of environmental variation. For regulators, the prefix homeo- applies, as in homeo- thermic and homeo-osmotic. Some animals may be homeo- in one environmental range and poikilo- in another range. At the ends of a mid-range, mechanisms of regulation break down (dotted lines in Fig. 1). In general, conformers vary over a wider range of internal state, whereas in regulators the internal state varies little while the environment varies considerably.

The various patterns of conformity and regulation are well illustrated for osmotic adaptations of aquatic animals. Figure 2 shows the patterns of conformity (D), of regulation over a wide range (B), and of regulation in low osmotic concentrations (A) and in high concentrations (E).

FIGURE 2. Diagrams of several patterns of internal osmoticity (O.C._i) as a function of osmoticity of the medium (O.C.₀). A, Strong hyperosmoregulators live in freshwater and are limited in capacity to live in brackish water; B, animals that are hyperosmotic in freshwater and hypo-osmotic in seawater (e.g., euryha- line fish); C, weak hypo-osmotic regulators live in estuaries and do poorly in seawater (e.g., estuarine crabs); D, osmoconformers are marine invertebrates that maintain slight hypertonicity above and below seawater concentration; E, strong hyperosmotic regulators at low external concentrations and hypo-osmotic regulators at high external concentrations (e.g., terrestrial crabs); F, line of osmotic equality. (From Prosser, Adaptational Biology, 1986.).