Energy and Evolution
The concept of the extended phenotype is incomplete without an understanding of the underlying physiology. Genes can act outside the bodies of the organisms containing them only if they can manipulate flows of matter and energy between organisms and their environment. Let us now build upon this fountain a more formalized framework of energy, evolution, and physiology that will support the rest of this book. The framework will include two elements: the energy streams that power physiology, whether inside or outside the organism;and the types of "external organs" and "external organ systems" that can manipulate these energy streams to power physiological work. I begin by making a baldly teleological assertion:
Anything an organism does is "for" one and only one purpose: to ensure that its genes are copied and passed on to progeny.
If organism X does this marginally better than organism Y, organism X will leave more progeny than Y, passing along whatever it did right to its progeny so that they can do it right also. This is simply natural selection restated.
Evolution by natural selection is predominantly a matter of genes and their transmission, but adapta tion and natural selection have an indissoluble link to physiology, and hence to energetics. We know that reproduction takes energy—energy must be used to make eggs and sperm, to get eggs and sperm together, and to get the resulting zygote to grow to the point where it can begin capturing its own energy to power its own reproduction. So part of the energy stream flowing through an organism must power reproduction, or more properly reproductive work (Fig. 3.2). Organisms that do more reproductive work will be selected in preference to those that do less.
Reproductive work requires infrastructure to support it, and building, maintaining, and operating this infrastructure also costs energy. Much of an organism's operating energy budget is devoted to providing the right physical environment for reproductive work to occur. Usually, this involves maintaining an environment inside the body that differs in some way from the organism's external environment. Accomplishing this task has some important energetic consequences.
The internal environment of an animal is described by various physical properties, like its temperature, salt and solute concentration, acidity, pressure. Conditions outside the animal are also reflected in these properties. Usually, conditions vary only slightly in the internal environment, while outside they vary more widely. For example, the daily excursion of environmental temperatures is about 10-20oC through the day, but the range can be much larger, depending upon the season and locale. Whether through active regulation or simple inertia, the range of body temperatures experienced by animals during the day will be somewhat smaller. For mammals and birds, at most about 5oC separates the highest body temperature commonly experienced from the lowest.4 Furthermore, environmental temperatures often exceed the maximum or minimum tolerated temperatures of ani
4. Obviously, I am leaving out hibernators and birds that go into torpor. Birds and small mammals in this state will allow body temperatures to drop to within a few degrees of freezing.
mals. Animals as a group tolerate body temperatures up to about 45oC, though the limit is higher in a few and may be as low as about — 2oC, lower if ice formation can be suppressed or managed somehow. In contrast, daytime surface temperatures in the tropics can exceed 70oC, and winter temperatures even in the temperate zones can fall well below — 10oC.
Such disparities mean that conditions in an animals' internal environment will frequently differ from conditions outside. These disparities, in turn, establish potential energy (PE) differences that can drive a flux of matter or energy across the boundary separating the organism and its environment (Fig. 3.3). For example, an animal that is warmer than its surroundings will lose heat at a rate proportional to the difference in temperature between the body and the surroundings.5 Similarly, an animal whose body fluids contain a concentration of a solute X higher than that in the environment will lose X from the body as it diffuses down the concentration difference.6 Differences in solute concentration or pressure can drive water across an animal's boundary with its environment, and so on. Matter and energy flow down such potential energy differences spontaneously, in conformity with the dictates of the Second Law. Consequently, we will call these kinds of fluxes thermodynamically favored fluxes (TFFs).
If an animal's internal environment differs from the external environment, it will experience a TFF (Fig. 3.3), and its internal environment will change—a loss of heat from the body will result in a drop of body temperature, for example. Maintaining the internal
5. More properly, the animal experiences a potential energy difference in the form of heat content, which is equivalent to the temperatures, the specific heats, and the masses of the respective bodies exchanging heat; that is, APE = ATc^M, where AT = a difference of temperature (K), cp is a specific heat (J K—1 kg-1), andM is the mass (kg).
6. The potential energy difference is related to concentration difference in the following way: APE = RTAC, where R = the gas constant (8.314 J mol-1 K—1), T = the temperature of the solution (K), and AC = the difference in concentrations (mol l—1) inside and outside the body.
Flux rate (PF)
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