Section 20.1: Biological Adaptation

The property of adaptation, from an evolutionary point of view, is often described by the equation adaptation = variation + heredity + selection. This distillation of ideas, known as neo-Darwinism, differs from strict Darwinism by explicitly making reference to a method of heredity. Breaking the equation down into basic terms reveals some interesting connections between adaptation and the fundamental computational issues highlighted earlier. For example, variation, which refers to how individuals in a population can differ from each other, is crucial to the neo-Darwinist view since evolution operates on no single individual but on entire species. Variations, by definition, can be expressed only in terms of multiple individuals; thus, parallelism and spatial multiplicity are essential ingredients in the algorithm of evolution. Similarly, heredity can be seen as a form of temporal persistence. When children inherit traits from their parents in discrete chunks of information, the traits can be seen to be iteratively passed down a time line. Thus, we have both parallelism and iteration as fundamental pieces of the biological equation for adaptation.

Now if we lived in a world of infinite resources where every organism was guaranteed an opportunity to reproduce, then that would be the end of the story; you and I would not be here since evolution would never have occurred in our world. So it is perhaps ironic that we are indebted to the finiteness of the universe. With limitations on available resources, reproduction is far from a sure thing since more organisms will exist than can reproduce. This brings us to the often misunderstood term ``survival of the fittest.'' This phrase has been criticized as being a tautology since it is really equivalent to ``survival of the survivors,'' a nearly meaningless phrase. The problem here seems to be with the word ``fittest,'' which is usually associated with physical characteristics independent of the ability to reproduce. But by ``survival of the fittest'' we really mean ``survival of the reproducers'' and nothing more. As Richard Dawkins is fond of saying, you and I can proudly make the claim that every one of our ancestors---without exception---survived long enough to reproduce. This may be an obvious statement, but if we consider the number of organisms that did not survive long enough to reproduce, and consider the exponential number of descendants that could have been, then we can be seen as members of a truly exclusive club.

Our ancestors may have been strong, fast, clever, or even sexy, and ``survival of the fittest'' seems to refer to some of these traits, but in truth these admirable traits are secondary. What really counts in natural selection is an organism's ability to reproduce, and nothing else. If strength, speed, intelligence, and desirability happen to increase an organism's chances of reproducing, then these traits do indeed correspond to being ``fit,'' but only in the context of reproduction. In fact, ``fitness'' can often be associated with a trait that is maladaptive in the sense that the trait may actually decrease the functionality of an organism. An example of this can be found in the peacock's gaudy tail feathers, which can hardly do anything but decrease a peacock's ability in the daily business of survival, but are selective for survival solely because peahens find them ``sexy,'' in that they are indicators of a peacock's overall health and fitness.

Adaptation, natural selection, and evolution are indeed very strange things. This is especially apparent when one factors in how organisms can have a recurrent relationship with their environments. The concept coevolution refers to how two species can mutually adapt to one another in such a way as to have a circular relationship, with one species' influence on another ultimately returning to the first species in a feedback loop. We see this in the coevolution of predator and prey species, such as lions and gazelles. As lions became better hunters over the past several millions of years, they exerted selective pressure on the gazelles that they preyed upon, which had the effect of increasing the speed and elusiveness of future gazelles. This in turn made it harder for the lions to get a meal, turning them into victims of their own success. This phenomenon is often characterized as a biological arms race.

An even more elaborate and illustrative example is found in the coevolution of bats and moths. Bats use a form of sonar, known as echolocation, to locate moths to eat. To accomplish this, bats emit very high-frequency sounds that bounce off moths and other insects, giving them an estimate of the prey's relative location. The process is actually far more complicated than one would think, since the bat-emitted sounds are often thousands of times more powerful than the returning echoes; hence, bats have evolved a technique to filter out the most powerful sounds so that they can concentrate on the faint return signals. By itself, this form of navigation and target identification is an extremely impressive and creative feat of evolution. But moths have coevolved a defense in the form of a soft covering on their bodies and wings that absorbs the bat chirps. Bats, in turn, have evolved new chirp frequencies that can be used to identify moths' fuzzy coating. In response, the moths have enhanced their stealth technology and, furthermore, have come up with a jamming technique that involves emitting their own sounds to jam the bats' return signals. This is often coordinated with elaborate evasive maneuvers. As if that were not enough, bats have evolved an elaborate flight pattern that can overwhelm a moth's senses, and also periodically turn off their echolocation, making the jamming technique less affective. And so the arms race continues (Wesson 1991).

The point is that every species partially molds its own environment, which makes the boundary between the selector and the selected somewhat indistinct. Hence, in many ways, Earth as whole may be best understood and appreciated as one enormous complex adaptive system.