Krogh's Principle - Philosophy and Applications

Philosophy and Applications

A central concept to Krogh's principle is evolutionary adaptation. Evolutionary theory maintains that organisms are suited to particular niches, some of which are highly specialized for solving particular biological problems. These adaptations are typically exploited by biologists in several ways:

• Methodology: (e.g. Taq polymerase and PCR): The need to manipulate biological systems in the laboratory has driven the use of an organismal specialization. One example of Krogh's principle presents itself in the heavily used Polymerase Chain Reaction (PCR), a method which relies on the rapid exposure of DNA to high heat for amplification of particular sequences of interest. DNA polymerase enzyme from many organisms would denature at high temperatures, however, to solve this problem, Chien and colleagues turned to Thermus aquaticus, a strain of bacteria native to hydrothermal vents. Thermus aquaticus has a polymerase that is heat stable at temperatures necessary for PCR. Biochemically modified Taq polymerase, as it is usually called, is now routinely used in PCR applications.

• Overcoming technical limitations: (e.g. large neurons in Mollusca): Two Nobel Prize winning bodies of study were facilitated by using ideas central to Krogh's principle to overcome technical limitations in nervous system physiology. The ionic basis of the action potential was elucidated in the squid giant axon, discovered by John Zachary Young. In 1958 Hodgkin and Huxley, developers of the original voltage clamp device and co-recipients of the 1963 Nobel Prize in Physiology or Medicine. The voltage clamp is now a central piece of technology in modern neurophysiology, but was only possible to develop using the wide diameter of the squid giant axon. Another marine mollusc, the opisthobranch Aplysia possesses relatively small number of large nerve cells that are easily identified and mapped from individual to individual. Aplysia was selected for these reasons for the study of the cellular and molecular basis of learning and memory which led to Eric Kandel's receipt of the Nobel Prize in 2000.

• Understanding more complex/subtle systems (e.g. Barn owls and sound localization): Beyond overcoming technical limitations, Krogh's principle has particularly important implications in the light of convergent evolution and homology. Either because of evolutionary history, or particular constraints on a given niche, there are not infinite solutions to all biological problems. Instead, organisms utilize similar neural algorithms, behaviors, or even structures to accomplish similar tasks. If one's goal is to understand how the nervous system might localize objects using sound, one may take the approach of using an auditory 'specialist' such as the barn owl studied by Mark Konishi, Eric Knudsen and their colleagues. A nocturnal predator by nature, the barn owl relies heavily on using precise information on the time of arrival of sound in its ears. The information gleaned from this approach has contributed heavily to our understanding of how the brain maps sensory space, and how nervous systems encode timing information.