Biomedical Engineering Reference
In-Depth Information
5
ROBUSTNESS IN BIOLOGICAL SYSTEMS:
A PROVISIONAL TAXONOMY
David C. Krakauer
Santa Fe Institute, Hyde Park Road, Santa Fe, New Mexico
Biology is a domain of tension: on the one hand, biology is concerned with transforma-
tion and the generation of diversity; on the other, biology is concerned with the persis-
tence of improbable structural regularities. The historical sciences in biology, principally
evolution, have focused on change. The mechanistic sciences in biology, principally
medicine, have focused on stability. Robustness, as a research program, aims to uncover
those evolved mechanisms promoting the persistence of regularities. Here I organize
mechanisms of robustness into a phenomenological taxonomy, grouping biological
mechanisms into principles of robust organization. These include: Redundancy, Purging,
Feedback, Modularity, Spatial Compartmentalization, Distributed Processing, and the Ex-
tended Phenotype. I present case studies in which mechanisms representative of each
principle are described. These case studies serve to illustrate the ubiquity of specialized
robustness mechanisms in all complex biosystems.
1.
A FUNDAMENTAL BIOLOGICAL DICHOTOMY
:
ROBUSTNESS AND EVOLVABILITY
Biologists have been motivated by two fundamental sets of questions. One
set is associated with the generation and maintenance of genotypic, phenotypic,
and functional diversity. The second set is associated with genotypic, phenotypic,
and functional invariance. Evolutionary theory, following Darwin (9,10),
Address correspondence to: David C. Krakauer, Santa Fe Institute, 1399 Hyde Park Road, Santa Fe,
NM 87501 (krakauer@santafe.edu).
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