Glossary of terms

Term	Definition
Attractor state	A particularly stable transcriptional state that attracts cells in less stable states toward it, that is, cells exhibiting violations of regulatory rules will tend to gravitate toward the nearest attractor state; a modern descriptor of a canalized state attributed to transcriptional networks
Canalization	The process of producing phenotypic constancy in the presence of varying conditions
Capacitor	Gene product involved in maintaining robustness to environmental stress
Chromothripsis	Multiple chromosomal rearrangements occurring within a single event either within a single chromosome or across multiple chromosomes; chromosome shattering
Epigenetic landscape	A metaphor describing the regulation of developmental trajectories by buffering mechanisms that dictate which phenotypes cells are likely to adopt; the shape of the landscape is derived in part due to the cell-intrinsic action of gene regulatory networks and is not equivalent with the modern term “epigenetics”
Evolvability	An organism's capacity to generate heritable phenotypic variation
Fitness	The propensity of an individual to exhibit reproductive success (if the individual is a cell, it is the number of daughter cells a parent cell is likely to produce); a quantitative metric for selection representing how well adapted an organism is to its environment
Gene regulatory network, or GRN	A collection of regulatory molecules that work in concert to determine expression of gene products, the transcriptional state, and, in turn, the final phenotype of a cell; a mechanistic descriptor for the buffering interactions within a cell that contribute to the final shape of Waddington's epigenetic landscape
Mutator phenotype	A phenotype exhibiting higher than normal mutation rates
Punctuated equilibria	An alternative hypothesis to gradualism, or slow and steady evolutionary change, in which periods of equilibria are disturbed by “rapid and episodic events of speciation”
Purifying selection	Selection resulting in reduction in frequency of individuals; negative selection
Robustness	The ability of a cell, tissue, or organism to maintain a constant phenotype despite various genotypic and environmental perturbations
Selection	A force that acts upon phenotype and results in preferential survival of individual organisms due to greater relative fitness within an environment; the key deterministic mechanism of evolution