Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 251

sudden appearance of changes in the fossil record, without intermediate fossil forms.
He used Ernst Mayr's concept of allopatric speciation in peripheral populations iso-
lated from the main stock of population to explain the concept and proposed that the
result could be explained by migration and isolation ( Eldredge, 1971 ).
Based on earlier observations as well as on their own observations of the gaps in
paleontological evidence, Eldredge and Gould (1972) developed the theory of punc-
tuated equilibria to explain these observations as reflecting a nongradual mode of
evolution that was characterized by long periods of stasis in the evolution of species,
which were interrupted by short periods during which new species suddenly sprung,
hence the term “punctuated equilibria”: “The norm of a species or, by extension, a
community, is stability. Speciation is a rare and difficult event that punctuates the
system in homeostatic equilibrium.” It is generally known as the theory of punctu-
ated equilibria. The theory handles evolution at the species level:
Punctuated equilibria is a model for discontinuous tempos of change at one biologi-
cal level only: the process of speciation and the deployment of species in geological
time.
Gould and Eldredge (1977)
There is abundant evidence to support the “punctuated” model of evolution: the
existence of microorganisms such as cyanobacteria that remained almost unchanged
( Schopf, 1994 ) since the dawn of the evolution of life on Earth more than 3 billion
years ago, or the existence of “living fossils” such as the gingko tree, Ginkgo biloba ,
believed to have remained virtually unchanged for 270 million years, and the coela-
canth, which has remained unchanged for 409 million years ( Figure 5.3 ). These are
paradigmatic examples of the extremely long periods of stasis in the evolution of
living forms, although gradual evolution can be illustrated with equivalent examples.
The Hypothesis of Facilitated Variation
Marc W. Kirschner and John C. Gerhart presented their hypothesis for the first time
in The Plausibility of Life (2005). It is intended to explain the evolution of meta-
zoans alone. Admitting the role mutations play in producing genetic variability, the
hypothesis posits that phenotypic variability and animal evolution depend primar-
ily on internal developmental processes. Besides changes in regulatory circuits, the
hypothesis acknowledges the possibility of involvement of rare neoDarwinian gene
mutations and of the Baldwin effect in the emergence of evolutionary changes.
The hypothesis posits that there is a group of core components, consisting of
the genetic toolkit and processes that generate and operate the animal's phenotype.
Although these core components and processes are highly conserved, the physiologi-
cal adaptability of animals provides them with a capacity to form “weak linkages”
between the elements of the causal chains of regulatory circuits. The capacity facili-
tates phenotypic variation. Central to the authors' illustration of the capacity to form
weak linkages is the phenomenon of the inherent developmental plasticity, sensu ,