Biology Reference
In-Depth Information
much more phylogenetically primitive sludge.
20
Generative entrenchment also
cannot grow beyond limit. At some point the mutation rate (and 'genetic load')
gets too great to preserve the structure, and we should expect an equilibrium
between entrenchment-building and entrenchment-breaking processes. Michael
Lynch et al. (1993) have analyzed this from one perspective (still within tradi-
tional population genetics) and described the behavior above the equilibrium as
'mutational meltdown'. Reduced absolute fitness from accumulating mutations
decreases population size, leaving fewer possibilities to find ameliorative muta-
tions, and the population goes extinct. His original application was to explain
why asexual reproduction was not more common, but similar problems can
arise for populations with mutation-sweeping sexual recombination if the overall
genetic load is too great. Selection cannot maintain indefinitely large genome
sizes, though various kinds of adaptations can enormously increase the size that
can be maintained. (Wimsatt & Schank, (1988, 2004) consider different kinds
of systemic adaptations involving generative entrenchment and genetic load that
can do so.) At some point the design architecture cannot grow more: It faces
a complexity catastrophe. The only escape from this is to start over again with
these systems as units to build larger differentiated structures. This is the route to
new levels of complexity, as Maynard Smith and Szathmár (1997) have argued,
and also the route to a new hierarchical systems biology.
REFERENCES
Aldana M & Cluzel P.
A natural class of robust networks
. Proceedings of the National Academy
of Sciences: 100 (15): 8710-8714, 2003.
Arthur W.
The Origin of Animal Body Plans
, New York: Cambridge University Press, 1997.
Bruggeman F.
Of Molecules and Cells: Emergent Mechanisms
. Academisch Proefschrift, Vrije
Universiteit Amsterdam, 2005.
Culp S.
Objectivity in Experimental Inquiry: Breaking Data-Technique Circles
. Philosophy of
Science: 62, 438-458, 1995.
Darwin C. 1859.
The Origin of Species.
Facsimile reprint, Harvard University Press, 1964.
Davidson E.
Genomic Regulatory Systems: Development and Evolution
. New York: Academic
Press, 2001.
Davidson E.
The Regulatory Genome: Gene Regulatory Networks in Development and Evolution.
New York: Academic Press, 2006.
Davidson E & Erwin D.
Gene Regulatory Networks and the Evolution of Animal Body Plans
.
Science: 311, 796-800, 2006.
20
A simulation at http://www.athro.com/evo/elephs.html exhibits Darwin's illustration of the power of geo-
metric growth with the reproduction of elephants (1859, p. 64). With a rate of growth you specify, you are
invited to estimate how many years it will take until there is a sphere of elephants expanding out beyond the
orbit of Pluto at the speed of light. (Not so long.) Long before that, of course, the center would have undergone
gravitational collapse and sucked it all in! (Any guesses on the Schwartzchild radius for elephants?)
