No natural process computes. When observing a physical dynamic, in
order
to associate it with a computation, one has to fix a "measure" (an
association of a state of the dynamics to a number, at the chosen
input
time), then let the process go. Finally, measure again the result at
the
chosen output moment and produce a number, according to the given
approximation. As a matter of fact, by principle, physical measure is
always an interval (even of conjugated variables, in Quantum
Physics):
it does not intrinsically produce a number in a discrete 01
universe.
Moreover, most natural processes are not "laplacian", as Turing calls
his own Discrete State Machine in 1950: most of them are subject to
the
"exponential drift", which breaks any approximation. This is one of
the
novel features he observes (and mostly cares of) in the "continuous
systems" for Morphogenesis, in 1952.
A computation instead is a matter of alphanumeric sequence matching
and
sequence replacement; it is a rewriting procedure handled by a human
(or by a suitably programmed Universal Turing Machine). As a matter
of
fact, in order to have a physical device compute, we had to invent:
 the alphabethic coding of language (from Mesopotamia, IVth
millennium
BC, to the Greek alphabet)
 the coding of the alphabeth by numbers (Gödel, 1931)
 a purely mathematical Logic Computing Machine (Turing, '36) for
alphanumeric terms rewriting and, then, its physical implementation
as
a Discrete State Machine (Turing '4650).
This makes Turing's Logical and Discrete State Machine a unique
physical
system, when materialized, an extraordinary invention for its
stability
and perfect iterability, away from of the natural world, like the
alphabet, a dualistic device based on syntax and semantics (or
hardware
and software).
References (see http://www.di.ens.fr/users/longo )
 Francis Bailly, Giuseppe Longo. Mathematics and Natural Sciences :
the
Physical Singularity of Life, 333 pages, Imperial College Press,
London,
2011.

Giuseppe Longo, Thierry Paul. The Mathematics of Computing between
Logic and Physics. Invited paper, "Computability in Context:
Computation and Logic in the Real World ", (Cooper, Sorbi eds)
Imperial
College Press/World Scientific, 2010.

Giuseppe Longo. Incomputability in Physics and Biology. Invited
Lecture, Proceedings of Computability in Europe, Azores, Pt, June 30 
July 4, LNCS 6158, Springer, 2010.

Giuseppe Longo. Critique of Computational Reason in the Natural
Sciences, In "Fundamental Concepts in Computer Science"
(E.
Gelenbe
and J.P. Kahane, eds.), Imperial College Press, pp. 4370, 2009.
