From: Stephen Friedman (sfriedma@cs.washington.edu)
Date: Tue Nov 30 2004 - 21:06:41 PST
The Evolutionary Origin of Complex Features by Richard E. Lenski,
Charles Ofria, Robert T. Pennock and Christoph Adami
The authors attempted to show that complex features could be
developed using the principles of evolution by applying those principles
to computer programs and evolving a complex logic function, then look at
the complete traces to see how they evolved, something that isn’t
possible with current biological evolution due to incomplete
information. Their first major claim is that complex features generally
evolve by modifying existing structures and functions. They illustrate
this showing that their complex function evolved in trials where simpler
functions are also rewarded, while the complex function did not evolve
in trials where it was the only rewarded function.
Another major claim is that are many different solutions to the same
problem. They base this on the observation that in their different
trials, many different paths were taken to each of the final organisms
which demonstrated the best traits. Swapping their assumptions and
conclusions, at the end of the paper they conclude digital organisms
allow for studying problems in biology which are can’t be studied in
organic organisms because of incomplete observability and impractical
time/space/resource requirements. However, to accept this conclusion,
one must assume the validity of the hypothesis that the theory they
demonstrated is indeed true in organic organisms, in order believe that
showing this organic trend in digital life validates the use of digital
life.
One flaw in the paper is that they took a very simplistic approach
to evaluating the number of “needed” instructions for operations. They
did this by simply replacing one instruction at a time with nop’s. They
begin to realize that this is not a good metric, and relax it to a way
of finding “minimum needed” instructions for a computational trait.
This is still not correct, for it is easy to imagine a pair of jump
instructions, one which jumps to the second, and the second jumping back
to the instruction after the first, which perform no valuable
computation and could be nop-ed out as a pair, but when nop-ed out
singly, the second jump appears to be a “needed” instruction. A more
accurate term for what they found was the susceptibility of EQU to
deletion via single instruction nop replacements. It is not entirely
clear to me what valuable information this type of metric provides.
They use it to claim in an ill-defined qualitative way that EQU is a
“fragile” function, and that it only sticks around because it is very
valuable.
The paper does prompt several questions for further research. The
first is that put forth in their discussion: what differences occur when
sexual vs. asexual reproduction is used. Next, their research implied
there were a large number of potential paths to evolve and organism
sporting all the computational traits, but is there a real way to get an
accurate calculation/estimate of the relative number of potential paths
compared to the total or total reproductively viable? It doesn’t take a
stretch of the imagination to see that this is just another method of
search. The space is that of all genome sequences, with the goal of
finding a viable path through genome sequences that leads to a sequence
with all useful traits. Thus, knowing abundance/scarcity of valuable
paths, we could identify problem domains with similar ratios and partial
solutions where using evolutionary principles would be promising. This
would also give us a better idea of how well evolution performs at
finding useful organisms – whether it is a good search heuristic or
simply taking advantage of dumb luck – and allow it to be compared to
known search algorithms. Also, they claim that EQU is a more complex
function, and they assign reward values to functions based on what they
consider the complexity, but where do those numbers come from. Should
there be a complexity theory of evolved attributes, and could it be
applied to organisms to say just how complex an eye is? You never know,
we may find that a little toe is more complex than an eye.
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