This paper is published in Proceedings of the Genetic and Evolutionary Computation Conference (v 2, p 1444), W. Banzhaf et al., eds., Morgan Kaufmann Publishers, San Francisco, 1999 [PDF]; and was presented at that conference, GECCO-99, in Orlando, Florida, 13-17 July 1999.|
In Real or Artificial Life, Is Evolutionary Progress in a Closed System Possible? What'sNEW | Lenski et al.
Assumptions are more striking than ideas — Alexander Hiam
Evolutionary progress in life on Earth is evident in the long series of steps that lead from prokaryotic life almost four billion years ago to the variety of multi-celled eukaryotic creatures with specialized organs, tissues, systems and features that exist here today. It is driven by the accumulation of new genes, the encoded instructions for life. We would like to understand this progress.
Energy reaches Earth from the sun, of course, but encoded instructions do not. We have long believed that evolutionary progress takes place in a biologically closed system, because we thought, until recently, that space was a perfect barrier to life, making our whole planet a closed biological system.
Today, however, we know that space is an imperfect barrier to life. We now know that cells can survive in space and could be delivered in viable form to Earth's surface (NASA, 1999). We now know that dormant bacterial spores can remain viable for at least 25 million years (Cano and Borucki, 1995); it is reasonable to suppose that they are immortal (Postgate, 1994). There is growing evidence that Mars once harbored bacteria, and that rocks containing them have reached Earth (McKay et al., 1996). It is no longer certain, nor even likely, that Earth's biological system is closed.
Closed-system demonstrations of evolutionary progress in biology are not difficult in principle, but they have not been convincingly done. The most ambitious demonstration to date is a series of experiments on E. coli that have cumulatively run for 24,000 generations. Although mutation and recombination were rampant, no new genes or suites of genes with new functions were reported to have evolved. Only microevolution or sideways adaptations by mutations that enabled, disabled, or slightly changed existing genes took place (Papadopoulos et al., 1999; Vulic et al., 1999).
Meanwhile, biologists are finding more and more evidence, like viral genes in humans (Sverdlov, 1998), indicating that the lateral transfer of genes is a ubiquitous process. The biological means to make evolutionary progress in an open system are becoming well known (Lake et al., 1999).
At this point, the case for evolutionary progress in a biologically closed system depends heavily on the remotest evidence of all, the new perfect barrier to life, the big bang. If the whole universe is a permanently closed system that began in a lifeless state a finite time ago, then evolutionary progress, including the origin of life, must have subsequently happened in it. But the big bang theory is plagued with frequent surprises (e.g. Glanz, 1998). In some versions, big bangs are preceded by other big bangs ad infinitum (Guth, 1997), and ways for life to persist through big bangs have been proposed (Frautschi, 1982; Krauss and Starkman, 1999). In any case, to understand evolutionary progress biology should be able to cite firmer and more immediate evidence than the big bang!
With its basis insecure and under revision, and with an alternative becoming apparent, the theory that life makes evolutionary progress in a closed system needs additional support.
Computers, like life, rely on encoded instructions. They also exhibit evolutionary progress. Accumulated improvements have made commercial software far more powerful today than only fifteen years ago. Of course, this evolution has occurred in an open system, because people installed the improvements. But computer experiments that attempt to model evolutionary progress in closed systems are under way (e.g. Ray, 1996). The work is called "artificial life" and various other names, and the experimental environment is not restricted to conventional software. Obviously, a closed-system model that exhibited lifelike, sustained evolutionary progress would have profound importance for biology. In fact, many closed-system computer models exhibit surprising behavior or solve preestablished problems. But in spite of much honest effort, none has achieved ongoing, open-ended evolutionary progress. They all remain confined within their original parameters.
Nevertheless, computer scientists are confident that an unquestionable demonstration of evolutionary progress in artificial life is imminent, because they think they are only trying to model a phenomenon already proven in biology. Many biologists, on the other hand, are under the impression that computer models have already corroborated evolutionary progress in a closed system.
Yet the phenomenon has not been unequivocally demonstrated in either medium. Until it is, one can reasonably doubt that evolutionary progress in a closed system is possible, in real or artificial life.
AcknowledgmentsThe author thanks Max Garzon, Chris Langton and Dan McShea for their advice and encouragement.
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