ACT Reading — Natural Science — Drill 1

NATURAL SCIENCE: This passage is adapted from the article "What Slime Molds Know" by Erin Calabrese (©2020, Science Today). Physarum polycephalum is not an animal, a plant, or a fungus. It is a slime mold—a single-celled organism capable of growing large enough to be visible to the naked eye and, under the right conditions, of solving problems that would challenge a moderately sophisticated computer algorithm. In a series of experiments that attracted wide attention in the scientific community, researchers at Hokkaido University placed oat flakes—a food source Physarum finds highly attractive—at positions corresponding to the locations of Tokyo and the surrounding cities on a scale map. They then introduced Physarum to the location representing Tokyo and observed what happened over the following days. The slime mold extended tentacle-like tubes in all directions, found the oat flakes, and then gradually eliminated the less efficient connections, reinforcing the routes that transported nutrients most effectively. After twenty-six hours, the network the organism had constructed closely resembled the actual rail network of the Tokyo metropolitan area—a system that had been designed by human engineers over more than a century. The slime mold has no brain, no nervous system, no centralized processing of any kind. It achieves this network optimization through a distributed biochemical process: tubes that carry high volumes of nutrients grow thicker and more stable, while underused tubes gradually contract and disappear. The result is a kind of bottom-up computation, a process that arrives at sophisticated solutions without anything resembling deliberate design. This finding has practical implications. Civil engineers and network designers have begun studying Physarum's optimization strategies as models for designing more efficient transportation and communication networks. The slime mold does not calculate; it responds. But the outcome of those responses can outperform human calculation in certain narrow domains. The deeper implication may be philosophical as much as practical. If a single-celled organism with no nervous system can produce solutions that rival human engineering, the question of what we mean by intelligence—and where it resides—becomes considerably more complicated.