Modularity (biology)

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Modularity (biology)

Modularity is the idea that biological systems are built from separate units called modules. These modules can work on their own but also fit together to form bigger systems. Modularity appears at many levels, from molecules and cells to whole organisms, and it helps networks run more efficiently and respond to change.

Evolution of Modularity

Scientists have debated how modularity starts and persists. One influential view by Günter Wagner suggests four main ideas:

1) Selection for the rate of adaptation: Different parts of a network can evolve at different speeds. The parts that evolve faster tend to reach stable changes sooner, pulling related genes to evolve together.

2) Constructional selection (pleiotropy): When a gene appears in multiple copies or has many connections, it can be kept because it supports many interactions. This can help maintain modular structure, especially after gene duplication.

3) Stabilizing selection: Very strong stabilizing forces can make it harder for new modules to form by keeping existing interactions intact. This can slow the evolution of new modularity.

4) Combined effect of stabilizing and directional selection: Stabilizing forces can create barriers, but directional selection can still push the system along a path toward an optimum, allowing some modular changes to occur.

A newer idea, from 2013, adds a different twist. Clune and colleagues showed that networks face connectivity costs: more connections can reduce overall performance. They built models where networks evolved with two criteria—performance alone, or performance plus a penalty for excessive connections. Networks that accounted for the costs formed modular, more efficient structures and performed better across tasks. This suggests modularity can arise because limiting connections makes networks faster and more reliable, not just because of selection for adaptation.