# Complexity by Subtraction

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## Abstract

The eye and brain: standard thinking is that these devices are both complex and functional. They are complex in the sense of having many different types of parts, and functional in the sense of having capacities that promote survival and reproduction. Standard thinking says that the evolution of complex functionality proceeds by the addition of new parts, and that this build-up of complexity is driven by selection, by the functional advantages of complex design. The standard thinking could be right, even in general. But alternatives have not been much discussed or investigated, and the possibility remains open that other routes may not only exist but may be the norm. Our purpose here is to introduce a new route to functional complexity, a route in which complexity starts high, rising perhaps on account of the spontaneous tendency for parts to differentiate. Then, driven by selection for effective and efficient function, complexity *decreases* over time. Eventually, the result is a system that is highly functional and retains considerable residual complexity, enough to impress us. We try to raise this alternative route to the level of plausibility as a general mechanism in evolution by describing two cases, one from a computational model and one from the history of life.

### Keywords

Evolution Complexity Constructive neutral evolution Irreducible complexity ZFEL## Notes

### Acknowledgments

The main ideas described in this paper originated at a catalysis meeting at the National Evolutionary Synthesis Center (NESCent) in Durham, NC, USA. They were developed further and finalized into the current paper during a subsequent short-term research visit of WH at, and supported by, NESCent. We thank Robert Brandon for suggesting the apt and evocative phrase “complexity by subtraction.” Finally, one of us (DM) would like to thank Benedikt Hallgrimsson for discussions decades ago, discussions that turned out to be foundational in the development of the ZFEL.

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