Years ago, someone here on HN compared evolution of the DNA to the evolution of large-scale code bases. This really stuck with me. One example that was brought up was: There is no single region in the DNA that's responsible for the human eye. More generally: The mapping between phenotype and genotype tends to be fuzzy and the individual role and impact of genes is often unclear. Now in a somewhat similar way, in large code bases the mapping between features and code can also become fuzzy, and the role of a single line of code can be surprising. There can be side effects and a ton of implicit/hidden dependencies across the code base, maybe even spooky action at a distance. As a result, product features will no longer be confined to individual code files or modules. Rather, their exact logic will spread out all across the code base, and a single line of code can affect many features at once.
