Patterning via differential activity

It is well known that soft dispersed and condensed phases can be patterned due to variations in some property X, where X = size, shape, adhesion, diffusion, growth, erosion, deposition, activity etc. I will discuss the last of these using two examples of differential-activity-driven patterning in biological systems:
1. A general framework that characterizes how binary mixtures e.g. a poroelastic or viscoelastic network, can phase-segregate due to differential activity.
2. A specific problem of limb patterning that seeks to understand the patterns of digitiation seen in vertebrates: horses have a single digit, pigs have 2 digits, chicks have 3 fore-digits (and 4 hind-digits), we have 5 digits, and fish have many.