Spatially-structured evolutionary algorithms (SSEAs) have allowed evolutionary search to be scaled up to increasingly larger and more difficult problems. While their use is becoming more widespread, the basic underlying theory behind them has some omissions. In particular, the effect of random genetic drift on spatial structures is largely unknown. This paper uses models derived from random walk theory to describe the behaviour of a specific class of SSEA. The match between the model and experimental findings is very good. It is the intention that this paper serves as the basis for a more abstract model of drift that encompasses all spatially-structured population models.