Structural change in an economy refers to variation in the distribution of activity and resources between economic agents. This thesis seeks to add to a long tradition of research that employs the s-shaped curve to explain and model structural change in the economy. The S-shaped curve has a tremendously rich history, ranging across biology, sociology, economics, marketing, thermodynamics and, latterly, the theory of evolutionary systems and games. It is argued that the evolutionary economics literature, in particular, correctly treats S-shaped phenomena as underlying processes which only sometimes are manifested as S-shaped curves in the data. The particular structural change problem that is addressed in the thesis is that represented by the Engel curve: the relationship between income and the structure of consumer expenditure. Since Ernst Engels seminal observations of family budgets in the 19th Century, there has been a long tradition of studies of the structure of consumer demand in the mainstream literature. However, it is argued that traditional methodologies (as exemplified by the Working-Leser model), while useful in an heuristic sense, have only limited theoretical underpinnings. Hence, given that the Engel curve is a non-linear model of structural change, the primary purpose of the thesis is to develop an alternative Engel curve that is inspired by theoretical insights from the literature on S-shaped curves. The main idea behind the alternative - or evolutionary - Engel curve developed herein is that the Engel relationship signifies inter-commodity competition for resources, and is therefore amenable to a methodology that is founded upon an explicit treatment of competition. The chosen methodology - the replicator equation is argued to be a fundamental and general representation of competition and structural change, as well as having some attractive properties in the expenditure system context. In attempting to apply replicator dynamics to the evolving structure of consumer demand, a number of conceptual and methodological problems are encountered, and the necessity to solve these is a major source of innovation in the thesis, and a major component of its contribution to knowledge. Along the way, the thesis augments a rather sparselypopulated area of the literature: evolutionary economics studies of the demand side of the economy; and empirical studies that use the replicator equation (of which there are only two at present). In the ultimate model, the basic replicator equation is significantly modified and extended, with the key development being the inclusion of a satiation effect. The model is applied to a system of 46 commodities based on data from the Australian Household Expenditure Survey. From this initial application, it is concluded that the replicator equation forms a potentially viable basis for an evolutionary Engel curve. The new model is well-behaved, and shows performance comparable with that of the foremost mainstream formulation of the Engel curve. However, more work is required to establish the generality of these results across different countries, demographics, and time periods. Keywords: Australia; consumer demand; diffusion; evolutionary economics; growth curves; logistic; replicator; replicator dynamics; S-shaped curves; structural change.
