The first reference to processes of social substitution in working-class neighborhoods can be found in the industrial England of the century (Engels, 1865). However, this process is typical of capitalism in the second half of the century. Although the first reference to the term is attributed to Ruth Glass in 1964, regarding a study on London, it was nevertheless Michael Pacione (1990) who defined the concept as it is understood today. Pacione delimited gentrification to processes in which there is a displacement of one group of inhabitants (lower class, generally) by the introduction of another, for its revaluation. It establishes three phases for the process: a phase of abandonment by the lower classes, a phase of repopulation by the upper-middle classes and a phase of economic revitalization. It is necessary to note that, although this sequencing fits most of the cases, the first phase could be completely dispensable, since it refers to a pre-capitalist state of the sector that does not necessarily have to occur. There are gentrified neighborhoods that were originally born as working-class neighborhoods in the capitalist stage of the city and that previously corresponded to demographic gaps.
Geographer David Ley, professor of geography at the University of British Columbia, presented the document Inner city resurgence units societal context in 1978, establishing a theory based on demand. In this way, Gentrification would be a consequence of the economic, sociocultural and demographic restructuring of the urban space. The changes in the economic structure of capitalism give rise to a new social group sponsored by the increase in the importance of the service sector and technical and white-collar workers with greater purchasing power than the classic working class. This would be the subject, which could materialize in a new social class, which bids for central residential spaces. The trend within the research established by Law, based mainly on consumption, has its main weakness in the little importance given to the role of supply and promoters. In this case, the supply would be a direct consequence of a pre-existing demand.
In response to D. Ley's proposals, another conceptual approach proposed by geographer Neil Smith, professor and professor of geography at Rutgers University in New Brunswick (New Jersey), emerges. Smith focuses his attention on the production of gentrifiable space, disregarding consumption as the driving force of the process. This author defends that the establishment of a theory on gentrification must include the study of demand and supply, but he gives priority to supply in his explanation, so that structural economic factors are predominant. The agents that act as the driving force of gentrification are those with the capacity to influence the real estate market, credit institutions, large developers, etc. The central point of Smith's theory lies in the rent gap or potential profit difference obtainable through a more lucrative use of the land. This approach is part of the so-called life cycle theory of neighborhoods, according to which neighborhoods experience phases of growth, decline and potential revitalization or renewal.[18].
Some scholars have come to describe the gentrification process as a diverse and chaotic phenomenon, difficult to define[19] and that can take and has taken many forms. The working-class neighborhood can be transformed into an upper-class neighborhood, but also into a middle-class neighborhood associated with groups of great cultural significance, with a predominance of elements of artistic expression, commercial premises where design and a bohemian atmosphere prevail. The degraded historic neighborhood can be transformed into a historic neighborhood, its rehabilitation focused on its use as a tourist attraction and incentive, in addition to the real estate sector, hospitality and commerce associated with tourism. On the other hand, we have the case of the old working-class industrial area that has remained anchored in the center of the city, with marginal productive activities and numerous premises in a state of abandonment that has become an upper-class residential area.
These types of gentrification may actually appear combined, however, this separation is not real. In all cases of gentrification, even if there is the presence of old industries or a notable historical value of the complex, the fundamental and common factor of the process is the existence of a humble population displaced by another with a higher level of income, that is, an invasion of space by upper-middle classes and the replacement of the population and to a large extent of the original hamlet, as well as a revaluation of previously devalued land.
The historical character and the presence of industry are inextricably associated with the industrial working-class neighborhoods developed in modern cities between the 19th century and the beginning of the 19th century; in the case of Seville, from the last two decades of the . Working-class neighborhoods that at a given moment stop fulfilling their function and are abandoned and degraded. Gentrification in these cases supposes the transition from the city as a support for productive activity, to the city as production, as a commodity. This commodity city responds largely to tourist exploitation, through the use of architecture, historical heritage, design, art, etc. It is the booming urban economy, postmodern and focused on the financial and services sector, which finds as its protagonist a middle class with a high cultural level and sufficient level of income.
It is therefore a question of a substitution of population and activity, of a population and productive activity in sharp decline for another that is booming. The factor that makes them compete for space is the situation of the population and the declining activity in a central space of interest, from which large capital gains can be extracted through its enhancement.
It is therefore a dynamic of capitalism that fits perfectly into the logic of the free market. The separation between different gentrification processes does not seem acceptable; Gentrification is a complex phenomenon that can take different forms, but it is delimitable and unique to the extent that it is produced by structural dynamics of post-Fordist capitalism and plays a fundamental role in the restructuring of urban space as a consequence of the productive and social restructuring of current capitalism.