Microevolution in Ferrara: isonymy 1890-1990

The distribution of surnames in the population of the town of Ferrara, as it existed in the memory banks of the Municipality Computer in June 1990, was studied by sex, age and place of birth of residents. Random isonymy was studied separately in persons born before 1901, and in persons born in the nine decades thereafter, ending with the period 1981-90. Isonymy was higher in the older age groups studied. Also other indicators of the abundance of surnames in the distribution, the common ecological indexes derived from entropy, were calculated and compared between age groups. It was found that redundancy, as isonymy, is larger in older age classes than in younger classes. Surname effective number was defined as the inverse of isonymy corrected for sample size, and it was observed that it is practically identical with Fisher's alpha. It was then possible to separate random isonymy into two components, so that for sample size N its formulation becomes Ir = 1/alpha + 1/N It was found that in Ferrara alpha increased significantly in time, indicating enrichment of surnames in the population in the century 1890-1990.