Isolation factors and kinship by isonymy in a group of parishes in northern Tuscany (Italy): Influence of within-parish similarity level on between-parish similarity pattern

Many studies have found surname analysis to be a powerful tool in the assessment of human microevolution, even though the method is based on a number of assumptions that are not likely to hold true in many cases (Crow 1980, 1983, 1989; Lasker 1985; Relethford 1988; Rogers 1991 and references therein). The effects of violating these assumptions (namely, monophyletic surname origins, roughly equal rates of migration for males and females, minimal changes as a result of surname mutations) are usually considered less important in studying population structure when the analysis concerns relative measures of kinship among subgroups of a single population.

Surname analysis has been applied to Italian populations to study temporal variation of inbreeding (Pettener 1985, 1990); many studies are related to large areas and concern migration (Barrai et al. 1987, 1989; Piazza et al. 1987; Lucchetti and Soliani 1989; Biondi et al. 1990; Beretta et al. 1993). Because surnames are extremely variable and geographically localized, they should maintain their heterogeneity even in small areas, making them a potentially useful tool for detecting migration patterns on small geographic scales. Few studies, however, concern surname analysis in small areas (Fuster 1986; Lasker et al. 1985; Presciuttini et al. 1990), and problems related to population structure at this hierarchical level are not yet well known.

Our purpose here is to study the influence of within-group similarity level on between-group similarity pattern when surnames are used to trace population movement at the low hierarchical level of parishes. The parishes studied, which have a well-documented and highly differentiated demographic dynamic during this century, are located in the Apennines of northwestern Tuscany and are administratively grouped into the Bagni di Lucca municipality.

MATERIALS AND METHODS

Biosocial Background. A mountainous landscape has resulted in population dispersion in 34 hamlets over an area of 164.65 km sup 2 , grouped into 21 parishes and located on the right and left sides of the Lima River, which flows through the valley of the same name. Five principal bridges cross the Lima River; the most ancient of these was built in 1317. The main demogeographic features of these parishes are reported in Table 1 and in Figure 1. (Table 1 and Figure 1 omitted) The actual age of these settlements is not well known, even though the oldest available documents, dating from the eighth to eleventh centuries, testify to their antiquity.

Most of the villages were located, for reasons of defense, on hilltops separated by deep and narrow valleys formed by the tributaries of the Lima River. Only during the nineteenth century, when difficult accessibility was no longer an advantage, did the number of inhabitants in the Villa, Ponte, and Fornoli parishes, located at the base of the principal valley, and their importance in the area increase.

The Lima River valley is bounded on all sides by steep mountains, which rise to over 1500 m and are crossed by few footpaths. There are no settlements further up the valley or across the neighboring mountains. Thus the region forms a well-bounded natural unit for isonymy and genetic analysis.

The geographic characteristics of the area and its poor economy, mainly based on agriculture, led in the nineteenth century to a high emigration rate, as documented by the substantial population decrease, which the whole municipality has undergone (Table 1). The present resident population should therefore have been affected only slightly by the generalized increase in population movement that occurred after World War II.

Data. The data used in the present study were collected from the archives of the Registry Office of the municipality of Bagni di Lucca. Only one surname for each family was analyzed to minimize possible redundancy resulting from close relationships within the sample. The surnames selected concern the head of each household; when the head of the household was a married or a widowed woman, the surname of her husband was selected. The 3052 surnames thus obtained, which represent all the families currently living in this municipality, were subdivided according to the parish of residence of the family.

Methods. the kinship values by surnames within and between parishes were evaluated according to the method of Relethford (1988) as one-fourth of the random isonymy coefficient according to the formula

(1) (Equation 1 omitted)

where

(2) (Equation 2 omitted)

n sub ik is the number of individuals in population i with surname k, N sub i is the total number of surnames in population i, and summation is over all surnames; and according to the formula

(3) (Equation 3 omitted)

where

(4) (Equation 4 omitted)

n sub ik and n sub jk are the numbers of surnames in populations i and j, N sub i and N sub j are the total number of surnames in populations i and j, respectively, and summation is over all surnames.

The obtained pattern of similarity was compared with the expected patterns based on geographic distance (road distances, directly measured by an odometer). Road connections between parishes were not present until after World War II; hence road distances should point out the impact, if any, of the new transportation facilities.