Density‐dependent migration and human population structure in historical Massachusetts

Abstract

Studies of population structure often focus on the effects of population size and migration rates on genetic variation. Few studies, however, have investigated the relationship between these two factors. The purpose of this paper is to determine the extent to which migration (and gene flow) is density-dependent (that is, affected by population size) for populations in historical Massachusetts. Data from 4,859 marriage records were analyzed from four populations in north-central Massachusetts during the time period 1741 to 1849. These data were placed into 29 samples defined in terms of population and time cohort. Within each cohort the overall exogamy rate was computed along with three estimates of gene flow based on marital migration: local migration (k), long-distance migration (m), and effective migration rate (me). Three samples show unusually low rates that reflect the history of settlement. Regression analyses were used with the remaining samples, and they show nonlinear density-dependent migration that is unrelated to temporal trends. Migration is highest in samples with small population sizes (less than 800) and large population sizes (greater than 1,600). Migration is lowest in medium-sized populations. Two processes are suggested to explain this curvilinear relationship of migration and population size. In small populations, the lack of suitable potential mates and/or availability of settled land leads to an increase in migration into the population. As population size increases, this migration decreases. After populations reach a certain size, migration increases again, most likely reflecting the economic pull of larger populations. These patterns could act to enhance, or counter, genetic drift, depending on the direction of density dependence.

Isonymy, inbreeding, and demographic variation in historical massachusetts

“…Previous papers have dealt with other aspects of population structure. Relethford (1986a) found nonlinear relationships between migration levels and population size. Estimates of differentiation among four of these populations has been analyzed by Relethford (1986bRelethford ( , 1988a) and compared to measures derived from migration matrix analysis.…”

“…The number of years since settlement was defined as the number of years since the year of settlement to the beginning of a time cohort (see Relethford, 1986a, for dates). Population size was measured as the midcensus value, following Reletkford (1986a).…”

“…These records contain information on births, deaths, and marriages from a town's earliest founding through the year 1849 (Hanson and Rutherford, 1981). Details of data collection and discussion of the accuracy of the marriage records have been published (Relethford, 1986a).…”

“…Each marriage was placed into one of eight possible time cohorts defined in terms of the year of marriage. Ten-year cohorts were originally selected for analysis, but several earlier cohorts were modified to ensure an adequate sample size (>50) for each town and cohort (Relethford, 1986a). Since some towns had later settlement dates than others, not every town is represented by either time cohorts.…”

Microdifferentiation in historical Massachusetts: A comparison of migration matrix and isonymy analyses

“…From vital records for these populations covering the years 1741-1849 (see Relethford, 1986a for details), 4,859 marriage records were obtained. The published vital records cover the time period from a town's initial settlement through 1849.…”