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  • Title: Density-dependent migration and human population structure in historical Massachusetts.
    Author: Relethford JH.
    Journal: Am J Phys Anthropol; 1986 Mar; 69(3):377-88. PubMed ID: 3518482.
    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. The extent to which migration and genetic variation is affected by population size is examined using data on historical populations in Massachusetts. The data are from 4,859 marriage records for the period 1741 to 1849. The results indicate that migration is more frequent among the smallest and the largest populations and less frequent among medium-sized populations. Reasons for this finding are given, and their genetic significance is assessed.
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