Relatedness and kin-structured migration in a founding population: Plymouth Colony, 1620-1633.

To test the common assumption of no genetic relationship in a founding population, we calculated average relatedness (r) for the emigrants to Plymouth Colony from Europe on seven voyages from 1620 to 1633. Of 355 individuals, 255 could be individually identified and 4 generations of genealogic depth accounted for. Mean r was 0.00399 (S.D. = 0.00395) and ranged from 0 for 76 individuals to 0.01574 for a single female. There was a strong sex difference in relatedness; 91 females had a mean r of 0.00557 (S.D. = 0.00428), and 161 males had a lower mean r of 0.00308 (S.D. = 0.00349) (t = 4.71, p less than 0.001). The major difference was in the lower proportion of females with no relations (n = 12, 13.2%) compared with males (n = 64, 39.8%) (t = 5.05; p less than 0.001). If relatedness is calculated only for those with relations, females still have a significantly higher mean r value (0.00641, S.D. = 0.00393) than males (0.00511, S.D. = 0.00312) (t = 2.38, p less than 0.05). Thus a higher proportion of females had relatives migrating to the colony, and relatedness is significantly higher among them. The majority of kin links (79.8%) were those within the nuclear family--parents, children or siblings. For the continuity of kin-structured migration, relatedness was highest among voyagers in the same boat load and lower between voyages, with no apparent time trend. The low relatedness value of the Plymouth migrants is a result of community history and recruitment practices for the new colony. For this European population average relatedness was approximately an order of magnitude below that of traditional groups, and the assumption of no genetic relationship is not unwarranted.