Swedlund, Alan C.

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Chapter 5, Summary and Conclusion

University of Massachusetts AmherstScholarWorks@UMass AmherstResearch Report 07: The Genetic Structure of anHistorical Population: a Study of Marriage andFertility in Old Deerfield, MassachusettsAnthropology Department Research Reports series12-2-2008Chapter 5, Summary and ConclusionAlan C. SwedlundPrescott CollegeFollow this and additional works at: https://scholarworks.umass.edu/anthro_res_rpt7Part of the Anthropology CommonsThis Article is brought to you for free and open access by the Anthropology Department Research Reports series at ScholarWorks@UMass Amherst. Ithas been accepted for inclusion in Research Report 07: The Genetic Structure of an Historical Population: a Study of Marriage and Fertility in OldDeerfield, Massachusetts by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contactscholarworks@library.umass.edu.Swedlund, Alan C., "Chapter 5, Summary and Conclusion" (2008). Research Report 07: The Genetic Structure of an HistoricalPopulation: a Study of Marriage and Fertility in Old Deerfield, Massachusetts. 7.Retrieved from https://scholarworks.umass.edu/anthro_res_rpt7/7Intl'ocIuction CHAPTER V SUMMAllT AND CONCLUS10NS 67 '!'h. .videnc. fr01ll. De.rfie1d whichi. pre.ented in the fORloin. chapters would t.nd to confin the obaervation of Sutt.r aftd ban-Nloc-Toan, (1957), that the departure of htllllm pOlnllationa fro. genetic mod.ls may oft.n be ,r.at. Thi. has b.en a •• battated }lat'ttetdarly in r.lard to population nu_.ra and the prob1e. of fertiU:ty and maration. Aa Nee1 (1958) pointed out mdtpub1er'i1&1 t4d.t.rated (1959) th.r. i. v.ry llttl.il1tOl'1lAticm 011 •• lutiM111 huaan popu-lations, y.t s.l.ction is a v.ry .ipific.t.tldrel •••• td.,.rture froa the conditions normally .ssu .. d in hU1l&1\ pOptd.-tic.a. Ob,erv.-tions and conclusions cone. mini the .... id.c. fr:OIl ' be.!"'!i.l. .Y' be su..ari •• d a. tallow.: population~·r· 1. A1thoulh the "effectiv. population ai .... t. au-.tu1 para-_t.r in conc.pt, it ia extre.1y d1fficulttoucually q.ou'ttl, in _1'1. '!'h. nature of int.rnal population .rOW'tht_~.ti •• ' ddpanera-tiona1 overlap in human beh.. JlUlkea eati_te. .ubjee11to a variety of errors. The Deerfield .aapl. cUa'llOtiatrate. that if c •• ti1t.tpopul.tion si •• 1a aasuaed, the tendency is to over.ati .. t. the .ffact.ivepopu1a-tion ai... Further, a failure to il1c1ude th •• f ·f.et. 0·£ tliataU.- will introduc. errors in .sti_tiua the .i.. of the bra.dt •• ;ellUl.tift and u1till&te1y the .ffective population si ••• 68 2. The ~mportance of knowing population numbers, in regard to genetic structure, is primarily for an understanding of the possibilities of non-random mating in the subject population. In the present study estimates of inbreeding, and the opportunities for drift, are baled on an analysis of isonomy, or the tendency for people of like lurname to marry. Observations for Deerfield include: (a) inbreeding tends to be low and mating is essentially random during the period studied; (b) there may be a tendency for exogamous mar-riages to commonly occur between related individuals; (c) close con-sanguineous marriages are less fertile than non-consanguineous mar-riages; and, (d)inbreedina tends to increase through time. This may confirm previous observations concerning effective population size, namely, that large family siae increales the likelihood of relatives to marry each other, which subsequently decreases rela-!!!! effective population size. 3. It would appear from the evidence from Deerfield and other communities studied (e.g. Alltrom and Lindelius, 1966; KUchemann et a1, 1967) that milration is the most significant variable in attempt-ing to define population numbers and that measurement of milration is very important in community studies. Migration 1. Exogamoul marriages occurring in time and apace form the basi. for a study of milration in Deerfield. The frequency of out-side marriages increases relatively and absolutely in time, and the mean urriage distance incre .. es only very slightly in time. By far the majority of exolamous marriages occur with members of nearby 69 neighboring communities. 2. The D.erfield population tends to be patrilocal but this is by no means an exclusive patt.rn. The general implications of the effect residence patterning has on sex-linked loci is discussed. If only females are exchanged between populations in gene flow, then the equilibrium frequ.ncy of newly introduced alleles will be achieved faster than if only males are exchanged, or if migration is equal with respect to sex. 3. The migration in Deerfield is plotted with respect to frequency over space and related to the geometriC and exponential curves. The exponential curve provides a slightly better fit and i. probably most often the best approximation of human migration. Several p .. t studies have found milration to show a leptokurtic distribution over spac., as is true of Deerfi.ld. 4. The nature of migration obs.rved for Deerfield c~ar.s closely with that found for historic Enllish villages (KOch.mann et aI, 1967). The concept of "neilhborhood knowledge" provides a formal explanation of the nature of migration and both population density and settlement pattern are important variables affectinl miaration. Selection 1. Crow (1958) has d.fined the Index of Total Selection which measures the aaximum possible amount of selection. Evidence fro. most studies (e.l. Spuhler, 1962; Kirk, 1966) suggests that the coapo-nent of fertility is greater than that of mortality in the total •• lec-tion intensity. However, in the Deerfield example and oth.rs ( •••• Hutt.rites), where family size is large and the ratio of mean f .. ily 70 size to variance i. small, the mortality component ia likely tobe greater. Aa the ratio of family size to varianca increases, as with many modern populations, the total index of aelection will tend to increase. 2. In an analYBis of aelection and heterosis it is found that endogamous qarriaae. in Deerfield are more fertile than exogamous marriaae.. However, if mean family size is measured at maturity, difference. in fertility become non-significant. Speculation in regard to these results is as follows: (a) Adaptation to local selective factors and maximum compatibility of all polymorphic allele. in the local population select positively for a larae mean family size at birth amona native marriages; (b) incompatibility of new allelic combination. and lower fitness to local conditions tend to increase fetal and neo-natal deaths among migrant matings, so that mean family size at birth is lower than for native matings; and. (c) certain new allele. or allelic combinations are favorably selected for and the viability of offspring of migrant matings who 8urvive birth is greater than that of native offspring, who may carry greater segregation loads and who do not possess favorable new alleles. This results in a tendency for mean family size at maturity to be closer than at birth for native and migrant matings. 3. Past studies on heterosis in man and animals are somewhat equivocal in regard to expected results. This suggests that no single outcome from out crossing is predictable, and that the outcome ia any specific situation will depend on the fitness of the two parent popu-lations to the selective environment in which the offspring are produced --- - - .-.. _ .••........ 71 and raised. Although this may appear to be a simplistic and obvious stat ... nt, it is seldom made by human geneticists. The study of Deerfield has brought to light an important recurring factor, this is the effect cultural patterns may have on population structure and ultimately genetic structure. In the present study evidence on human migration, residence practices, settlement patterns, and other aspects of mating behavior suggests significant non-random occurences. These events can have a definite effect on the distribu-tion of genotypes and the microevolution of a breeding population. This brings to the attention of physical anthropologists who are working on a population at a specific pOint in time, the importance of having knowledge of the demographic and ethnographic history of the subject population. I havaattempted, by using Deerfield as an example, to demonstrate the way in which actual populations may differ from the con~itions commonly assumed in genetics models. It would be overstatinl my case to imply that geneticists are not aware of the way in which actual popu-lations depart from the usimplifyinl assumptions," for much of the recent literature in human genetics is concerned with this very problem (see Schull and MacCluar, 1968; MOrton, 1969). It ia, however, very clear that the need for comprehensive studies of local populationa, includinl historic ones, is great. The expansion and refinement of mathematical-genetical models is dependent upon a better empirical base from which generalizations may be made. 72 

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Chapter 5, Summary and Conclusion

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