ISOLATION BY DISTANCE

Gene flow by immigrants into isolated recipient populations: a laboratory model using flour beetles

The effectiveness of immigrants as agents of gene flow was investigated in a laboratory model, using mutant marker strains of the flour beetle, Tribolium castaneum (Herbst). We show that immigrants had an advantage over residents. The proportion of hybrid offspring (PHO), resulting from immigrant mating with residents, was higher than expected from their frequency in the parental population. This advantage was observed regardless of immigrant sex and immigrant strain. The advantage seems to result from immigrant mating advantage (although not a rare-male phenomenon) and not from better survival of hybrid offspring. However, hybrid offspring seem to be more resistant to sporozoan infection, resulting in higher PHO in sporozoan-infected cultures.

Genetic variability and gene flow in geographical populations of Ceratitis capitata (Wied.) (medfly)

Two African populations of Ceratitis capitata (Kenya and Réunion Isl.) and two Mediterranean ones (Sardinia and Procida Isl.) have been studied for genetic variability at 25 loci by electrophoresis. Wright's FST, Slatkin's Nm* gene flow estimator, Nei's distance (D) together with measures of variability such as H, P, A have been used to compare the population from Kenya with the other three. Parameters using gene frequencies (FST, D, Nm*) indicate the presence of substantial geographic heterogeneity, largely attributable to genetic drift and correlated with dispersion of the medfly from its source area (Subsaharan Africa) to the periphery. The Kenyan population has high genetic variability (assessed by H, P and A), as might be expected given its native status. Significant gene flow estimates between Kenya and the derived Mediterranean populations supports the hypothesis of recent colonization. Part of the geographic heterogeneity is related to the presence of fixed alleles in the more differentiated Réunion population although it maintains the genetic attributes of the ancestral population. Selection or other forces may have played an important role in the differentiation of this population.

Influence of gene flow and breeding tactics on gene diversity within populations

Expressions describing the accumulation of gene correlations within and among lineages and individuals of a population are derived. The model permits different migration rates by males and females and accounts for various breeding tactics within lineages. The resultant equations enable calculation of the probabilistic quantities for the fixation indices, rates of loss of genetic variation, accumulation of inbreeding, and coefficients of relationship for the population at any generation. All fixation indices were found to attain asymptotic values rapidly despite the consistent loss of genetic variation and accumulation of inbreeding within the population. The time required to attain asymptotic values, however, was prolonged when gene flow among lineages was relatively low (less than 20%). The degree of genetic differentiation among breeding groups, inbreeding coefficients, and gene correlations within lineages were found to be primarily functions of breeding tactics within groups rather than gene flow among groups. Thus, the asymptotic value of S. Wright's island model is not appropriate for describing genetic differences among groups within populations. An alternative solution is provided that under limited conditions will reduce to the original island model. The evolution of polygynous breeding tactics appears to be more favorable for promoting intragroup gene correlations than modification of migration rates. Inbreeding and variance effective sizes are derived for populations that are structured by different migration and breeding tactics. Processes that reduce the inbreeding effective population size result in a concomitant increase in variance effective population size.

On the concept of effective number

A generalized conceptual basis for Wright's notion of effective size is presented. The concept is applied to the calculation of effective numbers based on the rate of change of genetic variability. With particular reference to the inbreeding, the eigen value, and the newly introduced "diversity" effective size, the use of the concept as a means for discrimination between and identification of various effective sizes is demonstrated.

Isolation by distance in a quantitative trait

Random genetic drift in a quantitative character is modeled for a population with a continuous spatial distribution in an infinite habitat of one or two dimensions. The analysis extends Wright's concept of neighborhood size to spatially autocorrelated sampling variation in the expected phenotype at different locations. Weak stabilizing selection is assumed to operate toward the same optimum phenotype in every locality, and the distribution of dispersal distances from parent to offspring is a (radially) symmetric function. The equilibrium pattern of geographic variation in the expected local phenotype depends on the neighborhood size, the genetic variance within neighborhoods, and the strength of selection, but is nearly independent of the form of the dispersal function. With all else equal, geographic variance is smaller in a two-dimensional habitat than in one dimension, and the covariance between expected local phenotypes decreases more rapidly with the distance separating them in two dimensions than in one. The equilibrium geographic variance is less than the phenotypic variance within localities, unless the neighborhood size is small and selection is extremely weak, especially in two dimensions. Nevertheless, dispersal of geographic variance created by random genetic drift is an important mechanism maintaining genetic variance within local populations. For a Gaussian dispersal function it is shown that, even with a small neighborhood size, a population in a two-dimensional habitat can maintain within neighborhoods most of the genetic variance that would occur in an infinite panmictic population.

Genetic evidence for three species within Pseudoterranova decipiens (Nematoda, Ascaridida, Ascaridoidea) in the North Atlantic and Norwegian and Barents Seas

Genetic variation of 1017 specimens of codworm, Pseudoterranova decipiens, collected from fish and seals at 23 sampling locations in the North Atlantic and Norwegian and Barents Seas, was analysed on the basis of 16 enzyme loci. Three reproductively isolated species, provisionally designated P. decipiens A, B and C, were detected, showing distinct alleles at the following loci: Mdh-1, 6Pgdh, Np, Pgm, Est-2 (between species A and B); Mdh-3, 6Pgdh, Np, Sod-1, Adk, Pgm, Est-2, Mpi (between A and C); Mdh-1, Mdh-3, Sod-1, Adk, Pgm, Est-2, Mpi (between B and C). One F1 hybrid was observed between P. decipiens A and B, but this apparently does not lead to any gene exchange between the two species, which do not show any evidence of introgression. No hybrids or introgressed individuals were observed between P. decipiens C and either A or B. Genetic distances among conspecific populations were low (average Nei's D 0.001-0.005), even though they were collected thousands of kilometres apart, indicating high levels of gene flow within each of the three species. The values of Nei's index D were 0.44 between P. decipiens A and B, 0.57 between B and C, and 0.79 between A and C. Estimated evolutionary divergence times, using Nei's formula, range from 2 to 4 million years. Differences between P. decipiens A, B and C were also found with respect to genetic variability, morphology, geographical distribution and hosts. Mean heterozygosity values of 0.08, 0.05 and 0.02 were obtained for P. decipiens A, B and C, respectively. Preliminary morphological examination of adult males, previously identified by multilocus electrophoresis, revealed differences in the relative size and pattern of caudal papillae. P. decipiens B is widespread in the study area, whereas P. decipiens A was found only in the North-East Atlantic and Norwegian Sea. In this area P. decipiens A is most common in the grey seal, Halichoerus grypus, while the common seal, Phoca vitulina, is the main host for P. decipiens B. In Canadian Atlantic waters, where P. decipiens A is apparently absent, P. decipiens B infects both grey and common seals; a few specimens were also found in the hooded seal, Cystophora cristata. The only definitive host so far identified for P. decipiens C is the bearded seal, Erignathus barbatus; P. decipiens C appears to be widespread, occurring in both the North-West Atlantic and Barents Sea.

Gene diversity and female philopatry

The effect of female philopatry on the apportionment of gene diversity within a population is evaluated. Even with random mate selection, the apportionment of gene diversity within and among social lineages (groups of related females) is inherently different than in classically defined demic groups. Considerable excess heterozygosity occurs within lineages without substantial changes in total or population heterozygosity. The proportion of genetic variance among lineages within the population was dependent on the lineage size and the number of male breeders per lineage. The greatest genetic differentiation among lineages was evident when there was one polygynous male breeding within a lineage of philopatric females, a common breeding tactic in mammalian social systems. The fixation indices depicting the genetic structure of the population were found to attain constant values after the first few generations despite the continuous loss of gene diversity within the population by genetic drift. Additionally, the change of gene correlations within individuals relative to the change within the population attains a state of dynamic equilibrium, as do the changes of gene correlations within lineages relative to the total and within individuals relative to within lineages. Comparisons of coancestries and fixation indices for philopatric versus randomly dispersing females indicate that philopatry and polygyny have probably not evolved independently and that promotion of gene correlations among adults rather than offspring has been of primary importance.

Skeletal biological distance studies in American physical anthropology: recent trends

Biological distance analysis, the dominant type of skeletal biological research during the 19th century, has become less visible in recent years. Although the proportion of American Journal of Physical Anthropology articles and published abstracts focusing on biodistance has remained fairly constant over the three decades between 1955 and 1985, the proportion of biodistance contributions relative to other skeletal biology studies has decreased. Emphasis in skeletal biology has shifted from the analysis of biological variation to investigations of health and diet, and within biodistance studies methodological issues have assumed prominence over purely analytical approaches. This paper investigates trends in biological distance analysis through a survey of articles and meetings abstracts published in the American Journal of Physical Anthropology from 1955 to 1985. The survey provides the historical context for five symposium papers on skeletal biological distance presented at the 1986 meeting of the American Association of Physical Anthropologists.

Mitochondrial and allozyme genetics of incipient speciation in a landlocked population of Galaxias truttaceus (Pisces: Galaxiidae)

Galaxias truttaceus is found in coastal rivers and streams in south-eastern Australia. It spawns at the head of estuaries in autumn and the larvae spend 3 months of winter at sea before returning to fresh water. In Tasmania there are landlocked populations of G. truttaceus in a cluster of geologically young lakes on the recently glaciated Central Plateau. These populations have no marine larval stage and spawn in the lakes in spring. Speciation due to land locking is thought to be a frequent occurrence within Galaxias. To investigate the nature of the speciation event which may be occurring within lake populations of G. truttaceus we studied the mitochondrial DNA (mtDNA) and allozyme diversity of both lake and stream populations. Using the presence or absence of restriction sites recognized by 13 six-base restriction endonucleases, we found 58 mtDNA haplotypes among 150 fish collected from 13 Tasmanian and one south-east Australian mainland stream populations. The most parsimonious network relating the haplotypes by site loss or gain was starlike in shape. We argue that this arrangement is best explained by selection upon slightly beneficial mutations within the mitochondrial genome. Gene diversity analysis under Wright's island model showed that the populations in each drainage were not genetically subdivided. Only two of these stream haplotypes were found among the 66 fish analyzed from four lake populations. Despite the extreme lack of mtDNA diversity in lake populations, the observed nuclear DNA heterozygosity of 40 lake fish (0.10355) was only slightly less than that of 82 stream fish (0.11635). In the short time (3000-7000 years) that the lake fish have been landlocked, random genetic drift in a finite, stable-sized population was probably not responsible for the lack of mtDNA diversity in the lake populations. We infer the lake populations have probably experienced at least one, severe, but transitory bottleneck possibly induced by natural selection for life-history characters essential for survival in the lacustrine habitat. If speciation is occurring in the landlocked populations of G. truttaceus, then it may be driven by genetic transilience.

Spatial autocorrelation of genotypes under directional selection

The spatial distributions of genetic variation under selection-mutation equilibrium within populations that have limited dispersal are investigated. The results show that directional selection with moderate strength rapidly reduces the amount of genetic structure and spatial autocorrelations far below that predicted for selectively neutral loci. For the latter, homozygotes are spatially clustered into separate areas or patches, each consisting of several hundred homozygotes. When selection is added the patches of the deleterious homozygotes are much smaller, in the range of 25 to 50 individuals. Selection also reduces temporal correlations. Also investigated are the effects of random replacement processes, such as mutation, immigration, and long-distance migration, on spatial and temporal correlations. The detection of natural selection through spatial pattern analysis is discussed, and applied to data from populations of the morning glory, Ipomoea purpurea.

Spatial subdivision of populations and estimates of genetic variation

Measures of variation in space are strongly affected by correlations between subdivisions used for sampling. Here we consider variation in gene frequencies across populations. Usually the variance of gene frequencies is standardized by dividing it by the mean gene frequency times one minus the mean (FST). Under the model of isolation by distance (usually called the "stepping stone" model), at the stationary state the correlation between the gene frequencies of two populations falls exponentially with the geographic distance between them. Using this model, we derive formulas for variances of blocks of populations of different sizes in one- and two-dimensional space and suggest that the theoretical results may be useful for understanding real observations, some examples of which are presented. We demonstrate how FST increases with the degree of subdivision among populations. We also show the effect of gaps between the sampled populations. Our results are valid, however, for traits other than gene frequencies, as long as their correlation with geographic distance falls exponentially. In the extension to 2-dimensional spaces, we present in closed form the distributions of distances between nodes of a lattice or of two lattices. These distributions might have applications in ecology.

Estimates of historical migration in County Durham

Twelve parishes in Northumberland and County Durham were used to estimate the accuracy of Anglican marriage registers as indicators of migration between birthplace and marriage. We compared entries in the marriage registers with the annotated baptismal registers which exist for most Northumberland parishes from 1798-1812, and which explicitly document the birthplaces of the parents of the child baptised. By linking individual marriages and subsequent baptisms we showed that the marriage registers systematically underestimate the amount of migration, with values of f (predicted by Wright's Island model) estimated from marriage registers likely to be two to four times the size of those indicated by the baptism registers.

Migration within the island of Korcula, Yugoslavia

For several centuries the village populations on the Yugoslav island of Korcula have remained essentially isolated from each other and from the mainland. Historical, linguistic and biological data indicate that systematic population pressure predominantly came from the coastal area of the Balkan peninsula westward onto the island. To examine short and long range migration on the island, data on the place of birth of parents of adult inhabitants (1168 subjects) were presented and migration matrices analysed applying a gamma function. Most examinees were born in the same village as their parents (86.39%); only 6.33% of the parents migrated between villages on the island; and village endogamy is quite high for the past four generations (75%). When inter-village migration has occurred, migration density is greatest at a distance of 8-12 km, rather than from the immediate neighbourhood. Short range migration occurs up to a mean distance of 33.98 km, long range migration over distances greater than 28 km.

Persistence of an infectious disease in a subdivided population

The transmission dynamics of a communicable disease in a subdivided population where the spread among groups follows the proportionate mixing model while the within-group transmission can correspond to preferred mixing, proportionate mixing among subgroups, or mixing between social and nonsocial subgroups, is analyzed. It is shown that the threshold condition for the disease to persist is that either (i) the disease can persist within at least one group through intragroup contacts, or--if (i) does not hold--(ii) the intergroup transmission is sufficiently high. The among-group transmission is computed as an average where each subgroup's reproductive number is weighted according to its intragroup activity level squared and the total number of cases that one infectious individual will cause through intragroup contacts. The model thus allows for a study of the relative importance of communitywide disease transmission and of disease transmission within geographically or socially separate groups.

Mitochondrial and nuclear gene dynamics of introduced populations of Lepomis macrochirus

A population of bluegill sunfish, Lepomis macrochirus, was introduced into three man-made ponds in 1966. Analyses of these ponds in 1984 and 1985 found significant mtDNA divergence without nuclear gene differentiation. The difference between nuclear gene and mtDNA measures of interpopulational divergence was very large and suggests that sexual asymmetries in life histories may be important considerations in mtDNA and nuclear gene rates of divergence. The rapid divergence of mtDNA haplotype frequencies suggests that sorting of previously existing variation may accelerate mtDNA divergence among recently isolated populations.

Inbreeding in the Utah Mormons: an evaluation of estimates based on pedigrees, isonymy, and migration matrices

Using a computerized genealogical database, inbreeding coefficients were calculated for a sample of 435777 Utah Mormons. The population was divided into ten ten-year birth cohorts (1846-1945) and 22 geographic subdivisions in order to assess temporal and spatial variation in inbreeding. The average inbreeding coefficient for this population is 0.000 106. The average within-groups random kinship coefficient is 0.000 312, reflecting consanguinity avoidance. Random kinship matrices were formed by estimating the average kinship within each spatial subdivision and between all pairs of subdivisions. These matrices were compared statistically with kinship matrices previously estimated using migration matrices and isonymy data. The isonymy approach consistently overestimates random and total inbreeding as well as Wright's Fst. This can be attributed primarily to the assumption of monophyletic origin of surnames. The migration matrix method underestimates random inbreeding and Fst. This is due mainly to the assumption that outside immigrants are derived from a genetically homogeneous population. While the absolute values of the kinship coefficients estimated by each method differ substantially, the patterns of between-groups kinship coefficients given by each method are highly congruent. Logistic and linear regression analyses of 85,235 marriages demonstrate that consanguinity is significantly dependent upon year of marriage, geographic distance between husband's and wife's birthplaces, and the population size of husband's and wife's birthplaces.

Microgeographic variation in rDNA intergenic spacers of Anopheles gambiae in western Kenya

The genetic population structure of Anopheles gambiae (Diptera: Culicidae) in western Kenya was investigated by hybridizing a rapidly evolving rDNA intergenic spacer sequence to restriction endonuclease digests of genomic DNA extracted from single mosquitoes from seven localities. Significantly different distributions of restriction fragment arrays were obtained from field sites less than 10 km apart, which suggests restricted gene flow and a subdivided population structure. Eight of twenty-one possible comparisons between pairs of populations yielded significant differences. An eastern Kenya coastal population did not share its restriction fragment arrays with any of the western populations, suggesting that isolation by distance can be complete on a relatively small geographic scale (700 km).

Spatial autocorrelation analysis of the distribution of genotypes within populations of lodgepole pine

Spatial autocorrelation analyses of point samples within two populations of lodgepole pine (Pinus contorta ssp. latifolia) indicate that single-locus mature tree and pollen genotypes are distributed in a nearly random fashion for most of the allozyme loci assayed. This lack of structure in the distributions of most genotypes is consistent with outcrossing rates that are very nearly 1.0 and with estimates indicating that both pollen and seed are dispersed over long distances in lodgepole pine. However, spatial autocorrelation of genotypes for a few loci suggests that genotypes at these loci may be under natural selection.

Blood biochemical polymorphisms as markers for genetic characteristics of wild Spanish and domestic rabbits

Seventeen blood proteins were studied in a sample of 412 Spanish wild rabbits and in 598 domestic rabbits belonging to various breeds. The wild rabbit populations showed a high level of genetic polymorphism. Six loci were monomorphic, while the remaining ten loci were segregating for at least two alleles. Two of the loci that were polymorphic in the wild rabbits were monomorphic in the domestic ones. Wright's inbreeding coefficient in the total Spanish wild rabbit population was F = 5.66, indicating subdivision of the total population. Inbreeding coefficients, estimated by Kidd et al.'s method (Anim. Blood Grps, Biochem. Genet. 11: 21-38), differed significantly from zero, being 15.62%, in wild rabbits and 6-12% in domestic breeds, indicating consanguinity. Genetic distances between wild rabbit populations showed that factors other than geographic distance (e.g., bottlenecks, barriers such as rivers, mountains, etc.) may explain the result that a northern population forms a cluster with two central populations whereas the northeastern populations form a different cluster with another central population. Populations of the first cluster are more closely related to the captive populations than others. There are three population clusters of domestic rabbits, namely (1) New Zealand White and a hybrid combination; (2) Spanish Common, Butterfly, Burgundy, and Californian; and (3) Spanish Giant.

On the theory of partially inbreeding finite populations. II. Partial sib mating

It is assumed that a population has M males in every generation, each of which is permanently mated with c-1 females, and that a proportion beta of matings are between males and their full sisters or half-sisters. Recurrence equations are derived for the inbreeding coefficient of one random individual, coefficients of kinship of random pairs of mates and probabilities of allelic identity when the infinite alleles model holds. If Ft is the inbreeding coefficient at time t and M is large, (1-Ft)/(1-Ft-1)----1-1/(2Ne) as t increases. The effective population number Ne = aM/[1 + (2a-1)FIS], where FIS is the inbreeding coefficient at equilibrium when M is infinite and the constant a depends upon the conditional probabilities of matings between full sibs and the two possible types of half-sibs. When there are M permanent couples, an approximation to the probability that an allele A survives if it is originally present in one AA heterozygote is proportional to FISs1 + (1-FIS)s2, where s1 and s2 are the selective advantages of AA and AA in comparison with AA. The paper concludes with a comparison between the results when there is partial selfing, partial full sib mating (c = 2) and partial sib mating when c is large.

Diversity of some gene frequencies in European and Asian populations. IV. Genetic population structure assessed by the variogram

Isolation-by-distance models of population structure predict an exponential decrease of genetic relatedness with distance. Under the Kimura-Weiss (1964) model the plots of gene frequency variance versus distance (variograms), computed at various loci, are expected to have equal slope and reach a common asymptote. The gene frequency distributions at eight loci in 192 European and Asian populations have been summarized by variograms. On the average, the Kimura-Weiss model seems to describe adequately allele frequency change up to 900 km, but gradients are apparent at greater distances for most markers studied. These patterns may result form either differential selection or long-range gene flow; however, the extensive clinal variation observed for glyoxalase, esterase D and 6-PGD cannot be entirely accounted for by the Neolithic radiation of early farmers in the Middle East and Europe.

A sampling theory of selectively neutral alleles in a subdivided population

Ewens' sampling distribution is investigated for a structured population. Samples are assumed to be taken from a single subpopulation that exchanges migrants with other subpopulations. A complete description of the probability distribution for such samples is not a practical possibility but an equilibrium approximation can be found. This approximation extracts the information necessary for constructing a continuous approximation to the complete distribution using known values of the distribution and its derivatives in randomly mating populations. It is shown that this approximation is as complete a description of a single biologically realistic subpopulation as is possible given standard uncertainties about the actual size of the migration rates, relative sizes of each of the subpopulations and other factors that might affect the genetic structure of a subpopulation. Any further information must be gained at the expense of generality. This approximation is used to investigate the effect of population subdivision on Watterson's test of neutrality. It is known that the infinite allele, sample distribution is independent of mutation rate when made conditional on the number of alleles in the sample. It is shown that the conditional, infinite allele, sample distribution from this approximation is also independent of population structure and hence Watterson's test is still approximately valid for subdivided populations.

EFFECTS OF DISTANCE BETWEEN POLLEN DONOR AND POLLEN RECIPIENT ON FITNESS COMPONENTS IN ESPELETIA SCHULTZII

The effect of flowering time and of distance between pollen donor and pollen recipient on the proportion of filled achenes, aborted seeds, and seedling survivorship was studied in populations of Espeletia schultzii in the Venezuelan Andes. Hand-pollinations were performed in two different years and at different times during the flowering season. Pollinations within-population included crosses between plants within a few meters to a maximum of 500 m apart and between-population pollinations included crosses between plants more than 10 km and up to 78 km away. Late in the flowering season, plants suffered a reduction in the proportion of filled achenes and an increase in aborted seeds. The proportion of filled achenes and aborted seeds did not vary significantly between the distances between pollen and ovules in crosses done early in the flowering season. However, the proportion of filled achenes in crosses between individuals within 30 m distance were very variable, either very successful or not successful. Beyond 30 m, for some of the populations, the frequency of unsuccessful crosses was almost 0. This pattern was observed at three elevation sites in two consecutive years. When crosses were done late in the flowering season, the proportion of filled achenes was lower in crosses between close individuals and between very distant ones. Seedling survivorship varied significantly between the crossing distances at the higher elevation only. The results show that a clear effect of the incompatibility alleles on the crossing success of the plants is present in the different populations examined, while they suggest that the effect of inbreeding or outcrossing depression is less clear but might still be present only under certain conditions. Late in the season, when plants might have fewer resources available for reproduction and at the high and intermediate site where cold and dry environmental conditions are less favorable for the plants, the discrimination among distances was stronger.

A model to estimate the increase of genetic variability due to electrophoretically cryptic alleles

To consider the problem of the increase of genetic variability due to electrophoretically cryptic alleles, the equally degenerate electromorph model is proposed. Mutation, random sampling drift and selection can be incorporated in this model. Simple formulas are obtained to show how genetic variability increases when cryptic alleles are distinguished. This model is extended to a two-population system to see the effect of cryptic allele variation on genetic differentiation.

Genetic differentiation among cottontails from isolated playa basins

Protein variation in 182 Sylvilagus floridanus from 19 playa basins in Castro Co., Texas was examined using starch-gel electrophoresis. Heterozygote deficiencies were noted for all populations. This heterozygote deficiency may be due to differential selection against heterozygous individuals over the winter months. Results of F-statistics indicated a significant degree of population differentiation at six loci. Nei's genetic distance between populations ranged from 0.20 to 0.388 and a significant association between genetic distance and linear geographic distance among playas was found. These results suggest that genetic exchange and long-distance dispersal may be hindered by expanses of unsuitable habitat.

Isolation by distance in Middle Dalmatia-Yugoslavia

Parameters of Malécot's isolation-by-distance model are estimated for biological (anthropometric head and body dimensions, morphometric dimensions of metacarpal bones, quantitative and qualitative dermatoglyphic traits, and physiological/cardiorespiratory/variables) and linguistic distances and migrational kinship on the island of Korcula and the Peljesac peninsula in Middle Dalmatia, Croatia, Yugoslavia. Resulting parameters and the fit of the model are compared, for both regions, as well as with results of similar analysis in other parts of the world. The fit of the model is highly significant for migrational kinship and linguistic distances and less so for biological traits. Differences between these two populations, which live under basically similar ecological conditions, are explained by variation in biological and sociocultural history.

Admixture in a biologically African caste of black Americans

Social and historical factors account for much of the variation in European ancestry among different Black American populations, including that of McNary, Arizona. The Black population of McNary is socioculturally and geographically isolated. Admixture estimates based upon reflectometry and serological data suggest that this population has less than 5% European ancestry. Anthropometric and hemoglobin data also suggest that this population is more African in ancestry than other Black American populations. Admixture estimates for the population are complicated by several factors. Genetic drift has probably affected Black McNary; estimated effective population size (Ne) is 52.11 and the coefficient of breeding isolation is less than 50. Frequencies of the alleles B, O, and r support this hypothesis; they are quite atypical for a Black American group. Selective migration and occupational selection may also have influenced the current genetic composition of Black McNary. Over 80% of the Black residents of McNary were born in backwoods lumbering towns in the American South. Most Black families in McNary trace their economic reliance on lumbering back several generations. Historical sources and demographic data from Black McNary suggest that Southern Black millworking families formed an endogamous unit that produced this caste, which has a relatively small amount of European ancestry.