LINDA JACKSON University of Florida
Sociocultural and Ethnohistorical Influences on Genetic Diversity in Liberia
In the Republic of Liberia where falciparum malaria remains highly endemic, four phenotypes reported to providc an innate resistance to malaria are nonunifOrmly distributed among the coun- tsy s geographical regionr and indigenous ethnic groups. Significant regional variation is ob- served in the prevalnce of sickle cell trait and nonsignificant but ostensible variation is seen in the distributbns ofbeta thalassemia trait, elevatedfetal hemoglobin, and intermediate red cell GGPD d e j a m y . It appears that particular cultural and historical factors may exert important infiences on genetic diversity in this area of West Afnca. A study of the regional and ethnic variation in these four phenotypes among 961 indigenous children and mothers suggests that the sociocultural variables most likely to increase assortive mating and nonrandom g m flow include (I) member- sh@ in the Poro and Sande traditional initiation societies, (2) linguistic afiliation, (3) dietary staple, and (4) religious preference. Ethnohistorical events most likely to explain the origins of this diversity are discussed in light of the populations pre-Liberian origins, time and direction of major migrations, nature of interethnic hostilities, evidence of assimilation, and enslavement patterns.
N WEST AFRICA, AS ELSEWHERE IN THE TROPICS, finding genetic variability is not I difficult. Tropical ecosystems are noted for their high rates of bioevolutionary change and intra- and interspecific diversity (Dobzhansky 1950). When the diversity at the re- gional and ethnic group levels involves variant human hemoglobins and red blood cell enzymes, researchers usually explain differential distributions in terms of variations in genetic fitness, local adaptations, disease resistance, infertility, and other factors. Yet, in many situations, the hereditary variation evident in particular regions or among certain ethnic groups appears to be equally responsive to cultural and historical variables which modulate the pattern of assortive mating and direct gene flow, and predispose a popu- lation to the random effects ofgenetic drift. In this study I examine the particular impact of specific sociocultural variables and ethnohistorical events on the contemporary distri- bution of four specific phenotypes: sickle cell trait, beta thalassemia trait, elevated fetal hemoglobin, and intermediate red cell G6PD (glucose-6-phosphate dehydrogenase) de- ficiency. Each of these phenotypes has been proposed to provide an innate resistance to falciparum malaria, either influencing ones susceptibility to infection or subsequent clin- ical response (Konotey-Ahulu 1972; Nurse 1979; Pasvol et al. 1976; Luzzatto 1979).
The Republic of Liberia was selected for study for several reasons. As the site of many of the earliest and most important studies on variant hemoglobins in West Africa (Liv- ingstone et al. 1960; Nee1 et al. 1961), as well as the location of more recent assessments of variant hemoglobins and red cell G6PD levels (Willcox 1975; Simbeye 1977), some- thing was already known about the distribution of these phenotypes in the country. In addition, the efforts of past researchers permitted a unique longitudinal perspective on genetic variation in this country. The high endemicity of falciparum malaria in contem- porary Liberia was also well documented (Swamy, Du, and Mason 1976; Division of Vi- tal and Health Statistics 1977; Hedman et al. 1979; Jackson 1985), with high rates of
LINDA JACKSON is Associate Professor, Department of Anthropolou, University of Florida, Coincsuilk, I;L 32611.
826 AMERICAN ANTHROPOLOGIST [88, 1986
malaria transmission reported since the early 1800s and the confirmed high prevalence of falciparum malaria since the 1930s (Strong 1930; Barber, Rice, and Brown 1932; An- igstein 1937).
From 1978 through 1979, research was conducted among indigenous children and mothers to determine the current distribution of these four phenotypes and their rela- tionship to endemic malaria in nonhospitalized individuals (Jackson 198 I ) . The regional and ethnic variation in the distribution of these phenotypes across Liberia appeared to be nonrandom, confirming and elaborating earlier studies (Livingstone 1958) and sug- gesting that the pervasive presence of malaria in this biome provided only a partial ex- planation for the observable genetic diversity between geographical regions and among various ethnic groups. In an effort to better understand the origin, pattern, and mainte- nance of this diversity, a number of sociocultural variables and ethnohistorical events were investigated. The cultural variables of greatest interest were those most likely to operate synergistically with other genetic and environmental factors and contribute to- ward more assortive matings. These included membership in the Poro and Sande tradi- tional initiation societies, linguistic affiliation, dietary staple, and religious preference. The ethnohistorical events scrutinized for their potential impact on genetic drift and gene flow included the pre-Liberian origins of various groups, time of migration into the coun- try and settlement pattern in various regions, and past interethnic hostilities and alli- ances.
Geographical and Ethnohistoricd Setting The Republic of Liberia is located on the west coast ofAfrica between latitude 4"2" and
8"35" north and longitude 7"30" and 11'30" west of Greenwich. Positioned at the south- west corner of the great Atlantic bulge of the African continent, this small country of approximately 1.8 million lies within the tropical rain forest biome, extending nearly 563 km (350 miles) along the coast and 257 km (160 miles) into the interior. The total land area is approximately 98,251 sq. km (37,950 sq. miles), and the country is bounded on the northwest by Sierra Leone, on the north by the Republic of Guinea, on the southeast by the Ivory Coast, and on the west and the south by the Atlantic Ocean. Today, 17 major ethnic groups reside in Liberia, speaking over 20 local languages and dialects of the Niger-Congo linguistic family and English, the national language. Figure 1 depicts the geographical distribution of these major ethnic groups within the country.
Tables 1 through 4 summarize important ethnohistorical data on these ethnic groups by principal geographical region. In the northwest region of Liberia (Table 1) are found the Gissi, Gbandi, Loma, Mende, and Belle ethnic groups. Many of these groups extend into Sierra Leone and Guinea; all are exogamous and patrilineal. All except the Gissi and Belle are derived from the Mande-speaking Mane invaders of the 16th century.
In the western region of Liberia (Table 2) live the Gola, Vai, Dei, and Mandingo. The Gola and Vai are also found in Sierra Leone, while the Mandingo of western Liberia are but a segment of the Mandingo people, numbering over 1.5 million, who also reside else- where in Liberia and throughout West Africa. All groups of this region are exogamous and patrilineal and the important Poro and Sande societies find their origins among the Cola.
In the more culturally and historically heterogeneous central region of Liberia (Table 3) reside the Kpelle, Bassa, Mano, Gio, and Congo ethnic groups. All of these are exog- amous and patrilineal. The Kpelle and Bassa are the country's two largest ethnic groups, with the Kpelle extending into Guinea. The Mano and Gio in the northern part of this region also extend into neighboring Guinea and the Ivory Coast. The Congo, at times referred to as Americo-Liberians or Settlers, are the descendants of repatriated Ibo, Congo, and other ethnic Africans from elsewhere on the continent and settlers of African descent from the Western Hemisphere.
In the less densely populated southeast region (Table 4) are found the Krahn, Kru, and Grebo ethnic groups. These too are patrilineal and exogamous; all are Kwa-speaking and extend into the Ivory Coast.
Jackson] GENETIC DIVERSITYIN LIBERIA a27
Figure 1 Regional distribution of major ethnic groups in Liberia.
Methods and Materials Sample Characteristics
The sample consisted of 961 Liberians (472 children and 489 mothers) attending rou- tinely scheduled well-baby and under-five clinics at five sites throughout rural and urban Liberia. Adult men were not included in this study because most were unavailable for testing during the usual clinic hours. Low-income children and mothers were recruited after a brief presentation of the aims and objectives of the study. An effort was made to study Liberian child-biological mother pairs and in this sample all but 17 individuals reflect this relationship. The relatively large size of the sample permitted all major indig- enous ethnic groups to be represented at proportions significantly similar to their pro- portions in the larger population (rho = .751, @= 16, p < .01).
The geographical distribution of the home villages and towns of sample children and mothers was nonrandom and tended to be clustered by ethnicity following the geograph- ical pattern depicted in Figure 1. Since all groups are patrilineal and patrilocal, ethnic identity and home village or town are that of ones father.
Childrens current age was determined after consulting with their mothers, birth cer- tificates, and clinic personnel. Children ranged in age from newborns to 9 years I 1 months with the mean age of children around 10 months. 461 children (98%) were less than 5 years of age and of these, 361 (79%) were under 1 year of age. No significant gender or age differences were observed among ethnic groups. Mothers ages ranged from 14 to 47 years with the mean age at 24.2 years. No significant differences in age statistics were