Sociocultural and Ethnohistorical Influences on Genetic Diversity in Liberia

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 observed 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 regional 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 population to the random effects ofgenetic drift. In this study I examine the particular impact of specific sociocultural variables and ethnohistorical events on the contemporary distribution of four specific phenotypes: sickle cell trait, beta thalassemia trait, elevated fetal hemoglobin, and intermediate red cell G6PD (glucose-6-phosphate dehydrogenase) deficiency. Each of these phenotypes has been proposed to provide an innate resistance to falciparum malaria, either influencing ones susceptibility to infection or subsequent clinical 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 contemporary 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.

825

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 relationship 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 suggesting that the pervasive presence of malaria in this biome provided only a partial explanation for the observable genetic diversity between geographical regions and among various ethnic groups. In an effort to better understand the origin, pattern, and maintenance 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 toward more assortive matings. These included membership in the Poro and Sande traditional 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 country and settlement pattern in various regions, and past interethnic hostilities and alliances.

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 elsewhere 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 exogamous 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 indigenous ethnic groups to be represented at proportions significantly similar to their proportions 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 geographical 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 observed among ethnic groups.


Table I Ethnohistorical overview of the major ethnic groups of northwest Liberia.

Belle

Gbandi

Gissi

Loma

Mende

Migrants from eastern Liberia (Johnson 1969)

Time of Past interethnic Ethnic Pre-Li berian migration into Settlement hostilities and group area of origin Liberia pattern alliances

Unknown-share Patrilocal, isolated Hostilities with ancestral origins in forest regions, Gola resulted in with Krahn marriage to wives scores of Gola (Johnson 1969) of maternal uncle women becoming

prohibited (John- captives of the son 1969) Belle (Johnson

1969); alliances with Vai for com-

Hinterland of modern Ghana (Kup 1960; Den- nis 1972)

Upper Niger (Kup 1960; Schulze 1973)

Ancient Mali (Condor 1979)

Hinterlands of modern Ghana and Ivory Coast (Rodney 1970)

A.D. 1500

Pre-1500s with additional major migrations to A.D. 1700 (Condor 1979)

A.D. 1500

Late 1500s (Kup 1960; Schulze 1973)

Patrilocal, levirate and sororate, marriages permitted

Patrilocal, hierar- chical political system influences residence

Patrilocal

Patrilocal, traditional orientation toward Sierra Leonean Mende community

mercial purposes, dodi status with Krahn (Schroder and Seibel 1974) Hostilities with Gissi; alliances with Loma and Mende (Rodney 1970)

Hostilities include raiding, conquer- ing, and subjuga- tion of Gbandi (Holse, dAzev- edo, and Gray 1969; Moore 1955) Alliances with Gbandi and Mende

Alliances with Lorna, Gbandi, and Gola

Genetic Determinations

With the consent of participants and local authorities, peripheral blood samples were obtained from all children and mothers to determine the phenotype prevalences of sickle cell trait, beta thalassemia trait, elevated fetal hemoglobin, and intermediate red cell GGPD deficiency. Cellulose acetate and citrate agar electrophoresis (Schneider 1973; Schmidt and Brosious 1976) were used to detect the presence of hemoglobin S (sickle hemoglobin), elevated proportions of hemoglobin F (fetal hemoglobin) in individuals over six months of age, and beta thalassemia (as evidenced by elevated levels of herno- globin A2). Red cell GGPD enzyme activity levels were determined semiquantitatively using a commercial colorimetric kit (Sigma Chemical Co., St. Louis, MO).

Analytical Methods

Chi-square is the primary statistical test reported in this study (Snedecor and Cochran 1978). Values ofp less than .05 are considered statistically significant. In most cases

Jackson] GENETIC DIVERSITY IN LIBERIA

Table 2 Ethnohistorid overview of the major ethnic groups of west Liberia.

Ethnic Pre-Liberian area of orinin

Dei Unknown

Cola Migrants from Kongba region of western Liberia (Karnga 1926)

Mandingo Sudanic empires of West Africa (Condor 1979)

Vai Western Sudan; descendants of Mandingo of Bambara clan (Johnson 1954)

Time of Past interethnic migration into Settlement hostilities and Liberia pattern alliances Unknown, possi- Patrilocal, polyga- Alliances with Cola bly pre-Cola mous (Hoke, dAzev-

edo, and Gray 1969), also Man- dingo and Congo

500-100 B.C. Patrilocal, unlim- Hostilities with Dei (Johnson 1961; ited polygamy and Mende result- Schulze 1973) permitted; first ing in increased

cousin and soror- interethnic mar- ate marriage per- riage: Dei men/ mitted Uohnson Cola women and 1969) Mende men/Gola

women during late 19th century (Johnson 1969); later hostilities with Vai (Moore 1955), conflicts with Mandingo resolved with Mandingo men/ Cola women inter- marriages.

Mid-20th century Patrilocal, polyga- Intermarriage mous, large num- with Cola and bers near commer- Kpelle women cialhading cen- (Hoke, dAzev- ters edo, and Gray

1969)

A.D. 1500-25 (Schulze 1973) with Prince Dua- mani Kamara (B. T. Moore, personal communication, 1979) linked with 16th-century Sumatra cohort of the Mane (Rod- ney 1970)

Patrilocal, polyga- Alliances with mous Mandingo and

Sierra Leonian Mende Uohnson 1954) involvement in trans-Atlantic European slave trade increased Vai contact with interior ethnic groups; contemporary Vai quite heterogeneous (B. T . Moore, personal communication, 1979)

means are reported plus or minus ( f ) the standard error of the mean. Within the sample, ethnic groups totaling 30 or more individuals are considered large while those with less than 30 representatives are designated small.

830 AMERlCANANTHROPOLOClS7 [88, 1986

Table 3 Ethnohistorical overview of the major ethnic groups of central Liberia.

Time of Past interethnic Ethnic Pre-Liberian migration into Settlement hostilities and group area of origin Liberia pattern alliances Bassa

Congo

Gio

Kpelle

Mano

Interior regions of central Liberia, up St. John River

Modern Nigeria, Zaire, Republic of the Congo, Ga- bon, Benin, and elsewhere (B. T. Moore, personal communication, 1979; Fage 1969), the Americas

Modern Ivory Coast

Modern Guinea and Ivory Coast

Modern Ivory Coast; former rear guard of 16th-century Mane (Rod- ney 1970)

Among earliest migrants (Condor 1979); proto-Bassa present A.D. 1450-1 650 (Schulze 1973)

Early to mid- 1800s with later migrations in 1900s; sporadically deposited on Liberian shores by British and Amer- ican navies during 19th century

Two major migrations: A.D. 1600 andA.D. 1650- 1700 (Schulze 1973); one minor migration A.D. 1860 (Condor 1979) Shortly before A.D. 1600 (Schulze 1973)

A.D. 1600, 1650, 1800 (Rodney 1970; Schulze 1973)

Patrilocal, polyga- Hostilities with mous, reside only Kpelle during in Liberia 19th century (E.

Jallah, personal communication, 1979)

Patrilocal (now), Hostilities with var- numerically con- ious coastal centrated along groups (e.g., Vai, the coast of central Gola, Kru, Grebo, and southeast Lib- Bassa) (Holse eria 1971); intermar-

riage with Gola women (Martin 1961); adoption of Gola children into Congo households

Patrilocal No information

Patrilocal, migra- Hostilities include tion formed broad enslavement by wedge between the Bassa during Belle and Bassa 19th century (E.

Jallah, personal communication, 1979)

Patrilocal, rela- No information tively isolated until recently

Results

Extent $Regional and Ethnic Genetic Diversity

Table 5 summarizes the regional and ethnic distributions of the four phenotypes examined in this study.

Sickle Cell Trait. T h e distribution of this phenotype is quite dramatic in Liberia with children and mothers showing a significant regional gradient in prevalence (Chi-square = 28.83, df = 3, p C .005). Across Liberia sickle cell trait attains highest frequency in the northwest region, is a t slightly lower levels in the west, is lower still in the central region (except among the Kpelle), and is lowest in the southeast. Among the larger ethnic groups represented in the sample (N greater than 29), high sickle cell trait frequencies

Jackson] GE~~ETIC DIVERSITYIN LIBERIA 83 1

Table 4 Ethnohistorid overview of the major ethnic groups of southeast Liberia.

Time of Ethnic Pre-Liberian migration into Settlement POUP area of origin Liberia pattern Grebo Coastal areas of Origins in Gbobo Patrilocal, unlim-

modern Ivory chiefdom of ited polygamy, Coast Krahn (Johnson son may inherit

1963); additional fathers wives (ex- migrations into cept mother), Liberia shortly nephew may in- after A.D. 1700 (Schulze 1973) Uohnson 1963);

herit uncles wives

sororate marriage prohibited

Krahn Areas to the north Several waves of Patrilocal, mar- and northeast of immigration be- riage between Liberia (Schroder tween A.D. 1650- members of the and Seibel 1974) 80 (Schroder and same clan prohib-

Seibel 1974) ited

Kru Central Africa by Linked with 16th- Concentrated near way of Ghana (E. century Sumba the coasts and in Nagbe, personal cohort of Mane close proximity to communication, (Rodney 1970); major commercial 1979); southern may have origi- trading ports end of Sahara de- nated from a sec- sert (Johnson tion of the Krahn 1963) (Zetterstrom

1969)

~

Past interethnic hostilities and alliances Alliances with na- tive Bakwe-speakers from the Ivory Coast and the Gbobo have pro- duced the modern Grebo (Schulze 1973)

Alliances with Belle (dodi or sibling group status); con temporary Krahn result of intermarriage between northern and eastern Krahn, Sapo subgroup of the Krahn, and the northern Crebo; in the past (pre- 1800s) unre- deemed female war captives and their offspring were integrated into the Krahn Alliances with Fanti of Ghana (from whom was learned deep sea fishing) (E. Nagbe, personal communication, 1979); acted as in- termediaries in the trans-Atlantic Eu- ropean slave trade and its abolition (McAllis ter 1896; B. T. Moore, personal communication, 1979)

(higher than the sample norm of 0.12) are observed among the Loma, Kpelle, Vai, and Gbandi. Large ethnic groups with low frequencies of sickle cell trait include the Grebo, Kru, Gio, Krahn, Mano, and Bassa.

O f the four phenotypes studied, evidence of beta thalassemia trait displayed the lowest overall prevalence in the sample. Perhaps because of this low prevalence, regional variation in elevated hemoglobin A, levels is statistically insignifi-

Beta Thalassmia Trait.

Tab

le 5

R

egio

nal and e

thn

ic g

enet

ic v

aria

tion in s

elec

ted

phen

otyp

es.

03

w

N

Ele

vate

d fe

tal

Inte

rmed

iate

GG

PD

Sick

le ce

ll tr

ait

Bet

a th

alas

sem

ia tr

ait

hem

oglo

bin

defic

. N

umbe

r in

R

egio

nIE

thni

c gr

oup

sam

ple

N

Freq

. N

Fr

eq.

Nlto

tal'

Freq

. N

Fr

eq .

Nor

thw

est

Bel

le

7 1

.I4

0 0

015

0 1

.I4

Gba

ndi

40

5 .I

3 2

.05

5/30

.I7

6 .I

5 G

issi

88

10

.I 1

1 .o 1

6/

70

.09

11

.13

Lom

a 71

20

.2

8 0

0 5/

47

.I1

14

.20

Men

de

23

7 .3

0 1

.04

1/15

.07

3 .I

3 R

egio

nal t

otal

s 22

9 43

.I

9 4

.02

1711

67

.I0

35

.I5

Wes

t D

ei

4 0

0 0

0 1/

3 .3

3 1

.25

Gol

a 26

4

.I5

0 0

1 122

.0

5 I

.04

Vai

61

12

.2

0 2

.03

2/42

.0

5 5

.08

Reg

iona

l tot

als

119

21

.I8

3 .0

3 5/

81

.06

8 .07

Cen

tral

B

assa

12

9 8

.06

1 .o 1

11

/95

.I2

20

.I6

Gio

56

2

.03

1 .0

2 41

4 1

.I0

9 .I

6

Man

o 39

2

.05

0 0

3/29

,I

0 6

.I5

Reg

iona

l tot

als

412

47

.I1

6 .o 1

32

/305

.I

0 64

.I6

Sout

heas

t G

rego

94

1

.o 1

0 0

8/66

.I

2 15

.I

6 K

rahn

34

1

.03

0 0

1/35

.0

3 5

.I5

KIU

73

2

.03

1 .o 1

9/

48

.I9

10

.I4

.I5

Tot

als

961

1 I5

.I2

14

.o 1

72/7

02

.I0

137

.I4

Man

ding

o 28

5

.I8

1 .04

1/14

.07

1 .04

Con

go

19

1 .0

5 0

0 01

1 1

0 0

0

Kpe

lle

169

34

.20

4 .0

2 14

1129

.I

1 29

.I7

- R

egio

nal t

otal

s 20

1 4

.02

1 .o 1

18

/149

.I

2 30

8: c W 01 UI 'T

otal

for

ele

vate

d fe

tal h

emog

lobi

n ph

enot

ype

refle

cts

only

indi

vidu

als

six

mon

ths

of a

ge a

nd o

lder

.

Jackson] GENETIC DIVERSITY IN LIBERIA 833

cant. The phenotype is higher than the sample norm ofO.O1 among the numerous Gbandi and Vai but absent among the large number of Grebo, Mano, and Krahn.

Regional differences in the frequency of this phenotype are not statistically significant, however, the frequency of raised fetal hemoglobin levels among children over six months of age and among mothers is ostensibly highest in the southeast region, particularly among the Kru (although lowest among the neighboring Krahn) and in the northwest region, particularly among the Gbandi. Lower frequencies are observed in the western region among the Vai.

Regional variation in this phenotype is also not statistically significant although the highest frequencies apparently occur in the southeast, central, and northwest Liberia with partially inactive enzyme levels most common among the Loma, Kpelle, Bassa, Grebo, and Gio. Lower frequencies of the phenotype, averaging less than half those observed elsewhere in the country, are found in the western region (as among the Vai).

Interface of Sociocultural Variables and Genetic Variation

Figure 2 presents the frequencies of sickle cell trait, beta thalassemia trait, elevated fetal hemoglobin, and intermediate red cell G6PD deficiency when the sample is parti- tioned into various sociocultural subcategories.

The largest and most extensive traditional male and female initiation societies in Liberia, Poro, the male society, and Sande, its female counterpart, are actually educational institutions that consummate the transformation of the individual from child to adult (Dennis 1972). Where they are found, they reaffirm the traditional way of life and are crucial to the continuity of ethnic identity. In the traditional frame- work, completion of school entitles individuals to marriage, procreation, and the respon- sibilities of adulthood. Ethnic group membership in these institutions transcends linguistic barriers and international boundaries (as do often the ethnic groups themselves) and facilitates inter- and intraethnic group cooperation.

In Liberia, the Poro and Sande are quite prevalent among the Cola, Vai, and most Mandingo of the western region, the Mende, Loma, Gbandi, Gissi, and Belle of the northwest region, and the Kpelle, some Bassa, and some Mano of the central region. In the sample, 513 children and mothers (53%) came from ethnic groups or clans that participate in Poro and Sande. Among other central and southeast region groups, Poro and Sande is either partially present (N = 168, 17%) or absent (N = 280, 29%).

Poro and Sande membership status is significantly associated with the distribution of sickle cell trait (Chi-square = 47.75, df = 2, p < .005). Among those ethnic groups with these specific societies the mean frequency of sickle cell trait is high at 0.19 & .007. For the Bassa and Mano who maintain some Poro and Sande clans and other clans without these particular societies, the mean frequency of sickle cell trait is lower (0.06 f .004). The lowest mean prevalence of sickle cell trait (0.03 & .003) is observed among the non- Porolnon-Sande ethnic groups.

Poro and Sande membership status is also significantly associated with the distribution ofthe beta thalassemia trait (Chi-square = 7.23, d f = 2,p < .05). Mean beta thalassemia trait frequencies are higher (0.02 2 .002) among those groups with Poro and Sande but less than 0.01 (.005 f .004) among the groups that only partially participate in these societies or lacked the Poro and Sande institutions altogether (.005 2 .001).

Poro and Sande membership status is not significantly associated with the distributions of either elevated fetal hemoglobin or intermediate red cell G6PD deficiency.

Linguistic Afiliation. All four major language groups of Liberia are represented in the sample. 487 children and mothers (51%) are Mande-speakers (either Mande-fu [ N = 3981 or Mande-tan [ N = 891) and they generally come from home villages or towns in northwest, northcentral, or west Liberia. Kwa-speakers (N = 341, 35%) are nearly uni- formly residents of southeast, central, and southcentral Liberia. Two small Kwa-speak-

Elevated Fetal Hemoglobin.

Intermediate Red Cell GGPD Dejicamcy.

Initiation Societies.


I- C A O U V A OOCD DEClC

DICTTUV OICKLI CELL OETA THAUOOIYIA ILIVATED lNTERYEDI4TI OTABLC TRAIT TRAIT CITALHL

I I I

Figure 2 Variation in phenotypic frequencies by sociocultural subcategories.

Jackson] GENETIC DIVERSITYIN LIBERIA 835

ing isolates, the Belle in northwest Liberia and the Dei in western Liberia, are exceptions. The Gola and Gissi are speakers of a Me1 dialect and, at 12% of the sample, are also linguistically isolated in northwest and west Liberia, respectively. Nineteen Liberians identified English as their first language. Traditionally these individuals tended to be classified as Congo whose home areas are usually limited to coastal regions of central, west, and southeast Liberia.

Linguistic affiliation is significantly associated with the distribution of sickle cell trait (chi-square = 33.74, df = 3, p < .005). The highest mean prevalence of sickle cell trait occurs among Mande-speakers (0.18 * .01) and the lowest mean prevalences are among the Kwa-speakers of central and southeast Liberia (0.04 f .008), the (Mande-speaking) Mano and Gio (0.05) of northcentral Liberia, and the English-speaking Congo (0.05) of central Liberia. Mel-speakers maintain a mean frequency of 0.12 f .01.

Language group is also significantly associated with the distribution of beta thalassemia trait (chi-square = 9.51, df= 3,p < .025). The lowest mean prevalence of this phenotype is among English-speakers (0), Kwa-speakers (.003 f .001, and Mel-speakers (.005 f .004). Slightly higher frequencies are observed among Mande-speakers (0.02 * .002), particularly among Mande-tan (0.03 f .004).

Linguistic affiliation was not significantly associated with the distributions of elevated fetal hemoglobin or intermediate red cell G6PD deficiency.

Dietary Staple. Although rice and cassava are grown throughout Liberia, two major dietary patterns are evident in this sample: most children and mothers from ethnic groups of the northwest and west regions and certain central region groups (the Kpelle and Congo) tend to consume upland rice (Oryza sativa) as their primary dietary carbohydrate, while children and mothers of other central region groups (Bassa, Gio, Mano) and those from ethnic groups of the southeast region are more likely to consume cassava roots (Ma- nihot esculnta) as their principal carbohydrate staple.

A significant divergence in sickle cell trait prevalence is associated with the north to south dietary shift from rice to cassava as the staple (chi-square = 42.72, d f = I , p < .005). Among the 536 children and mothers (55.8%) for whom rice is the dietary staple, the mean frequency ofsickle cell trait is 0.18 ? .008. Among the 425 children and mothers (44.2%) consuming cassava as their principal staple, the mean frequency of this phenotype is much lower at 0.04 f .003.

Dietary staple is also (statistically) significantly associated with the distribution of beta thalassemia trait (chi-square = 6.30, df = 1, p < .025). Rice eaters have a mean prevalence of 0.02 -+ .002 while the same phenotype among cassava eaters has a mean frequency of less than 0.01 (.007 +- .001).

Neither the distributions of elevated fetal hemoglobin nor intermediate red cell GGPD deficiency is significantly associated with dietary staple.

Religious Prcfcrme. For most Liberians, traditional West African religious beliefs con- tinue to permeate daily life. Often, the expanding influence of Islam and Christianity is superimposed upon this traditional pattern and creates a situation in which delineating singular and exclusive religious preferences among the sample is somewhat problematic. There is some degree of uniformity in the plethora of traditional belief systems in Liberia; all encompass the concept of a supreme creator, the practice of divination by sorcery, communication with ancestors, herbalism, and magic. Their focus is usually at the individual, family, clan, and (in some cases) ethnic group levels rather than being univer- sal. Thus, in contrast to both Islam and Christianity, conversion is not an important goal. As a consequence, it is often possible for Liberians to appear relatively pluralistic in their religious affiliations. In most instances, however, usually one belief system or category will predominate most spheres of life and become an important part of one's identity.

Children and mothers from ethnic groups of western Liberia and parts of northwest Liberia are predominantly Muslim (N = 138,14'/0), especially the Vai, Mandingo, Gola, and Mende. A large proportion of children and mothers from central and southeast Lib- eria are Christian (N = 488, 51%), particularly the Congo, Bassa, Grebo, Kpelle, Kru,


and Dei. Traditional religions appear to maintain their greatest impact among the Mano, Gio, Gissi, Loma, Gbandi, Krahn, and Belle of the more interior regions of the country

Religious preference is associated with significant differences in the distribution of sickle cell trait (chi-square = 10.61, @= 2, p < .005). The phenotype attains highest mean frequency among Muslims (0.20 5 .016), is lower among Traditionalists (0.12 ? .013), and is lowest among Christian children and mothers (0.09

Religious preference is not (statistically) significantly associated with the distributions of beta thalassemia trait, elevated fetal hemoglobin, or intermediate red cell GGPD deficiency.

(N = 335,3570).

.015).

Discussion Relative InJklenct of Sociocultural Variables

Of the four sociocultural factors examined in this study, the factor most strongly linked to variation in sickle cell trait frequencies is the presence, partial presence, or absence of the Poro and Sande societies. Membership status also appears sensitive to variation in the distribution of beta thalassemia trait. However, the distribution of elevated fetal hemoglobin and intermediate red cell GGPD deficiency was not statistically significantly associated with these initiation societies.

The sharp regional and ethnic delineations in sickle cell trait (and to a lesser extent beta thalassemia trait) frequencies may have to do with the time and location the hemoglobin S and elevated hemoglobin A, genes first appeared in the Liberian population (either through mutation or gene flow). If these genes were introduced subsequent to the introduction of genes coding for elevated fetal hemoglobin levels and intermediate red cell GGPD deficiency, historical factors may account in part for the observed divergences. This would be especially important if the introduction of the responsible genes coincided with the spread of the Poro and Sande societies. Since Poro and Sande membership directly influences mate selection, this cultural factor would be a convenient vehicle for promoting assortive mating and a nonrandom distribution of hemoglobin S (and hence sickle cell trait) and elevated hemoglobin A, (and hence beta thalassemia trait) in Libe- ria.

The linguistic affiliation of the sample also yielded significant variation in sickle cell trait and beta thalassemia frequencies. The high frequency of sickle cell trait and (com- paratively high) beta thalassemia trait among Mande-speakers suggests that they may have been the linguistic group in which the hemoglobin S gene either initially appeared, as suggested 26 years ago by Livingstone (1958), or achieved early high concentration. The much lower frequency of sickle cell trait among Kwa-speakers suggests that the hemoglobin S gene may not have been part of the early genetic repertoire of these groups and that its contemporary presence may reflect more recent admixture with Mande- and Mel-speakers. Beta thalassemia trait frequencies were also higher than expected among Mande-speakers. Statistically insignificant differences separated Kwa-, Mende-, and Mel-speaking ethnic groups in elevated fetal hemoglobin and intermediate red cell GGPD deficiency frequencies.

Dietary staple, as a cultural factor, also appears to confirm significant variation in sickle cell trait frequencies. In addition to the contribution of dietary patterns to assortive mating, one staple, cassava, contains dietary precursors of cyanates which may interact with hemoglobin S, red cell GGPD, and other important body proteins (Cerami 1972; deFurin et al. 1972; Gillette, Manning, and Cerami 1971; May, Bellingham, and Huehns 1972; Glader and Conrad 1972; Jackson et al. 1985). In primarily rice-eating ethnic groups, cassava root is eaten sporadically, on a seasonal basis, although cassava leaves may be consumed in low to moderate quantities as a cooked green vegetable. In primarily cassava-eating ethnic groups, both the root and leaves are eaten throughout the year in high quantities with great consistency. The inverse relationship of cassava consumption to sickle cell trait frequencies in Liberia may be the result of two dosage-dependent in-

Jackson] GENETIC DIVERSITYIN LIBERIA 837

teractions: (1) low to moderate seasonal ingestion of dietary-derived cyanates as observed among northwest, northcentral, and western ethnic groups may improve the genetic fitness of homozygous sicklers by increasing red cell survival and oxygen-carrying capacity and thus increase these individuals chances for survival, reproduction, and boosting the prevalence of this gene over generations, and (2) high doses of dietary-derived cyanates as observed among central and southeast ethnic groups may directly interfere with the growth and development of Plannodiumfalciparum, causative agent of falciparum malaria (Nagel et al. 1980). This would place a ceiling on the intensity of falciparum malaria infection and actually select against hemoglobin S in these groups. Furthermore, cyanate has been demonstrated to inhibit red cell GGPD enzyme activity (Glader and Conrad 1972; Jackson, Chandler, and Jackson 1986). In this study dietary staple was not statistically significantly associated with the distribution of intermediate red cell GGPD deficiency, however, the ostensibly higher frequencies of this phenotype among groups consuming cassava as their staple may reflect this biologically important interaction. The distribution of elevated fetal hemoglobin is also not statistically significantly associated with dietary staple in this sample, yet recent studies have confirmed that hemoglobin F levels are also malleable by exogenous factors. It is possible that additional biochemical and parasitological interactions may contribute to the biologically higher frequencies of both intermediate red cell GGPD deficiency and elevated fetal hemoglobin levels among cassava eaters.

Grouping the phenotypes by the religious preferences of the sample demonstrates significant variation in sickle cell trait distribution and statistically insignificant but anthro- pologically interesting differences in beta thalassemia trait and intermediate red cell GGPD deficiency distributions among Muslims, Christians, and adherents to traditional West African belief systems. Christians and Traditionalists expressed highly convergent mean frequencies while Muslims diverged from these groups in each phenotype studied. This variation may reflect the relatively recent (early 1800s) broad presence of Christi- anity in the region. This recent presence can be contrasted with Islams older and deeper historical roots, and more long-standing biological and cultural impact on the population. The observed variation in phenotypes by religious affiliation also suggests that the highest proportion of Christian converts in Liberia come from the Traditionalists pool rather than from the Muslim pool. As a result, there is substantial overlap between the gene pools (and hence phenotypes expressed) of Christian and Traditionalist communi- ties. With the exception of the Congo of central Liberia, the adherents of these two religious categories are essentially derived from the same ancestral population of Tradition- alists. The close concordance of these groups in phenotype prevalences and their collec- tive divergence from the frequencies observed among Muslims are thus not surprising. Islam provides sanctions against the marriage of Muslims to polytheist Traditionalists and, although marriage to Christian women is permitted, the prevalence of either kind of union appears low in the sample. While religious preferences merely reemphasize the significant variation in sickle cell trait frequencies apparent elsewhere, the statistically insignificant but bioculturally interesting lower frequencies of intermediate red cell GGPD deficiency and higher frequencies of beta thalassemia trait among Muslims (Fig- ure 2) suggest that there may be religion-associated cultural practices that affect the survival value (fitness) of these phenotypes in some groups. Since nongenetic factors may contribute to the expression of these phenotypes, further studies along these lines may prove useful.

Ethnohistorical Perspectives on the Observed Genetic Variation

The pattern of genetic diversity evidenced in contemporary Liberians suggests that much of this variation is long-standing and may reflect the impact of various historical events. Although the indigenous literary histories of these groups are limited, their oral histories are rich sources of information on population movements, interethnic hostilities, evidence of assimilation and enslavement, all factors of potential significance in directing gene flow or predisposing groups for the random effects of genetic drift.


Sickle Cell Trait. In the northwest and west regions of Liberia two historically com- patible clusters of high sickle cell trait frequencies are observed: one among the Loma, Mende, and (to a lesser extent) Gbandi, and a second among the Vai and Mandingo. The close historical and cultural ties of these groups and their migration patterns (Tables 1 and 2) suggest that the hemoglobin S gene may have first entered Liberia between A.D. 1500 and 1530 in conjunction with the invasions of the Mane from the hinterlands of present-day Ghana and Vai-Mandingo settlers originally from the area around Tim- buktu, Mali. As these waves of Mande-speaking invaders swept through the northwest and west regions, earlier residents such as the Gola, Gissi, Dei, and Belle were either assimilated, displaced, or isolated (dAzevedo 1969). In many instances, cultural exchange served as vehicle for increased ethnic exogamy and gene flow in these regions. Most notably among these was the introduction and dissemination of the Gola-based Poro and Sande societies among these recent Mane immigrants. In the face ofwidespread Mane immigration, earlier resident groups devised other genetically and culturally significant survival strategies as well: the Gola allowed their women to marry out but em- phasized the continued Gola cultural and political identity of these women and their children; the Belle tended to isolate their women from exogamous unions with the Mande- speakers yet developed high levels of linguistic fluency in the Mande languages of their neighbors; the Gissi maintained hostile relations with certain Mane-derived groups, particularly the Gbandi (Dennis 1972), among whom some residual animosity continues (B. T . Moore, personal communication, 1979). In addition to these strategies, intraethnic hostilities in early Liberia undoubtedly influenced gene flow since the voluntary and in- voluntary exchange of women frequently initiated and resolved such conflicts. In the northwest region, for example, gene flow between the Belle and adjacent non-Belle (Loma, Mende, Gbandi, Gola, and Kpelle) likely increased with the war-related incorporation of non-Belle women into the Belle community.

Prior to their arrival in western Liberia in the early 1600s, the Vai were still a subgroup of the Mandingo. With later migration into the western region, the Vai in particular have become much more heterogeneous. This heterogeneity and its historical affinities to the more interiorly located northwest groups is suggested by the role played by the Vai during the centuries of the European slave trade. Between the mid-1600s and the middle to late 1800s numerous contingents of northwest Liberians were marched through Vai territories on their way toward the coast. Along the way, varying proportions of these captives ultimately became assimilated into the Vai community. Today, when the Vai practice exogamy, they frequently select the same ethnic groups sought exogamously by the Liberian Mandingo: most often other western or northwestern groups. Thus, the diffu- sion of the hemoglobin S gene and possibly the gene(s) responsible for elevated hemoglobin A, throughout northwest, west, and certain sections of central Liberia (particularly among the more north and westerly Kpelle) corresponds to the current resident populations degree of hereditary relatedness, and, in most cases, cultural affinity to these early Mane invaders and Vai-Mandingo settlers.

The two ethnic groups that are at variance with this historical pattern are the Gio and Mano of north-central Liberia. Although these two groups are historically associated with the Mane (Rodney 1970), their current dietary reliance on cassava as their principal carbohydrate staple may have a depressing effect on sickle cell trait frequencies.

Elsewhere in central and southeast Liberia, the very low frequency of the hemoglobin S gene may be associated with two important historical events: first, the protracted isolation of these groups in the interior forests of these regions and second, the introduction and rapid incorporation of cassava cultivars as central components of their traditional diets. The first event ofgenetic importance, isolation, was initially in response to the mass immigration of Mande-speakers from the north and east. The second event, increased dietary use of cassava, spread among southeast and central groups in the 1600s, quickly replacing in importance the assortment of yams that had previously provided the bulk of dietary carbohydrates for those groups that practiced agriculture (E. Nagbe, personal communication, 1979). It should be noted that yams also contain dietary precursors of

Jackson] GENETIC DlVERSITYlN LIBERIA 839

cyanates, although in much smaller quantities than cassava, and may also have been of some molecular importance in influencing the genetic profiles of southern and central groups.

Be& Thalassnnia Trait. The low overall frequency of this phenotype in the sample makes it difficult to reconstruct specific historical influences upon its distribution exclusive of the historical influences on hemoglobin S distribution. In spite of this limitation, it is clear that the contemporary ethnic foci of beta thalassemia trait are observed among the Vai and Mandingo groups and the Mende and Gbandi groups (Table 5). The cul- tures and ethnic histories of these groups overlap to a large extent. The phenotypes limited distribution suggests the operation of genetic drift rather than a deterministic pro- cess. Historically, polygamy may have provided an appropriate vehicle for drift. Surname analysis (Stevenson, Brown, and Schanfield 1983) in western and northwestern Liberia, particularly among the ethnic groups with the highest current frequencies of this phenotype, should provide important details on its origins.

In the sample the distribution of this phenotype demonstrates two geographical areas of highest frequency: (1 ) among the Kwa-speakers of southeast and central Liberia and (2) among the Mande-speakers of the northwest region. In view of the cultural and historical dissimilarities of these areas (Tables 3 and 4), it is likely that the two foci of biologically highest frequency represent at least two instances of independent mutations for this trait.

This study was not able to determine the precise molecular basis or bases of the elevated fetal hemoglobin phenotype, however, it is possible that the continued production of high level of gamma hemoglobin chains into adulthood is associated with an altered response to some other environmental agent. Equally likely, the perpetuation of this phenotype in contemporary Kwa-speaking groups may also be the result of genetic drift and may reflect their historical geographical isolation in southeast and south-central Li- beria.

As previously mentioned, this isolation was precipitated by the mass immigrations of Mande-speakers from the north and east into formerly Kwa-controlled territories. In the 1600s the forerunners of the Kpelle, Mano, and Gio invaded north-central Liberia. The extensive radiation of these immigrants pushed the earlier Kwa-speakers, such as the Bassa, further into the forests and toward the coasts ofcentral and southeast Liberia. The movements of the Kpelle in particular created a linguistic and cultural wedge through central Liberia which significantly curtailed contact between Kwa-speakers in the south and Mel- and Mande-speakers in the north and west (Schulze 1973). Although the 1800s brought Western Hemisphere African-Americans and repatriated African settlers to coastal central and southeast Liberia, their penetrations into the interior sections were limited. In fact, until very recently the traditional Kwa-speaking interior areas of the south-central and southeast remained tmderdeveloped, with few roads and sparse educational or medical facilities which would have tended to encourage interethnic contact and increase the probability of gene flow.

In the northwest region the elevated fetal hemoglobin phenotype may have increased in frequency in conjunction with the introduction of the hemoglobin S gene. Among these Mande-speakers the principal adaptive significance of persistently elevated fetal hemoglobin production is its likely ability to ameliorate the normally deleterious condition of sickle cell disease (Wood et al. 1977; Dover and Boyer 1981). The positive colinearity of elevated fetal hemoglobin with sickle cell homozygosity within the sample supports this notion. A historical association of this sort also suggests that the distribution of the elevated fetal hemoglobin phenotype in northwest Liberia may be modulated by similar cultural and historical factors that have influenced the distribution of sickle hemoglobin.

The rather uniform distribution of partially inactive red cell GGPD enzyme raises more ethnohistorical questions than provides an- swers. Since phenotypic rather than genotypic determinations were made in this study, future isozyme typing may serve to distinguish the likely dissimilar historical and genetic

Elevated Fetal Hemoglobin.

Intermediate Red Cell G6PD De$aency.


origins of intermediate red cell GGPD deficiency among northwest groups versus southeast and some central groups. Furthermore, the observed higher frequency of this phenotype among the Loma needs further explanation: perhaps it is an artifact ofsome cultural practice rather than a reflection of altered population genetics. As is the case with beta thalassemia trait, surname analysis may provide significant elaboration.

Conclusions

While our knowledge of the basis and extent of human genetic variation is often quite rich at the molecular, cellular, organ, and individual levels, our understanding of this same variation at the regional and ethnic group levels of analysis frequently lacks a similar sophistication. Biocultural anthropologists able to integrate genetic, physiological, cultural, and historical data have an important contribution to make toward illuminating the origin and maintenance of human genetic diversity. In Liberia where malaria remains highly endemic, the contemporary distributions of sickle cell trait, beta thalassemia trait, elevated fetal hemoglobin, and intermediate red cell GGPD deficiency suggest that their presence and perhaps their origins reflect the complex interactions of specific sociocultural variables and ethnohistorical events. In this study I explore the possible roles of initiation societies, linguistic affinities, dietary staples, and religious preferences in mod- ulating the direction of gene flow and predisposing various segments of the population to the random effects ofgenetic drift. I also present a plausible historical scenario suggesting major genetic implications for specific events in the ethnic histories of the Liberian people.

Acknowledgments. I am most grateful to the people and government of the Republic of Liberia for allowing me to conduct this research in their country. Special thanks go to Assistant Minister of Information Bai T. Moore, Augustus Caine of the University of Liberia, and W. Boyuou, E. Jallah, and E. Nagbe of the Ministry of Health and Social Welfare. Field and laboratory investigations were supported by grants from the Huber Foundation, National Fellowships Fund, Sigma Xi, and Cornell University.

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Documents

Sociocultural and Ethnohistorical Influences on Genetic Diversity in Liberia