J. L. Serre* Social Blending and Gene&al Blending

lnrtitut National d’Etudes DLmographiques, Paris For many years now the study of more or less isolated populations has

been carried out throughout the world.

P. Lefevre-Wittier The population of IdelPs has this particular characteristic of being

recently formed. This is the sedentarisation of groups from the Sahara

Centre d’Ht?motypologiqie du C. N. KS., Toulouse

of various origins. As a result of their being few in number, the demo- graphical means are powerless to reflect the evolution and rates of evolution of this population itself divided into socio-ethnic groups.

The study of the probability of the origin of the genes allows to answer some questions in so far as it was possible to draw up genea- logical trees which is impossible for a large population but quite feasible at Idek

This survey shows that the mixing of the genes can be far greater than that of the ethnic aspects of sacro-cultural habits, and that genetic exchange, albeit small, remains, through their differences, one of the oldest exchanges between human groups.

1. Introduction

The population of Idelb, a village near Tamanrasset (Algerian Sahara), has this par- ticular characteristic of being recently formed. This is the sedentarisation of groups from the Sahara of various origins, especially of Twaregs who used to be wandering tribes, and are now farmers. This new way of life and blending of different populations on ethnic or cultural levels gives rise to a new kind of social relations. But it was interesting to ask ourselves whether this sociological blending was giving rise to a new population and how this genetical blending, if it exists, evolved?

2. Materials

After several missions (1968, 1969, 1970, 1973, 1976), a great number of demographic, sociological and biological data were available. We could also draw up the family-trees from the settlement of Idel&s. The total number of individuals is 1502 over six generations.

Previous studies had shown that six groups could be defined among the population.

(if The first group: M’rabtines, except the Kel Tit family. (ii) The second group: the Kel Tit family, who was at the origin of Idelb by inter-class

marriages with Twaregs. M’rabtines are muslims and arabic speaking. (iii) The third group: Twaregs Isseqamarhnes. (iv) The fourth group: Twaregs of other tribes. (v) The fifth group: Zklans (ancient slaves of African origin, previously employed as

servants by wandering tribes), and Artisans (a traditional group of African societies).

(ui) The sixth group: Harratines (traditional oasis farmers in the Sahara).

We tried to estimate the importance on qualitative and quantitative levels of relations between groups, firstly by a demographical approach, secondly by a genetical approach developed by A. Jacquard (1970, 1972) who used the concept of probability of origin of genes.

3. Demographical Results

The results of the study of the nature of marriages are shown by the Table 1 and more significantly by Figure 1. The children’s class is their father’s one except if the father is

*Present address: Genktique, Tour 42, let &age, Universitt Pierre et Marie Curie, 4 Place Jussieu, 75230 Paris Cedex 05, France.

Journal of Human Evolution (1979) 8, 789-793

0047-2484/79/080789+05 $02.00/O @ 1979 Academic Press Inc. (London) Limited

790 J. L. SERRE AND P. LEFEVRE-WITTIER

Table 1 Marriages between classes and inside classes for men and women

& 1 2 3 4 5 6 7

1 32 16 10 9 3 1 4 2 13 37 14 6 1 2 3 3 7 103 2

4 2 21 3 5 2 76 11 : 6 12 34 4 7 11 5 9 1 56 9

Figure 1. Exchange of wives between classes (marriages inside classes are between brackets).

unknown. Unknown fathers and sometimes mothers, have been gathered in a seventh class.

As a first conclusion it can be said that endogamy is more often the rule, and that preferential rules exist for exogamy, between classes.

The population appears to be divided in four groups:

(i) The group of classes 5-6 which have a mutual exchange of wives, giving only a few to other classes, getting none from them. Most of the children of unknown

fathers are in this group, especially in class 5. (C) The group of classes l-2 which have a mutual exchange of wives, giving a few only

to classes 3 and 4, but getting wives from all classes. (ii;) The class 3 on one hand and the class 4 on the other hand give more wives to the

class l-2 than they receive. Endogamy seems to be important in both classes and though they are both Twaregs there is not an exchange of wives between them.

We should have to take into account demographical parameters like wedding age, fecundity, mortality, because it is obvious that the wife exchange cannot by itself bear out

SOCIAL AND GENETICAL BLENDING 791

the evolution of the population. But demographical means need large samples and are inapplicable in the study of small human groups.

That is a reason why we calculated the probabilities of origin of genes.

4. Genetical Results

The knowledge of all the family-trees is the knowledge of the real evolving of the popula- tion (marriages, number of descendants). It is a way of studying small groups when demographical means are inapplicable.

The probability of origin of genes is a summary of the whole information of the genea- logy which is not by itself obvious when there are 1500 individuals over six generations, divided into seven classes.

This concept of probability of origin of genes has been defined by A. Jacquard who analysed several isolated populations.

“The knowledge of all the family-trees let us calculate, for each individual living at a precise time, the percentage (in expectation) of his genetical heritage issuing from each ‘founder’. A founder is an ancestor the ancestry of whom is unknown.”

The results of this calculation are summarized by Figures 2 and 3 which show the result of the evolving after six generations.

Figure 2. Genetical heritage of each class at the sixth generation. The origin of genes k geometrically figured, the generation of entrance in the class is given by the different numbers a, Class 1; ffl, dads 2; ~,~la~~3;~,cla~~4;~,class5;~,classb;~,claas7unlcnown.

It can be said (Figure 2) that Twaregs are much more isolated than other classes. But about one-sixth of their genes are already issuing from other classes, almost all from classes l-2.

The classes 5 and 6 are much less isolated, most of their foreign genes are issuing from a reciprocal exchange. We must also notice that it would be interesting to know the real origin of genes from unknown fathers.

The classes 1 and 2 have respectively 60% and 42% of their genes issuing from all the other classes; they appear to be a melting pot of the whole genetical heritage of the population.

These results are also obvious in Figure 3. We can notice here that the intensity of

792 J. L. SERRE AND P. LEI’EVRE-WITTIER

Figure 3. Genetical exchanges between classes of IdeMs after six generations. In black inside each class: the percentage of genes issuing from other classes.

Figure 4. Genetical melting-pot in Id&s.

genetical exchanges does not square with the intensity of wife exchange, because of the differences in fecundity.

The main genetical exchanges are summarized in Figure 4. As we know that half of the

SOCIAL AND GENETICAL BLENDING 793

unknown genes in classes 4 and 5 are issuing from “unknown fathers” of the four other classes, we can say that a true turn over will lead these classes to share, through the differences of social and cultural habits, a mixing of all their ancestral genes.

The genetical exchange seems to be far greater and faster than the sociological one.

Adapted from: Serre, J. L. & Ldevre-Wittier, P. (1977). Echange sociologique et &change g&&ique. Population 35 361-372.