Variation,Sex, and Social Cooperation: Molecular Population Genetics of the Social Amoeba Dictyostelium discoideum

Dictyostelium discoideum is a eukaryotic microbial model system for multicellular development, cell–cell signaling, and social behavior. Key models of social evolution require an understanding of genetic relationships between individuals across the genome or possibly at specific genes, but the nature of variation within D. discoideum is largely unknown. We re-sequenced 137 gene fragments in wild North American strains of D. discoideum and examined the levels and patterns of nucleotide variation in this social microbial species. We observe surprisingly low levels of nucleotide variation in D. discoideum across these strains, with a mean nucleotide diversity (π) of 0.08%, and no strong population stratification among North American strains. We also do not find any clear relationship between nucleotide divergence between strains and levels of social dominance and kin discrimination. Kin discrimination experiments, however, show that strains collected from the same location show greater ability to distinguish self from non-self than do strains from different geographic areas. This suggests that a greater ability to recognize self versus non-self may arise among strains that are more likely to encounter each other in nature, which would lead to preferential formation of fruiting bodies with clonemates and may prevent the evolution of cheating behaviors within D. discoideum populations. Finally, despite the fact that sex has rarely been observed in this species, we document a rapid decay of linkage disequilibrium between SNPs, the presence of recombinant genotypes among natural strains, and high estimates of the population recombination parameter ρ. The SNP data indicate that recombination is widespread within D. discoideum and that sex as a form of social interaction is likely to be an important aspect of the life cycle.     

The Xilixana Yanomami of the Amazon: History, Social Structure, and Population Dynamics

The Xilixana Yanomami of the Amazon: History, Social Structure, and Population Dynamics. John D. Early and John F. Peters. Gainesville: University Press of Florida 2000 323 pp.     

Numbers, Turnover, and Social Structure of the Isle Royale Wolf Population

Observations of wolves on Isle Royale are reported for 1961–66,with interpretations including the earlier 3-year period describedby Mech (1966). On this 210-square-mile island the fully protectedwolf population varied from approximately 22 to 28 in midwinter.The major and minor foods were moose and beaver, respectively. The main pack varied in number between 11 and 22 with aboutthree breeding pairs believed present. The population remainedrelatively stable; mating occurred every winter; and adult mortalityappeared to be low. High mortality among pups seemed to be thepoint of population control. Socio-economic factors may havecontrolled the size of the large pack. Availability of foodduring the period of parturition and rearing probably was criticalto survival of young. Recruitment of young appeared to take place in years of highproduction of moose calves. Numbers in the large pack probablywere curtailed through the progressive exclusion of aged andsocially subordinate individuals. Under harassment these animalsseparated and became pack-following scavengers, then probablytrue loners ranging outside the area used by the pack. Smalleraggregations of two or three non-breeders were seen each winter,as were the loners, some of which appeared thin and weak. The only known breeding outside the big pack was in a groupof five present in the winter of 1965. This group was probablya family unit which separated from the main pack. A year laterthe male had disappeared, and remains of a pup, probably theirs,were found. In the winter of 1966 the alpha male of the largepack became lame and apparently was killed. This backgroundappears favorable for further changes in social organization.     

The Mushroom Crowd: Social and Political Aspects of Population Pressure

The meaning of population pressure is discussed and, with the help of a formula, its five main components (people, rate of increase of people, food-producing acreage, protein nutrition and industrialization) are analyzed.Factors which have eased population pressure in the past are considered under the headings of political violence (war and genocide), natural disaster (famine and disease), and social action (increased local protein supply, emigration, industrialization and birth control).Birth control is the first prerequisite to an easing of population pressure; the effectiveness of the Japanese and the Indian programs is discussed.The crippling burden of preventable disease, shown to be a major obstacle to the development of sophisticated industries, presents a challenge to Western medical men who so far have done little to meet it.     

Fate and Fortune in Rural China: Social Organization and Population Behavior in Liaoning, 1774-1873

Fate and Fortune in Rural China: Social Organization and Population Beliavior in Liaoning, 1774-1873. James Lee and Cameron Campbell. Cambridge: Cambridge University Press, 1997. 280 pp.