Biomedical subjects

H Harpending

Publications and source records attributed to H Harpending.

8 recordsLinked to original sources

PubMed1992

Population growth makes waves in the distribution of pairwise genetic differences.

Episodes of population growth and decline leave characteristic signatures in the distribution of nucleotide (or restriction) site differences between pairs of individuals. These signatures appear in histograms showing the relative frequencies of pairs of individuals who differ by i sites, where i = 0, 1, .... In this distribution an episode of growth generates a wave that travels to the right, traversing 1 unit of the horizontal axis in each 1/2u generations, where u is the mutation rate. The smaller the initial population, the steeper will be the leading face of the wave. The larger the increase in population size, the smaller will be the distribution's vertical intercept. The implications of continued exponential growth are indistinguishable from those of a sudden burst of population growth Bottlenecks in population size also generate waves similar to those produced by a sudden expansion, but with elevated uppertail probabilities. Reductions in population size initially generate L-shaped distributions with high probability of identity, but these converge rapidly to a new equilibrium. In equilibrium populations the theoretical curves are free of waves. However, computer simulations of such populations generate empirical distributions with many peaks and little resemblance to the theory. On the other hand, agreement is better in the transient (nonequilibrium) case, where simulated empirical distributions typically exhibit waves very similar to those predicted by theory. Thus, waves in empirical distributions may be rich in information about the history of population dynamics.

Biological Evolution↗

PubMed1991

African populations and the evolution of human mitochondrial DNA.

The proposal that all mitochondrial DNA (mtDNA) types in contemporary humans stem from a common ancestor present in an African population some 200,000 years ago has attracted much attention. To study this proposal further, two hypervariable segments of mtDNA were sequenced from 189 people of diverse geographic origin, including 121 native Africans. Geographic specificity was observed in that identical mtDNA types are shared within but not between populations. A tree relating these mtDNA sequences to one another and to a chimpanzee sequence has many deep branches leading exclusively to African mtDNAs. An African origin for human mtDNA is supported by two statistical tests. With the use of the chimpanzee and human sequences to calibrate the rate of mtDNA evolution, the age of the common human mtDNA ancestor is placed between 166,000 and 249,000 years. These results thus support and extend the African origin hypothesis of human mtDNA evolution.

Africa↗

PubMed1990

Estimating parity of parents: application to the history of infertility among the !Kung of Southern Africa.

We derive a method for interpreting information about the reproductive performance of mothers of a sample of informants and apply it to the history of low fertility among the !Kung of the Kalahari desert of southern Africa. The method formalizes the commonsense procedure of weighting mothers with 1 birth by 1, with 2 births by 1/2, with 3 births by 1/3, etc. An expression for the confidence interval of the estimate of mean completed fertility of fertile women in the mothers' cohort is given. Several independent studies show that !Kung women have low fertility, with a population average completed family size of slightly over 4. Our method, applied to the cohort of old informants, shows that fertility is comparable among !Kung women, mothers of old informants, who were born in the late nineteenth and early twentieth centuries. If the cause is an endemic infectious disease of long standing in this region, there are important implications for the history of southern and central Africa.

Africa, Southern↗

PubMed1989

Mitochondrial DNA sequences in single hairs from a southern African population.

Hypervariable parts of mitochondrial DNA (mtDNA) were amplified enzymatically and sequenced directly by using genomic DNA from single plucked human hairs. This method has been applied to study mtDNA sequence variation among 15 members of the !Kung population. A genealogical tree relating these aboriginal, Khoisan-speaking southern Africans to 68 other humans and to one chimpanzee has the deepest branches occurring amongst the !Kung, a result consistent with an African origin of human mtDNA. Fifteen cases of unrelated individuals having identical sequences in the most variable parts of the mtDNA control region were found within populations of !Kung, Western Pygmies, and Eastern Pygmies, but no cases of identity were evident among these populations. This and other evidence of geographic structuring of the mitochondrial diversity in Africa, together with knowledge of the rate of accumulation of base changes in human mtDNA, implies that the average rate at which female lineages have moved their home bases during hunter-gatherer times could be as low as 13 meters per year. The technique of enzymatic amplification and direct sequencing applied to readily collected, highly stable biological materials such as hairs makes it possible to examine with high resolution many representatives of virtually any population.

Africa, Southern↗

PubMed1988

Fitness and fertility among Kalahari !Kung.

In this paper we develop a model that examines fertility and childhood mortality patterns and their relationship to environmental variables. Interactions among environmental variables can account for different fertility patterns and different mixes of these variables can produce similar patterns of fertility. Our model attempts to quantify the idea that there is a trade-off between producing a few children likely to survive to reproductive age and producing a greater number of children with lower chances for survival. The optimum mix of these strategies depends on environmental characteristics. We use the model to make predictions about fertility and mortality patterns among two Bushmen populations of southern Africa--the Ghanzi and Ngamiland !Kung--using data collected by Harpending in 1967-1968. The results do not support explanations of the low fertilities observed among !Kung Bushmen women, in whom it is thought that fitness is maximized by limiting fertility, and show no relationship between mortality and family size in either !Kung population. Instead, the number of offspring reaching reproductive age in both populations increases as their completed family size increases. We examine the effects of sex, birth order, and paternal investment on mortality. No sex ratio differences and no differences in mortality by sex or birth order are present. Infant mortality among women who married more than once is significantly higher than among women who married once, suggesting that paternal care has a significant effect.

Adult↗

PubMed1987

On Wright's mechanism for intergroup selection.

A haploid model of Wright's (1945) intergroup selection is described. Several approximate analyses as well as Monte Carlo simulation show that Wright's mechanism leads to powerful selection for group advantageous traits in the face of strong local selection against them. Intergroup selection is favored by small local subpopulation size and by high, not low, rates of immigration into local groups, although the dependence on migration rate is not strong. These results and others cast doubt on the primacy of genic selfishness as a tenet of Darwinian theory.

Animals↗

PubMed1986

Primate population structure: evaluation of models.

Genetic markers among macaques on Cayo Santiago island were analyzed in an attempt to infer aspects of mating structure. Several models that included high levels of gene flow among groups could not be distinguished, but the data are clearly incompatible with group endogamy and with high variance in male fitness. Drift effective size is approximately one half census size in this population.

Animals↗

PubMedDate not supplied

Effect of infertility on the population structure of the Herero and Mbanderu of southern Africa.

Analysis of the fertility of Herero and Mbanderu pastoralists of the northern Kalahari Desert of Botswana indicates that they have suffered from infertility. The smoothed population pyramid constructed from a recent census shows waves of births occurring about every 22 years. Since generation times in human populations are typically longer, we suggest that infertility is responsible for prematurely terminating the reproductive spans of women, resulting in a reduced generation time. The eigenvalues of a series of Leslie matrices indicate that the periods of oscillation in the population have increased from 22 to 29 years following the recovery from infertility. Increases in fertility since the 1950's have also transformed the Herero from a slowly declining population to one growing at a rate of nearly 3.5 per cent per year.

Adolescent↗