POPULATIONS GENETICS OF HUMANS
The aim of population genetics is to model the dynamics of evolutionary change within and between populations i.e. a group of individuals who exist together in time and space and are capable of interbreeding. In human DNA approximately 0.08% of the nucleotide base pairs varies among individuals and thus populations genetics has been trying to establish why this is so. Four basic evolutionary forces responsible for genetic diversity in populations have been identified: mutation, natural selection, genetic drift and gene flow. Mutations are copying errors during DNA replication and transcriptions, which introduce new alleles into the population. Natural selection is the differential transmission of alleles into the next generation due to the consequences of functional differences on an individual’s survival and reproductive success. Genetic drift is the differential transmission of alleles into the next generation as a result of random sampling and has the greatest potential impact in small populations. Gene flow spreads alleles from one population into another via migration, making them more genetically similar to each other, and countering genetic differentiation by drift. I am going to examine the contribution of genetic drift, gene flow and natural selection to the levels of diversity and composition of genetic polymorphisms in different human populations. Further I am going to examine why some populations have greater diversity than others and compare the patterns of genetic diversity of humans and chimpanzees.
Genetic drift is a process through which gene frequency changes (over generations) are produced as a result of the finite number of individuals present in a population, and such changes are intrinsically random; i.e. non-systematic. Random genetic drift was introduced to population genetics through the work of Sewall Wright. He believed that the random properties of allele frequency change were important not only in understanding the exact changes from one generation to the next, but also in the way dif allowed evolution to proceed in novel directions which might otherwise be opposed by direct selection. The change in allele frequency due to genetic drift, while unpredictable in individual populations, on average causes populations to change in a predictable way. Drift on average causes there to be fewer alleles within populations, and variable allele frequencies over time. Two immediate consequences of drift effects are: (1) a gradual decay of within-population variation, ultimately leading to either fixation or loss of specific alleles and (2) genetic differentiation between populations can be created by genetic drift alone. (Whitlock, 2001; Chakraborty, 2001)
When one allele becomes so common in a population that no others exist, that allele is said to have ‘fixed’ in that population. One of the consequences of genetic drift is that, if there is no other source of new genetic variation such as mutation or migration from other populations, allele frequencies will randomly increase and decrease until one or another of the alleles in the population is fixed, and the others lost. As drift is a random process, the fixation of an allele in one population is independent of its fixation in another population. As a result, over time populations will become more and more different from each other due to the effect of genetic drift. This effect is not merely the result of fixation of alternate alleles in different populations, but also happens earlier as the result of the random change in one population not being exactly the same as that somewhere else, such that different populations become different from one another. Natural populations that are isolated from one another because they rarely exchange migrants would be expected to diverge from another solely on the basis of genetic drift unless some other evolutionary force, such as natural selection, is acting to maintain the populations in a similar state. Nevertheless, the most immediate consequence of genetic drift is that the allele frequencies in on generation are not likely to be the same as those in the previous population. Drift changes allele frequencies without regard to their adaptive properties and therefore is non-adaptive. Furthermore, drift is as likely to increase allele frequencies as to decrease them, and is therefore non-directional and it is not correlated across generations. (Whitlock, 2001; Chakraborty, 2001)
The most important predictor of the magnitude of genetic drift is the number of breeding individuals in a population – its population size. As the number of breeding individuals gets smaller, the magnitude of genetic drift increases. Other properties of the species, in particular variation in the amount of reproductive success among individuals of that population, also affect the amount of genetic drift. These effects are often encapsulated into the concept of the ‘effective’ population size, which is often written as N e. This refers to the number of individuals that would be in an ideal population, with equal probabilities of reproduction of each individual in the population, which would have the same amount of genetic drift as the population under study. Most natural populations have effective population sizes that are much smaller than their actual population sizes, and therefore genetic drift is more important than the large apparent population size of many species would seem to indicate. The long-term patterns of genetic drift are also very sensitive to periods of small population size. A single generation where the population size drops to a low value (e.g. ‘population bottleneck’ or ‘founder effect’) can cause more change in allele frequency than many generations at a large population size. (Whitlock, 2001)