Population Genetics and the Problem of Diversity

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Population Genetics and the Problem of Diversity

Overview

The extent to which genes differ between individuals, races, and species has been the central theme of population genetics in the twentieth century. New experimental methods developed in the 1960s allowed the first estimates of the degree of genetic variation in natural populations of human and non-human species. The unexpected finding was that on average, at least one out of every three genes in a species had more than one molecular form, revealing substantial genetic variation among members of the same species. In humans related studies revealed that the genetic variation between individuals of the same race was much more pronounced than that between races. These findings called for a re-thinking of the role of natural selection in evolution and brought a deeper understanding of the close symbiosis between genes and the environment. The inadequacy of racial classifications in humans became clear, with far-reaching implications for the use of racial distinctions in human society.

Background

Natural selection, which is sometimes referred to as "survival of the fittest," formed the cornerstone of the theory of evolution formulated by Charles Darwin (1809-1882). Species evolve due to hereditary variations that favor their survival and reproduction. It is an organism's phenotype, or outwardly expressed characteristics like physiology and behavior, that is subject to selection by the environment. By contrast, what is actually transmitted from one generation to the next is the genotype, the collection of genes that is inherited by the organism. The phenotype of an organism is determined both by its genotype as well as the environment in which it grows. Human skin color, for instance, varies with the amount of exposure to ultraviolet sunlight, with darker skin having a selective advantage for people who live in lower latitudes.

Each gene can have one or more molecular forms, giving rise to the notion of genetic polymorphism. Population genetics deals with characterizing the frequency with which one or other form of a gene prevails in a population. A polymorphic gene is one that has more than one variant present in significant amounts—usually a few percent—in natural populations. The earliest example of genetic polymorphism in humans was found in studies of blood types. Different forms of a gene gave rise to different types of blood cells that were incompatible with one another.

For many years, it was thought that most genes were not polymorphic. This was in accordance with the assumption that natural selection would weed out unfit variants of a gene, leaving a relatively homogenous gene pool in each species. This view was challenged in 1966 by Richard Lewontin (1929- ) and Jack Hubby, who worked with fruit flies, and by Henry Harris, who worked with humans. Their method relied on the known link between proteins and genes—that the sequence of molecules making up a protein was a translated copy of the gene sequence. By studying the protein sequence, they could make inferences about the gene sequence and distinguish one form of a gene from another. They used a technique called gel electrophoresis, in which proteins of different electric charge migrated different distances in a gel. Even a single variation in the protein sequence could be detected in this way. The main advantage of this method over blood group studies was that a nearly random selection of genes, not affiliated with a particular trait, could be analyzed for polymorphism.

Lewontin and Hubby found that 7 out of 18 genes in their fruit fly samples were polymorphic, while Harris found that 3 out of 10 genes in his samples taken from the human population were polymorphic. Subsequent studies in other species, including other primates, amphibians, rodents, and birds, reached the same conclusion: on average, at least one out of every three genes in a species harbored polymorphic variants. In humans, this meant that any two individuals taken at random from a population are likely to have no more than two-thirds of their genetic material in common. It is natural to ask whether it matters if these individuals are chosen according to their racial or ethnic affiliation.

Although individual traits such as blood type or disease susceptibility were known to show marked racial variation, no single trait could be used to determine a person's race with certainty, and no two traits were seen to yield similar racial groupings. What was needed was a random sampling of the human genome to determine the extent to which humans of different races diverged from one another genetically. Harris, Lewontin, and others used electrophoretic studies in the early 1970s to estimate racial differences in humans. These studies relied on a measure of genetic variation that was more precise than polymorphism, called heterozygosity. In sexually reproducing organisms, genes occur in pairs, one of each pair inherited from each parent. Heterozygosity measures the probability that a gene pair has dissimilar genes for an individual taken at random from the population. While polymorphism does not indicate the number of variant forms of a gene (other than to say that such variation exists), heterozygosity would be higher for a gene with more variants.

A famous study by Lewontin in 1972 used 18 polymorphic genes in 7 races, including Africans, Caucasians, and Mongoloids. He found that 85% of the heterozygosity in the human species was already present in a single nation or tribe. Race contributed about 7% of the decrease in total heterozygosity, while nationality contributed the remaining 8% of the decrease. In other words, genetic diversity in humans was most pronounced at the individual leve, rather than at the national or racial level.

Impact

The discovery of large amounts of genetic variation in humans and non-human species revolutionized the understanding of the evolutionary theory of natural selection. Polymorphism is introduced into a species by mutation, the random errors that occur in the replication and transmission of genes from parents to offspring. Once introduced, it was thought that these mutant genes would either become prevalent or die out under the forces of natural selection, in either case moving toward a reduction in variation. The continued maintenance of so much variation in a species called for new evolutionary assumptions, including some that departed from traditional Darwinian evolution.

In 1968 Motoo Kimura (1924- ) introduced the idea that perhaps a large fraction of the genetic variants observed were selectively neutral—that is, they were immune to the forces of natural selection. He assumed that these variations were introduced into a population by mutation and maintained by random genetic drift. The latter refers to the statistical fluctuation in the frequencies of gene variants transmitted from one generation to the next. This implied a less deterministic view of evolution and one that was less adaptive to the environment.

Advocates of natural selection, on the other hand, considered mechanisms of selection that maintained, rather than reduced, genetic variation. Selection could be frequency-dependent, favoring high frequencies of polymorphism in the species. Heterozygosis could also have a selective advantage, as in the case of the sickle-cell gene in humans. This gene causes anemia but offers resistance against malaria, so having one member of the gene pair of the sickle-cell type optimizes the chances of survival in areas where malaria is prevalent. That genetic variation in a species could be preserved by natural selection seems contrary to Darwinism, although Lewontin takes the view that Darwinian evolution is essentially the conversion of variation within populations to variations between populations in space and time.

The neutralist-selectionist controversy of the 1970s left its mark on evolutionary theory, with elements of both views gaining currency with new experimental evidence. In the end, it was an effort to come to terms with the immense genetic diversity inherent in every species. Various factors were identified that could sustain this diversity and still allow for deterministic evolution. Genetic diversity implied a variation in the observed characteristics of an organism, and this link between the genotype and the phenotype was a necessary ingredient in determining the effects of natural selection. Apportioning causes to genes and the environment in determining the phenotype of an organism was at the heart of another controversy that has had a long history in science, the problem of nature versus nurture. Apart from the genetic contribution, the immense range of environments available to an organism further added to the diversity of phenotypes observed.

In some sense, the discovery of genetic diversity among humans came as no surprise, since it has often been said that "no two individuals look exactly alike." By the same token, the very features that are used to distinguish individuals—skin color, hair form, eye shape, nose shape, etc.—are the basis for the perception of distinct races in human society. The discovery in the 1970s was that, at the biochemical level, races were not nearly as homogeneous as we perceived them to be, and racial distinctions were, at best, superficial. Racial groupings continue to be used in anthropological studies, but more as a means of describing the findings rather than explaining them. Population biologists continue to study the variation of human traits across populations, defined as groups of interbreeding individuals, without inferring any racial taxonomy per se.

Although the race concept has lost its utility in these fields of study, it still informs many issues in public policy, especially in multi-racial societies. This is because race can be associated with social inequalities among people that are attributable to their perceived racial differences in the society. Even genetic differences between races continue to raise complex public policy concerns. One issue is whether companies should be allowed to screen individuals for racespecific genetic diseases such as sickle-cell anemia. Another is whether law enforcement agencies can rely on deoxyribonucleic acid (DNA) fingerprinting for incriminating individuals, given the possibility that such techniques may not be racially unbiased. It is noteworthy that the average genetic differences between races that were studied in the 1970s do not obviate such concerns.

New developments in DNA technology in the 1980s made it possible to map individual DNA sequences in each gene directly. This provided the impetus for mapping the entire human genome, an international ongoing effort known as the Human Genome Project. An offshoot of this project was the Human Genome Diversity Project (HGDP), a proposed survey of the genetic variation in humans from all racial and ethnic groups in the world. The urgency of this project is based on the assumption that the genetic isolation of the world's populations is fast disappearing today due to the rapid merging of peoples through migration. Still in its infancy, the HGDP continues the trend of studying the racial diversity and unity preserved in the human genome that was inaugurated by the electrophoretic studies in the 1970s.

ASHOK MUTHUKRISHNAN