28 Causes of Human Variation

Dr. Prasanna Kumar Patra

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Contents

1. Introduction: what is human variation and what are its causes?

2. Historical views of Human Variation

3. Monogenism and Polygenism

4. Adaptive significance of human variation

5. Forms of variation:

6. Causes of human variation

(a) Mutation

(b) Gene flow

(c) Genetic drift

(d) Natural selection

7. Contemporary Interpretations of Human Variation

 

Learning objectives

 

  1. To provide an introductory overview of human variation and its causes
  2. To provide an historical overview of causes of human variation
  3. To familiarize the student with the adaptive significance of variation
  4. To discuss the various forms of variation and genetic factors that shape human variation
  5. Finally, to provide the contemporary interpretation of human variation

 

Introduction: what is human variation and what are its causes?

   In this module, we shall find out some of the ways and means by which humans differ from one another, both within and between populations. No two humans are genetically identical, even the monozygotic twins, who develop from one zygote. Therefore, we can imagine the extent and scope for difference or variation in and within a population.

 

   All humans are different from each other, and much of this difference has a genetic basis: differences in phenotype caused by differences in genotype. Some of these differences are easily observable, and we know from our own experiences that they run in families – hair, eye, and skin colour, stature, and some morphological features (Jobling et.al. 2014). Humans are one of the most morphologically variable species of animals, and part of the reason this variation exists is because members of our species occupy many diverse habitats. Yet the exact genetic basis of this variation must be quite small. Biochemical evidence shows that humans and chimpanzees differ genetically by between 1-2 % andthat this amount of difference was accumulated over a period of approximately 7 million years (Boaz and Almquist, 1997).

 

   The issue of human variation is very complicated, and the biological and cultural factors that have contributed to that variation and that continue to influence it are manifold.

 

Historical views of Human Variation

Historically, when different groups of people came in contact with one another, they offered explanations for the phenotypic variations they observed. It was because, some phenotypic characters, such as skin colour was obviously noticeable. Skin colour was one of the more frequently explained traits, and most systems of racial classification were based on it. As early as 1350 B.C., the ancient Egyptians had classified human on the basis of skin colour: red for Egyptian, yellow for people to the east, white for those to the north, and black for Africans from the south (Gossett, 1963). In the sixteenth century, after the discovery of the New World, Europe embarked on a period of intense exploration and colonization in both the New and Old Worlds. Resulting from that contact was an increased awareness of human diversity (Jurmain et.al 2001).

 

   The first scientific attempt to categorize the newly discovered variation among humans was Linnaeus’ taxonomic classification, which placed humans into four separate groupings (Linnaeus 1758). Linnaeus assigned behavioural and intellectual qualities to each group, with the least complimentary descriptions going to black Africans.Johann Friedrich Blumenbach (1752 – 1840), a German anatomist, classified humans into five categories, or races. Although Blumenbach’s categories came to be described simply as white, yellow, red, black, and brown, he also used criteria other than skin colour. Moreover, he emphasized that divisions based on skin colour were arbitrary and that many traits, including skin colour, weren’t discrete phenomena (Jurmain et.al, 2005). In 1842, Anders Retzius, a Swedish anatomist, developed a cephalic index as method of describing the shape of the head. The cephalic index, derived by dividing maximum head breadth by maximum length and multiplying by 100, gives the ratio of head breadth to length. Individuals with an index of less than 75 had long, narrow heads and were termed dolichocephalic. Brachycephalic individuals, with broad heads, had an index of over 80, and those whose indices were between 75 and 80 were mesocephalic.

 

   In discussions on human variation, people have traditionally clumped together various attributes, such as skin colour, shape of the face, shape of the nose, hair colour, hair form (curly, straight), and eye colour. People possessing particular combinations of these and other traits have been placed together into categories associated with specific geographical localities. Such categories are called races. While race is usually a term with biological connotations, it is also one with enormous social significance. Moreover, there is still a widespread perception that there is an association between certain physical traits (skin colour, in particular) and numerous cultural attributes (such as language, occupational preferences, or even morality). Therefore, in many cultural contexts, a person’s social identity is strongly influenced by the manner in which he or she expresses those physical traits traditionally used to define “racial groups”.

 

   Objections to racial taxonomies have also been raised because classification schemes are typological in nature, meaning that categories are discrete and based on stereotypes or ideals that comprise a specific set of traits. Thus, in general, typologies are inherently misleading, because there are always many individuals in any grouping who do not confirm to all aspects of a particular type. Prior to World War II, most studies of human variation focused on visible phenotypic variation between large, geographically defined populations, and these studies were largely descriptive.

 

   Since World War II, the emphasis has shifted to the examination of differences in allele frequencieswithin and between populations, as well as adaptive significance of phenotypic and genotypic variation. This shift in focus occurred partly as a result of the Modern Synthesis in biology and because of advances in genetics. Racism is based on the false belief that intellect and various cultural attributes are inherited along with physical characteristics. Such beliefs also commonly rest on the assumption that one’s own group is superior to other groups (Jurmain et.al, 2005).

 

   On the matter of human origin, can the religious doctrine of original sin (as in the Abrahamic religion, such as in Christianity) be reconciled with what contemporary biology says? The doctrine requires descent from a single original ancestor, whereas contemporary biologists hold that the genetic evidence indicates that modern humans descended from a population of at least several thousand individuals.

 

Monogenism and Polygenism

The dominant debate concerning human origin and variation before and around Darwin’s time was between supporters of the unitary (monogenic) and of the separate (polygenic) origin of the human species. Debate centred on whether the differences seen in modern populations were of such a degree that different human groups should be considered separate species. Monogenists believed that all human races were descended from one pair (Adam and Eve), but they differed from one another because they occupied different habitats. It emphasized the similarities among populations and pointed to the criterion of inter-fertility between individuals of the different groups. This concept was an attempt to explain phenotypic variation between populations, but did not imply evolutionary change. The polygenist view, on the other hand, argued that human races or all populations were not all descended from Adam and Eve. Instead, there had been several original human pairs, each giving rise to a different group. Also, polygenists saw such a wide gap in the physical, mental, and moral attributes between themselves and other peoples that they were sure that outsiders belonged to different species. Thus, human races were considered to be separate species (Jurmain et.al. 2001, Boaz &Almquist 1997).

 

Adaptive significance of human variation

   Human variation is viewed, by physical anthropologists, as a result of evolutionary factors such as, mutation, genetic drift, gene flow, and adaptation to environmental conditions, both past and present. We must also recognise that cultural adaptations have also played an important role in the evolution of human. Adaptation is defined as a functional response to environmental conditions in populations and individuals. In a narrower sense, adaptation refers to long-term evolutionary (i.e. genetic) changes that characterize all individuals within a population or species. Biological anthropologists today recognize three levels of adaptation; (a) genetic, (b) short term, and (c) developmental. The sickle-cellexample (see the section on natural selection)is an example of genetic adaptation. The way our bodies adapt to cold illustrates a short-term physiological response. There is evidence to show that the capacities of adult physiology to adapt to new environmental stresses are narrower than the original genetic potential of the child (Little 1995). In any case, our bodies remain biologically equipped to adjust automatically to a range of temperatures. This physiological flexibility, which allows us to respond to environmental stresses such as temperature changes, is called plasticity.

 

  Shivering is an example of acclimatization, a change in the way the body functions in response to physical stress. Some acclimatization responses disappear when the physical stress disappear: we stop shivering when we put on more clothes or the air around us warms up. Other forms of acclimatization are longer lasting than the shivering response: Some environments in which human populations live, such as the highlands of the Andes Mountains in South America or the highlands of Himalayas in Asia, are characterised by hypoxia; that is, less oxygen is available to breathe than at lower altitudes. Studies have shown that people who grow up in high altitudes adapt to lower oxygen levels by developing greater chest dimensions and lung capacities than do people living at low altitudes. These changes – a form of developmental acclimatization – are apparently not due to genetic factors. Instead, they occur when the human body is challenged by a low level of oxygen in environment. Studies have shown that individuals who were not born in such an environment increased in chest dimensions and lung capacity the longer they lived in such an environment and younger they were when they moved there (Greska 1990).

 

Forms of variation:

Like any other living organisms, human beings show variation. If we consider almost any characteristic, we will find differences between various people (or other animals or plants) in a population. There are two forms of variation: continuous and discontinuous variation.

 

Characteristics showing continuous variation vary in a general way, with a broad range, and many intermediate values between the extremes. Height is an example of a continuously variable characteristic, as long as we consider a consistent sample, for example a large number of people of a particular age and sex. It is usually difficult to give a straightforward explanation of the genetic basis for these continuously variable characteristics because they result from a combination of genetic factors as well as environmental influences.

 

    Characteristics showing discontinuous variation fall into a few very distinct classes. The ability to roll the tongue, and blood groups, are examples of discontinuous variation. These characteristics can be explained much more easily by simple rules of genetics and are less likely to be affected by other factors. Some of the characteristics possessed by an individual in a population can be said to be inherited – i.e. derived from the previous generation. These characteristics are passed on, in a fairly predictable way, as a result of sexual reproduction. Sexual reproduction also introduces an element of randomness, so that variation is brought about in a population. These two almost contradictory factors – dependable inheritance of characteristics from parents, and variation within the population – are essential to an understanding of the process of evolution (Steane, n.d.)

 

  Now, we shall discuss the genetic causes of variation or otherwise, genetic factors that shape the micro evolutionary processes which brings about variation at population level and eventually are responsible for macro level evolution.

 

Causes of human variation

(a) Mutation

Mutation is defined as a change in DNA (deoxyribonucleic acid) that can occur either as a single base substitution or at a larger, chromosomal level.

 

Fig-1: Types of Mutation (Ref: http://www.biologyisfun.com/genetics/mutations)

 

      From an evolutionary perspective, mutation is the only way totally new variation can be produced. Effects on any gene should be minor, however, since mutation rates for any given locus are quite low (estimated at about 1 per 10,000 gametes per generation). In fact, because mutation occurs so infrequently at any particular locus, it would rarely have any significant effect on allele frequencies. Certainly, mutation occurs every generation, but unless we sample a huge number of subjects, we are unlikely to detect any noticeable effect.

     However, because we each have many loci (estimated at about 100,000), we all possess numerous mutations that have accumulated over recent generations. Most of these are not expressed in the phenotype, but are ‘hidden’ as recessive alleles. Some examples of such a recessive mutation are the alleles of PKU (phenylketonuria) cystic fibrosis, albinism, sickle cell anaemia and thalassemia. About of 1 in 12,000 babies born in the United States and 1 in 18,300 babies born in India carry this PKU allele in homozygous form and thus are affected phenotypically. Without early detection and treatment, this condition leads to severe mental retardation. Several dominant alleles that produce phenotypic effects are also well known. Some examples of such a condition are: a type of dwarfism called achondroplasia, brachydactyly (shortened fingers and toes), and Huntington disease (progressive degeneration of nervous system accompanied by dementia and seizures.

 

(b) Gene flow

   Gene flow is the movement of alleles between populations. The term migration is frequently used synonymously with gene flow; however, migration, strictly defined, means movements of people, whereas gene flow refers to the exchange of genes– which can occur only if the migrants interbreed (Jurmain et.al. 2001).

Fig-2: Gene flow (Ref: https://www.studyblue.com/notes/note/n/bio-1b-study-guide-2013-14-feldman/deck/8700412)

 

     Biological relationships between inter-breeding human groups are best understood in terms of gene flow between superficially distinct populations whose gene pool overlap considerably. For example, we know that individuals from European and African populations have interbred considerably since Europeans brought the first Africans to the New World as slaves. Similar processes have mixed the genes of these and other in-migrating populations with the genes of indigenous American populations. These are examples of gene flow among populations of a single species that had experienced relative isolation in the past but that continues to exchange enough genes often enough with neighbouring populations to preserve an overall species identity (Schultz and Lavenda,1998). However, we must keep in mind that, it would be a misconception to conclude that human gene flow can occur only through large-scale movements of groups. In fact, significant alterations in allele frequencies can come about through long-term patterns of mate selection. If exchanges of mates are consistently in one direction over a long period of time, allele frequencies will ultimately be altered. Due to demographic, social, or economic pressures, an individual may choose a mate from outside the immediate vicinity (Jurmain et.al, 2001).

 

(c) Genetic drift

Genetic drift is the chance factor in evolution and is tied directly to population size. The term drift is used because, as a completely random process, the allele frequencies can change in any direction. A particular kind of drift seen in modern populations is called founder effect or bottleneck effect. Founder effect operates when an unusually small number of individuals contributes genes to the next generation, making for a genetic bottleneck. This phenomenon can occur when a small migrant band of ‘founders’ colonizes a new and separate area away from the parent group. Small founding populations may also be left as remnants when famine, plague, or war ravages a normally larger group. In another sense, we can say that, each generation is the founder of all succeeding generations in any population (Jurmainet.al,2001).

 

 

Fig-3: Genetic Drift (bottleneck effect). Ref: http://evolgen.wikispaces.com/Group+19

 

   One of numerous examples of genetic drift that have occurred in human history began early in 19th century when British soldiers occupied the islands of Tristan da Cunha in the Atlantic Ocean. Eventually the soldiers withdrew, leaving only a single married couple who were later joined by a few other settlers. Throughout the 19th century, the population of Tristan da Cunha never grew much beyond 100 individuals. Over the 20th century, the population has grown to as many as 270 people, all of whom owe an enormous proportion of their genes to a very few individuals. It was calculated that more than 29 percent of those living on the island in 1961 had genes contributed by two of the original founding population (Schultz and Lavenda1998, Roberts 1968, Underwood 1979).

 

(d) Natural selection

   Natural selection is a mechanism of evolutionary change first articulated by Charles Darwin. It refers to genetic change, or to changes in frequencies of certain traits in populations due to differential reproductive success between individuals.

 

Fig-3: Natural Selection (Ref: http://missevrardbio1.blogspot.in/2012_05_01_archive.html)

 

        The best-documented case, in case of humans, concerns the sickle cell allele, which is the result of a single amino acid substitution in the haemoglobin molecule. In many human populations, only one allele – haemoglobin A (HbA) – is present. In other populations, however, mutant forms of the haemoglobin A may also be present. One such mutant allele, known as HbS, alters the structure of red blood cells, distorting them into a characteristic sickle shape and reducing their ability to carry oxygen. When individuals inherit the HbS allele from both parents, they develop sickle-cell anaemia. About 85 per cent of those with the HbSHbSgenotype do not survive to adulthood and hence do not reproduce.

 

      Because the HbS allele seems to be harmful, we would expect it to be eliminated through natural selection. But in some populations of the world, it has a frequency of up to 20 per cent in the gene pool. Why should that be? Geneticists might have concluded that this high frequency was the result of genetic drift if it were not for the fact that the areas with a high frequency was a result of genetic drift if it were not for the fact that the areas with a high frequency of HbS are also areas where the mosquito-bornemalaria parasite is common. There is, in fact, a connection. People exposed to malaria have a better chance of resisting the parasite if their haemoglobin genotype is HbAHbS rather than the normal HbAHbA. This is an example of what geneticists call a balanced polymorphism, in which the heterozygous genotype is fitter than either of the homozygous genotypes. The HbA and HbS alleles are co-dominant, with the result of a single HbS allele changes the structure of red blood cells enough to inhibit malarial parasites but not enough parasites but not enough to cause sickle-cell anaemia (Schultz and Lavenda 1998).

Table-1: Effects of the four evolutionary processes on variation within and between populations(Ref. Schultz and Lavenda, 1998, p.52)

 

Evolutional process Variation within populations Variation between populations
Mutation Increases Increases
Gene flow Increases Decreases
Genetic drift Decreases Increases
Natural selection Increases or decreases Increases or decreases

 

Contemporary Interpretations of Human Variation

    Physical anthropologists and other biologists who study modern human variation have largely abandoned the traditional perspective of describing superficial phenotypic characteristics in favour of measuring actual genetic characteristics. Those traits that differ in expression among various populations and between individuals are most important in contemporary studies of human variation. Such characteristics with different phenotypic expressions are called polymorphisms.

 

     Human blood type is one genetic trait with different alleles. Each person has one of four blood types— A, B, AB, or O—and these four types comprise the ABO blood group system, first discovered in 1900. Because it has more than two variants, a genetic trait such as this one is called a polymorphism (Greek poly, meaning “many”; Greek morph, meaning “form”). Each person has one A, B, or O allele on one chromosome of the homologous pair and another A, B, or O allele on the other chromosome of that pair. The combination determines the person’s blood type.

 

    Some of the most exciting contemporary DNA research has revealed a whole new array of genetic markers. Showing a tremendous amount of variation within and between human populations, these single nucleotide polymorphisms (SNPs) are known from well over 1,000,000 sites on the human genome. Closer examination of the human genome has also revealed that DNA segments are often repeated, sometimes many times, and for no apparent reason. These microsatellites are highly individualistic, forming a unique DNA signature for each person. Microsatellites have quickly become the most important tool for individual identification, and they have proven especially valuable in forensic science.

 

    For a long time, the general impression about genes has been that they represent specific locations of DNA coded to produce specific proteins. Much of research genetics,including in anthropological genetics, is based on this “one gene—one protein” model. However, the relationship between genes and their physical expression—especially their phenotypes—turns out to be more complex than previously thought. Many traits are polygenic, determined by genes at two or more loci; however, the genes cannot be identified individually, and the physical manifestations are influenced by environmental factors. In humans, thousands of traits such as height, skin colour, head form, tooth size,and eye shape have multiple genetic components and are strongly influenced by environmental factors. For some of these traits, scientists can determine only the relative proportions of genetic and environmental contributions. A trait’s heritability, the proportion of its variation that is genetic, can be calculated this way:

    Heritability estimates are presented as values ranging from 0, where none of the variation is genetic in origin, to 1, where all of the variation is genetic. In traits with heritability estimates greater than .5, most of the variation is genetic. For example, in the United States the heritabilities for height and weight are estimated to be .6 and .3, respectively. Tooth size has among the highest levels of heritability, about .7, and brain size and fingerprints are even higher, at .9. Physical anthropologists and other evolutionary biologists are very interested in heritability for one simple reason: because only heritable traits respond to natural selection, they are the primary driving force of evolution.

 

   Measurement of heritability, however, is complicated by pleiotropy—a single allele can have multiple effects. In fact, most complex traits are polygenic and pleiotropic

 

Summary points

   1. Much of the differences between humans have a genetic basis: differences in phenotype caused by differences in genotype. Some of these differences are easily observable, and we know from our own experiences that they run in families – hair, eye, and skin colour, stature, and some morphological features.

 

2. Historically, skin colour was one of the more frequently explained traits, and most systems of racial classification were based on it. Then, taxonomic classification by Linnaeus, cephalic index by Anders Retzius, various physical attributes like such as skin colour, shape of the face, shape of the nose, hair colour, hair form (curly, straight), and eye colour.

 

3.  Since World War II, the emphasis has shifted to the examination of differences in allele frequencies within and between populations, as well as adaptive significance of phenotypic and genotypic variation. This shift in focus occurred partly as a result of the Modern Synthesis in biology and because of advances in genetics.

 

4. Human variation is viewed, by physical anthropologists, as a result of evolutionary factors such as, mutation, genetic drift, gene flow, and adaptation to environmental conditions, both past and present.

 

5. Lately, physical anthropologists and other biologists who study modern human variation have largely abandoned the traditional perspective of describing superficial phenotypic characteristics in favour of measuring actual genetic characteristics. Those traits that differ in expression among various populations and between individuals are most important in contemporary studies of human variation. Such characteristics with different phenotypic expressions are called polymorphisms.

 

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References

  • Boaz NT and AJ Almquist (1997). Biological anthropology – a synthetic approach to human evolutionPrentice Hall, New Jersey.
  • Greska LP (1990). Developmental responses to high-altitude hypoxia in Bolivian children of European ancestry: a test of the developmental adaptation hypothesis. Americal Journal of Human Biology, 2: 603-612.
  • Jobling M, E Hollox, m Hurles, T Kivisild and C Tyler-Smith (2014). Human Molecular Genetics (2nd edition). Garland Science, New York.
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  • Schultz EA and RH Lavenda (1998). Anthropology – a perspective on the human condition (2nd edition). Mayfield Publishing Company, London.
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  • Underwood JH (1979). Human variation and human microevolution. Englewood Cliffs, NJ: Prentice-Hall.