#1. Observations
Human beings are morphologically diverse. Compared to other primates, human beings show less variation. We differ considerably in our external appearances (eye color, hair color, skin color, hair texture, skin texture, blood group types- A,B, AB and O, height, build, facial features, etc) and our physiology. However despite all these variations, the genome of each and every human being is 99 percent identical (Lai CQ, 2010). It is also interesting to note that humans and chimpanzees share 98 percent of the genome. The study of these genetic variations is very important to geneticists. The allelic differences between African populations are far greater than the differences between European and Asian populations (Barbujani G, 2010).
#2. Ask questions
What is the cause of genetic variation between species and also within a species? How is global genetic variation patterned?
#3. Hypothesis
Genetic variations cause variations in human populations even though all human beings share 99 percent of their DNA in common with each other. Differences in DNA sequences are the genetic basis of variability. Single nucleotide polymorphisms (SNPs) are responsible for genetic variations in different populations (Hinds DA, 2005).
#4. Predictions
Genetic variations at the nucleotide level in DNA are responsible for phenotypic variation in different populations since overall 99 percent of the genome in all human beings is the same.
#5. Controls
Human populations in different geographic regions of the world are the controls that could be used to test the hypothesis and confirm the observations.
#6. Exploration
Individual differences in DNA sequence are the genetic basis for human variation (Hinds DA, 2005). Environmental changes are the primarily responsible drivers for changes in population variation (Lai CQ, 2012).
Out of Africa explanation: Africa plays an important role in global genetic variation. Several thousands of years ago, the first group of humans to successfully move out of Africa towards the Arabian Peninsula towards the Indian sub-continent and eventually reaching South China (Lai CQ, 2012). A sub-group of the same population travelled towards Europe after recovering from the ice-age. Molecular evidence has provided crucial support for this explanation. Mitochondrial DNA and the DNA from Y chromosomes show little recombination between genetic markers and both can serve as a molecular clock for history of human migration (Lai CQ, 2012).
The allelic differences between African populations are far greater than the differences between European and Asian populations (Barbujani G, 2010). The study of these genetic variations is very important to geneticists. Differences in DNA sequences are the genetic basis of variability. Single nucleotide polymorphisms (SNPs) are responsible for genetic variations in different populations (Hinds DA, 2005). Genetic variation is derived from a wide assortment of genes and alleles (Nature Education Website on Genetic variation, 2014 and Essentials of Genetics Book Chapter).
The alleles found outside of Africa are a sub-set of the allele pool found in populations within Africa. Continent-specific alleles or haplotypes are rare in general but are far more prevalent in Africa than in any other continent (Barbujani G, 2010).
Big cities such as New York or London which usually contain a mix of different original populations (different races and ethnicities) in one geographical area can be used to test the hypotheses of genetic variation in human populations. India is a country whose population shows tremendous genetic variation due to its geography, language barriers due to the caste system and a mixture of distinct gene pools.
Adaptation to climatic variation: Global climates are categorized into 4 different types: dry, polar, humid tropical and humid temperate (Lai, CG, 2012). Human populations show adaptations to climatic variations and its many parameters such as sunlight, humidity and rainfall. There are 70 different loci that are responsible for skin pigmentation. There are also many different metabolic pathways that are responsible for the production of 2 main types of pigments, eumelanin and pheomelanin. The basic skin color depends on the proportion of these pigments, the size of melanosomes and their location in the epidermis (Barbujani G, 2010).
African populations nearer the equator show darker skin pigmentation while European populations at higher latitudes show have very light skin pigmentation. Thus, the human phenotype is highly correlated with latitude of the location where human beings inhabited (Lai, CG, 2012). There are also changes in genetic variations in response to humidity and temperature which are key factors in climatic variation (Lai, CG, 2012). Adaptation occurs when humans move to another location and survive in an unfamiliar environment.
Positive or negative selection can occur for humans to adapt to new environments and challenges (Lai, CG, 2012). Negative selection removes deleterious mutations that can make people susceptible to disease or having difficulty surviving in a particular environment. Genetic variations which cause an increased risk to disease in modern times probably gave our ancestors a survival advantage. Over the course of evolution, these genetic traits occur with an altered frequency resulting in positive selection. Thus, positive selection drives new mutations to higher frequencies which confer a survival advantage to individuals in the population (Lai, CG, 2012).
Climate is considered to be the most important environmental factor for adaptation since it controls several other environmental factors. Parameters such as temperature, rainfall, solar radiation, humidity are climate parameters in response to which genetic adaptation occurs (Lai, CG, 2012). The stronger the correlation of genetic adaptation to climate variation the more is the pressure of the selection.
Several other key questions to be explored are as follows: What about disease risks in relation to genetic variations in different people? Can the genetic variations help promote flexibility and survival in the face of changing environmental circumstances? How does genetic variation increase or decrease? How does genetic variation affect susceptibility to medical conditions and response to treatment? What effects do fluctuations in genetic populations have over time?
Agricultural activities and the lifestyle associated with it favored energy storage and resistance to starvation. Human lifestyles and activities have changed dramatically during the past 100 years due to industrialization and led to changes in modern civilization (Lai CQ, 2012). Diseases such as cancer, diabetes, cardiovascular problems, and strokes have also increased significantly in both the developed and developing world as a consequence in the change of lifestyle. Modern civilization relies on processed food and a sedentary lifestyle and related health consequences of this lifestyle are now the reality (Lai CQ, 2012).
African American populations in the US have a higher susceptibility to developing hypertension than other ethnic groups. Genetic variations have been implicated in different risks for susceptibility to developing hypertension (Lai CQ, 2012). A heat adapted and derived allele that is highly correlated with low latitude or hot and wet climate was implicated with risk for developing hypertension in African- American populations (Lai CQ, 2012). This allelic variant is also associated with blood pressure increase when salt intake is considered. Thus, this genetic variant helps in the explanation of higher hypertension risk among African American populations compared to other ethnic populations.
Carbohydrate metabolism is significantly different in Africans and in East Asian populations. Major carbohydrate groups include potatoes, milk and cereal foods. There are dietary adaptations based on allele-frequency at the genome level which contribute to metabolic differences (Lai CQ, 2012). In addition to genetic adaptation in response to carbohydrate metabolism, human populations also show adaptation to farming and adaptation to alcohol use in a cultural context. Culture usually co-exists with human evolution in response to genetic adaptation due to climate and environment (Lai CQ, 2012). Cultural practices can impose tolerance or intolerance to alcohol consumption thus resulting in the change in allelic frequency of variants in the alcohol dehydrogenase gene and be positively selected (Lai CQ, 2012). Humans also show adaptation to taste and cultural differences could favor genetic variation preference for certain foods.
The risk for developing age-related macular degeneration in the eye in different populations is another example of genetic variations affecting the risk for developing diseases based on genetics (Spencer K, 2012). Most genetic association studies have been conducted in European American individuals and the frequencies of various alleles varies among different ethnicities. Thus, predictive genetic testing varies among for complex diseases faces many challenges (Spencer K, 2012).
#7. Conclusion
Genetic variation in human populations is a very interesting topic. There are many possible explanations and theories to support the many phenotypic and metabolic differences observed among different populations. Even though all human beings share 99 percent of their DNA with each other the remaining 1 percent is responsible for the many differences observed in human populations. Changes in DNA at the nucleotide level such as single nucleotide polymorphism are often responsible for much of the genetic diversity (Hinds DA, 2005).
REFERENCES:
Lai CQ, “Adaptive Genetic Variation and Population Diversity”, Progress in Molecular Biology and Translational Science, Volume 108, 461-483, 2012
Barbujani G and Colonna V, “Human genome diversity: frequently asked questions”, Trends in Genetics, Volume 26, Number 7, 2010
Hinds DA, Stuve LL, Nilsen GB, Halperin E, Eskin E, Ballinger DG, Frazer KA, Cox DR, “Whole-Genome Patterns of Common DNA Variation in Three Human Populations”, Science, Volume 307, 1072-1079, 2005
Nature Education Website on Genetic variation, retrieved from website on Dec 7, 2014 http://www.nature.com/scitable/topicpage/the-genetic-variation-in-a-population-is-6526354
Essentials of Genetics, Unit 5.4 Book
A Brief History of Genetics: Defining Experiments in Genetics, Unit 9.4
Spencer KL, Glen K, Brown-Gentry K, Haines JL, Crawford DC, “Population Differences in Genetic Risk for Age-Related Macular Degeneration and Implications for Genetic Testing”, Archives of Ophthalmology, Volume 130, Number 1, 116-117, 2012
