Human variability

Other important factors of environmental factors include climate and disease. Climate has effects on determining what kinds of human variation are more adaptable to survive without much restrictions and hardships. For example, people who live in a climate where there is a lot of exposure to sunlight have a darker color of skin tone. Evolution has caused production of folate (folic acid) from UV radiation, thus giving them darker skin tone with more melanin to make sure child development is smooth and successful. Conversely, people who live farther away from the equator have a lighter skin tone. This is due to a need for an increased exposure and absorbance of sunlight to make sure the body can produce enough vitamin D for survival.[2]

Blackfoot disease is a disease caused by environmental pollution and causes people to have black, charcoal-like skin in the lower limbs. This is caused by arsenic pollution in water and food source.[3] This is an example of how disease can affect human variation. Another disease that can affect human variation is syphilis, a sexual transmitted disease. Syphilis does not affect human variation until the middle stage of the disease. It then starts to grow rashes all over the body, affecting people's human variation.[4]

Phenotypic variation is a combination of one's genetics and their surrounding environment, with no interaction or mutual influence between the two. This means that a significant portion of human variability can be controlled by human behavior. Nutrition and diet play a substantial role in determining phenotype because they are arguably the most controllable forms of environmental factors that create epigenetic changes. This is because they can be changed or altered relatively easily as opposed to other environmental factors like location.

If people are reluctant to changing their diets, consuming harmful foods can have chronic negative effects on variability. One such instance of this occurs when eating certain chemicals through one's diet or consuming carcinogens, which can have adverse effects on individual phenotype. For example, Bisphenol A (BPA) is a known endocrine disruptor that mimics the hormone estradiol and can be found in various plastic products.[5] BPA seeps into food or drinks when the plastic containing it is heated up and begins to melt. When these contaminated substances are consumed, especially often and over long periods of time, one's risk of diabetes and cardiovascular disease increases. BPA also has the potential to alter "physiological weight control patterns."[6] Examples such as this demonstrate that preserving a healthy phenotype largely rests on nutritional decision-making skills.

The concept that nutrition and diet affect phenotype extends to what the mother eats during pregnancy, which can have drastic effects on the outcome of the phenotype of the child. A recent study by researchers at the MRC International Nutrition Group shows that "methylation machinery can be disrupted by nutrient deficiencies and that this can lead to disease" susceptibility in newborn babies. The reason for this is because methyl groups have the ability to silence certain genes. Increased deficiencies of various nutrients such as this have the potential to permanently change the epigenetics of the baby.[7]

Genetic variation in humans may mean any variance in phenotype which results from heritable allele expression, mutations, and epigenetic changes. While human phenotypes may seem diverse, individuals actually differ by only 1 in every 1,000 base pairs and is primarily the result of inherited genetic differences.[8] Pure consideration of alleles is often referred to as Mendelian Genetics, or more properly Classical Genetics, and involves the assessment of whether a given trait is dominant or recessive and thus, at what rates it will be inherited.[9]  The color of one's eyes was long believed to occur with a pattern of brown-eye dominance, with blue eyes being a recessive characteristic resulting from a past mutation. However, it is now understood that eye color is controlled by various genes, and thus, may not follow as distinct a pattern as previously believed. The trait is still the result of variance in genetic sequence between individuals as a result of inheritance from their parents. Common traits which may be linked to genetic patterns are earlobe attachment, hair color, and hair growth patterns.[10]

In terms of evolution, genetic mutations are the origins of differences in alleles between individuals. However, mutations may also occur within a person's life-time and be passed down from parent to offspring. In some cases, mutations may result in genetic diseases, such as Cystic Fibrosis, which is the result of a mutation to the CFTR gene that is recessively inherited from both parents.[11] In other cases, mutations may be harmless or phenotypically unnoticeable. We are able to treat biological traits as manifestations of either a single loci or multiple loci, labeling said biological traits as either monogenic or polygenic, respectively.[12] Concerning polygenic traits it may be essential to be mindful of inter-genetic interactions or epistasis. Although epistasis is a significant genetic source of biological variation, it is only additive interactions that are heritable as other epistatic interactions involve recondite inter-genetic relationships. Epistatic interactions in of themselves vary further with their dependency on the results of the mechanisms of recombination and crossing over.

The ability of genes to be expressed may also be a source of variation between individuals and result in changes to phenotype. This may be the result of epigenetics, which are founded upon an organism's phenotypic plasticity, with such a plasticity even being heritable.[13][14] Epigenetics may result from methylation of gene sequences leading to the blocking of expression or changes to histone protein structuring as a result of environmental or biological cues. Such alterations influence how genetic material is handled by the cell and to what extent certain DNA sections are expressed and compose the epigenome.[13] The division between what can be considered as a genetic source of biological variation and not becomes immensely arbitrary as we approach aspects of biological variation such as epigenetics. Indeed, gene specific gene expression and inheritance may be reliant on environmental  influences.

Archaeological findings such as those that indicate that the Middle Stone Age and the Acheulean[15][16] – identified as a specific 'cultural phases' of humanity with a number of characteristics – lasted substantially longer in some places or 'ended' at times over 100,000 years apart, highlight a significant spatiotemporal cultural variability in and complexity of the sociocultural history and evolution of humanity.[17][18] In some cases cultural factors may be intertwined with genetic and environmental factors.

Measurement of human variation can fall under the purview of several scholarly disciplines, many of which lie at the intersection of biology and statistics. The methods of biostatistics, the application of statistical methods to the analysis of biological data, and bioinformatics, the application of information technologies to the analysis of biological data, are utilized by researchers in these fields to uncover significant patterns of variability. Some fields of scientific research include the following:

Demography is a branch of statistics and sociology concerned with the statistical study of populations, especially humans. A demographic analysis can measure various metrics of a population, most commonly metrics of size and growth, diversity in culture, ethnicity, language, religious belief, political belief, etc. Biodemography is a subfield which specifically integrates biological understanding into demographics analysis.[19]

In the social sciences, social research is conducted and collected data is analyzed under statistical methods. The methodologies of this research can be divided into qualitative and quantitative designs. Some example subdisciplines include:

• Anthropology, the study of human societies.[20] Comparative research in subfields of anthropology may yield results on human variation with respect to the subfield's topic of interest.
• Psychology, the study of behavior from a mental perspective. Does a lot of experiments and analysis grouped into quantitative or qualitative research methods.
• Sociology, the study of behavior from a social perspective. Sociological research can be conducted in either quantitative or qualitative formats, depending on the nature of data collected and the subfield of sociology under which the research falls. Analysis of this data is subject to quantitative or qualitative methods.[21] Computational sociology is also a method of producing useful data for studies of social behavior.[22]

Anthropometry is the study of the measurements of different parts of the human body.[23] Common measurements include height, weight, organ size (brain, stomach, penis, vagina), and other bodily metrics such as waist–hip ratio. Each measurement can vary significantly between populations; for instance, the average height of males of European descent is 178 cm ± 7 cm and of females of European descent is 165 cm ± 7 cm.[24] Meanwhile, average height of Nilotic males in Dinka is 181.3 cm.

Applications of anthropometry include ergonomics, biometrics, and forensics. Knowing the distribution of body measurements enable designers to build better tools for workers. Anthropometry is also used when designing safety equipment such as seat belts.[23] In biometrics, measurements of fingerprints and iris patterns can be used for secure identification purposes.[25]

Human genomics and population genetics are the study of the human genome and variome, respectively. Studies in these areas may concern the patterns and trends in human DNA. The Human Genome Project and The Human Variome Project are examples of large scale studies of the entire human population to collect data which can be analyzed to understand genomic and genetic variation in individuals, respectively.

• The Human Genome Project is the largest scientific project in the history of biology. At a cost of $3.8 billion in funding and over a period of 13 years from 1990 to 2003, the project processed through DNA sequencing the approximately 3 billion base pairs and catalogued the 20,000 to 25,000 genes in human DNA. The project made the data available to all scientific researchers and developed analytical tools for processing this information.[26] A particular finding regarding human variability due to difference in DNA made possible by the Human Genome Project is that any two individuals share 99.9% of their nucleotide sequences.[27]
• The Human Variome Project is a similar undertaking with the goal of identification and categorization of the set of human genetic variation, specifically variations which are medically pertinent. This project will also provide a data repository for further research and analysis of disease. The Human Variome Project was launched in 2006 and is being run by an international community of researchers and representatives, including collaborators from the World Health Organization and the United Nations Educational, Scientific, and Cultural Organization.[28]

Genetic drift is one method by which variability occurs in populations.[29] Unlike natural selection, genetic drift occurs when alleles decrease randomly over time and not as a result of selection bias.[30] Over a long history, this can cause significant shifts in the underlying genetic distribution of a population. We can model genetic drift with the Wright-Fisher model. In a population of N with 2N genes, there are two alleles with frequencies p and q. If the previous generation had an allele with frequency p, then the probability that the next generation has k of that allele is:[31][32]

${\displaystyle {2N \choose k}p^{k}q^{2N-k}}$

Over time, one allele will be fixed when the frequency of that allele reaches 1 and the frequency of the other allele reaches 0. The probability that any allele is fixed is proportional to the frequency of that allele. For two alleles with frequencies p and q, the probability that p will be fixed is p. The expected number of generations for an allele with frequency p to be fixed is:[33]

${\displaystyle {\bar {T}}_{\text{fixed}}={\frac {-4N_{e}(1-p)\ln(1-p)}{p}}}$

Where N_e is the effective population size.[34]

Single-nucleotide polymorphism or SNPs are variations of a single nucleotide. SNPs can occur in coding or non-coding regions of genes and on average occur once every 300 nucleotides.[35] SNPs in coding regions can cause synonymous, missense, and nonsense mutations. SNPs have shown to be correlated with drug responses and risk of diseases such as sickle-cell anemia, Alzheimer's disease, cystic fibrosis, and more.[36]

DNA profiling, whereby a DNA fingerprint is constructed by extracting a DNA sample from body tissue or fluid. Then, it is segmented using restriction enzymes and each segment marked with probes then exposed on X-ray film. The segments form patterns of black bars;the DNA fingerprint.[37] DNA Fingerprints are used in conjunction with other methods in order to individuals information in Federal programs such as CODIS (Combined DNA Index System for Missing Persons) in order to help identify individuals [38]

Mitochondrial DNA, which is only passed from mother to child. The first human population studies based on mitochondrial DNA were performed by restriction enzyme analyses (RFLPs) and revealed differences between the four ethnic groups (Caucasian, Amerindian, African, and Asian). Differences in mtDNA patterns have also been shown in communities with a different geographic origin within the same ethnic group[39]

Alloenzymic variation, a source of variation that identifies protein variants of the same gene due to amino acid substitutions in proteins. After grinding tissue to release the cytoplasm, wicks are used to absorb the resulting extract and placed in a slit cut into a starch gel. A low current is run across the gel resulting in a positive and negative ends. Proteins are then separated by charge and size, with the smaller and more highly charged molecules moving more quickly across the gel. This techniques does underestimate true genetic variability as there may be an amino acid substitution but if the amino acid is not charged differently than the original no difference in migration will appear it is estimated that approximately 1/3 of the true genetic variation is not expressed by this technique.

Structural variation, which can include insertions, deletions, duplications, and mutations in DNA. Within the human population, about 13% of the human genome is defined as structurally variant.

Phenotypic variation, which accounts for both genetic and epigenetic factors that affect what characteristics are shown. For applications such as organ donations and matching, phenotypic variation of blood type, tissue type, and organ size are considered.