Focused Assignment: Using sentences and paragraphs, identify two 2) important po

Question

Focused Assignment:

Using sentences and paragraphs, identify two 2) important points made about Human Variation by Houk (Chapter 4 Section 2) and then go on to compare and/or contrast them with any information from the Understanding Race website (http://www.understandingrace.org/ (Links to an external site.)Links to an external site.).

Grading as follows:

5 points for describing 2 important points about human variation from Houk Chapter 4, Section 2; 10 points for describing comparable or contrasting information from Understanding Race website (exact Web page address of website information required); 5 points for quality of written response. (((( LINK FOR QUESTION http://www.understandingrace.org/humvar/index.html)

Chapter 4 - Section 2

Human Adaptation and Variation

     One of the great tragedies of the human experience is the misinterpretation or misunderstanding of human phenotypic diversity. The issue is succinctly stated in the American Anthropological Association’s “Official Statement on ‘Race’” where it is written that “Biophysical diversity has no inherent social meaning except what we humans confer upon it” (1997:27) and history has shown that we have done a considerable amount of “conferring.” Virtually all cultural ideologies concerning “race” and phenotypic diversity are wrong as various myths and fantasies have been created a priori (i.e., based on “before the fact” assumption rather than on the a posteriori or “after the fact” gathering of empirical data and the subsequent analysis of those data) to support and legitimize presumptive notions of the “other.” To return to a common theme found throughout this textbook, nature is one thing, culture quite another, and “never the twain shall meet.” In this case, culture insists on seeing races where, in fact, there are none since distinct biological races among humans simply do not exist. (This is addressed more fully in Chapter VIII.) The cold, hard truth is that all modern humans belong to the genus and species, Homo sapiens, and we are all approximately 99.9 percent genetically similar. The phenotypic differences we do manifest can be attributed largely and primarily to adaptation to climate and environment, both of which vary considerably from place to place around the globe.

Skin Color

     Of all the many, many phenotypic traits that might be used to categorize and classify humans, skin color has easily been the most popular due primarily, no doubt, to its saliency (it is readily visible) and its variability (ranging from dark to light). In fact, the earliest attempts to construct a typology of humans virtually always relied on this one variable and the simplest schemes divided humans into four “races” based on four colors, black, red, yellow, and white. Of course, once one then attaches a behavioral significance to these colors, racism and the potentially harmful by-products of this prejudicial ideology follows. We will leave the topics of “race” and racism for later but what of variable skin color in humans? It does after all exist. It is natural, it is biological and, thus, needs explaining. Of course, any explanation will necessarily consider natural, biological phenomenon as primary factors since, as we have seen, cultural interpretations of natural phenomena are generally inaccurate.

     We will begin our discussion, oddly enough, with lice. Given the fact that we once shared a common ancestor with chimpanzees, it is thought that the earliest hominins possessed lots of dark body hair with relatively light skin as chimpanzees do today (Jablonski and Chaplin 2000). Thus, since thick body hair serves as a protection for the skin against the potentially harmful effects of the sun, before we can address the evolution of skin color we need to figure out when hominins became relatively hairless as modern humans are today. As it turns out, research conducted on the two forms of lice humans carry on their bodies, head lice (Pediculus humanus) and pubic lice (Pthirus pubis) has given us a pretty good idea when humans became hairless.

     The head louse has long made its home on the bodies of hominins and, in fact, genetic studies (using the molecular clock technique) have shown that the human head louse and the chimpanzee louse diverged approximately 6 mya (Reed et al., 2007) which is yet another confirmation of the 7-5 mya convergence date for the lines leading to modern humans and chimpanzees. More relevant for our discussion, however, is the pubic louse which, as it turns out, belongs to a different genus than the human head louse but, in fact, is closely related to the gorilla louse (Pthirus gorillae). The basic difference in the genomes of these two species of lice (human pubic and gorilla) points to a divergence date of around 3.5 mya (Reed et al., 2007). Apparently, as hominins were losing their body hair, the head louse gravitated to that dense area of hair atop the head leaving the rest of the body uninhabited by lice. It is thought that some sort of contact with gorillas (we won’t even go there) resulted in the migration of the parasitic gorilla louse to humans (parasites always look for opportunities to populate new hosts). Thus, it appears as though hominins began to lose their body hair around 3.5 mya. Actually, the loss of body hair and the appearance of exposed skin that produces perspiration greatly enhanced what is generally referred to as “persistence hunting” seen in the !Kung Bushmen, for example, that allows them to run great distances in high temperatures, which gives them an advantage over the animals they are pursuing. So, the loss of body hair is an adaptation that would confer an evolutionary advantage on those individuals that manifest that particular trait.

     So, as of around 3.5 mya we have relatively light-skinned hominins that are losing their body hair and, thus, are increasingly exposed to the intense sunlight of equatorial latitudes. The primary problem with exposure to high degrees of solar radiation is ultraviolet (UV) rays. UV rays can penetrate the epidermis (outer layer of skin) and adversely affect the DNA in our skin cells which can lead to mutations that cause skin cancer. Jurmain et al. (2010) note that this can impact human evolution since some albinos (individuals whose skin possesses no melanin or coloring pigments) in tropical areas of Africa have contracted skin cancer by age twenty. Thus, there is a potential deleterious impact on Darwinian fitness. More importantly, however, is the damage that UV radiation causes to the B vitamin folate. Jurmain et al. note that “insufficient levels of folate are associated with numerous fetal developmental disorders” (2010:453) which, again, would adversely affect reproductive fitness.

     It seems clear that once hominins lost their body hair, the relatively light skin possessed by the earliest hominins gradually evolved towards a darker color as the incidence of UV rays served as a selective factor whereby lighter skin would be selected against. This is due primarily to the melanin in our skin (which gives our skin its pigment or color) that serves to absorb UV rays. Simple logic tells us that the greater the intensity of sunlight, the darker the skin should be and that is precisely what is seen globally today. The darkest skin tones (we are actually talking about measures of reflectance here and not color per se) of indigenous peoples tend to be found in equatorial latitudes and the lighter skin tones are found as we move towards the poles.

     Humans can actually get sufficient amounts of vitamin D from dietary sources (fish oils and egg yolks for example) but this is not an option for those who don’t have access to these foods. Humans can also, however, synthesize their own vitamin D but this requires exposure to a sufficient amount of sunlight. In those areas where the incidence of sunlight is relatively weak (in northerly latitudes for example), having very dark skin would be disadvantageous since it would inhibit vitamin D production resulting in all the problems noted here. Thus, it appears that there was a strong selective pressure for lighter skin in northerly climes. Both darker and lighter skin tones, then, evolved as a result of natural selection favoring those traits that were conducive to reproductive fitness or, conversely, disfavoring those traits that inhibited reproductive fitness.

Body Shapes and Sizes
and Other Variable Phenotypic Features

     The British zoologist Carl Bergmann observed that those members of a species that were found in consistently colder areas were generally larger than their counterparts in warmer areas who tended to be smaller and more linear in body form. Today this principle is often stated in terms of a law (Bergmann’s Law), namely, body size of mammals in colder climes tends to be larger on the average than that of mammals living in more temperate climes. Why this is so can be understood by considering the fact that cold temperatures put a great deal of stress on mammals as mammals self-regulate their own body temperature which must be remain very close to an optimum value to maintain normal cellular functions. When faced with cold, humans, for example, must either generate heat or increase heat retention; the latter is the most biologically prudent, however, since it requires a much smaller investment of energy. It is this heat retention strategy that helps us understand the law stated above since the surface area of the human body decreases proportionately as body mass increases. Since heat in the human body dissipates at the surface (skin), less surface area means less dissipation.

     Hair texture is another variable phenotypic trait that ranges from the sparse, curly “spongy” hair found in sub-Saharan West Africa to the very strait and very dense hair found in Europe and Asia. It is thought that the differential densities of the two hair types could possibly be related to, once again, heat dissipation (in the case of the sparse, curly hair) and heat retention (in the case of the dense, straight hair). There is another interesting hypothesis, however, that considers the possible effect of UV rays. According to Iyengar (1998), straight hair tends to absorb more UV rays than curly hair. The argument here would be essentially the same as that used in regard to skin color: those in more northerly climes need to take advantage of what little solar radiation they may be exposed to so that they can synthesize sufficient amounts of vitamin D. Straight hair, according to Iyengar, would facilitate this process.

     There are many other phenotypic traits we might consider but a general pattern emerges as we consider each one: many of these traits are the result of an environmental adaptation in which certain traits were naturally selected for or against based on their potential to increase reproductive fitness. In some cases, however, for example the epicanthic fold, we could be dealing with mutations that, for whatever reason, made their carriers more desirable sexually. Sexual selection, then, has also had a significant influence on the phenotypic variability of humans.

Sexual Selection

     Selection (in the evolutionary sense) can either be natural or non-natural. In the case of natural selection, it is nature itself that is doing the “selecting” whereby those individuals that are reproductively the most successful are passing on more of their DNA to the next generation than their species counterparts. Of course, this process is not volitional or sentient, i.e., it occurs naturally without the involvement of any conscious entity whatsoever. On the other hand, however, non-natural selection does involve the intentional manipulation of phenotypic traits to bring about some desired end.

     Artificial selection, for example, has been practiced for centuries and Charles Darwin used the concept to explain natural selection. Examples of artificial selection are all around us from the cattle that have been purposefully bred for specific purposes, to the many breeds of dogs, to the huge, plump ears of corn that look nothing like their natural ancestors.

     The concept is really quite simple. Think back to Mendel’s pea plants. Let us say that one desired to produce only plants with purple flowers. In this case, the allele for purple is the dominant allele but there are, of course, recessive alleles for white that are hanging around in the gene pool. In this simple case of artificial selection, the desired results can be obtained quite easily by not allowing the white flowers to breed each generation. (This would be akin to forcing the process of genetic drift so that the purple allele becomes “fixed” at 100 percent in a very short time.) Since white flowers are no longer contributing to the gene pool, the parental generation consists of only purple flowers that are either homozygous dominant or heterozygous for this trait. As one continues to select against the white flowers, the proportion of white alleles decreases until the heterozygous genotype disappears completely; at this point, the white allele has been fixed at zero.

     These basic principles also apply in regard to another type of non-natural selection known as sexual selection. In this case, individuals are choosing to mate with partners that possess a certain constellation of phenotypic traits, which gives the possessor of those traits an incremental advantage over potential competitors that do not possess those traits. These traits, then, are being selected for since it is the individuals who possess these traits that will be contributing a disproportionate amount of DNA to the gene pool of the next generation.

     Whereas environmental adaptation can in fact explain most of what make up the phenotypic variability of humans cross-culturally, sexual selection does play at least a minor role. So, for example, Middle Eastern women tend to be attracted to men that are relatively hairy as opposed to east Asian women who do not choose hirsute mates. East Asian peoples also tend to select for dark hair as well. Thus, sexual selection acts to fine-tune, as it were, the phenotypic variability produced by tens of thousands of years of human evolution.

Explanation / Answer

two important variations are-

1- hairs : our ancesters got hair all over the body but as the time passes we are loosing hairs, hairs are divided in two parts head louse and pubic louse it is considered that head louse is because of chepangie downline but there is some connection too with the gorrila family because our pubic hairs are related to some how with them. this is a variation which is seen in human homo got the hairy body but with time this all changes and thih happens with the effect of diffrent climates in diffrent places some of us got heavy hairs in body and some of us got none.

2- skin complaction- skin complaction is a variation which is talked maximum times, but this variation took because of the diffrent climate and our skin changes its colour according to the need to resist the climate as the african countries are in a tropical place and these places are so hot too and moist because of which they are black and in northen region people are whicte because of cold in those areas.

Ans 2- As we are talking about race I think it should be talked frequently because seprating people because of colour is not humanity I saw some of the information of killing understanding race, but raceism is made by human it self. we should stop this becasue skin colour is not important than a life.

as conclusion we should know that we all are from same ancestor that means there is no diffrence between us and we should follow humanity this racism and conflicts are made by human beings to make others smaller to mae place in the society we should overcome from this,

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