Search This Blog

Monday, April 7, 2014

race and physiology

Foundation of Human Culture - 
Human Morphological Variation

Human variation stems from a number of sources which can be crudely classed as either genetic or environmental embedded in the process of evolution. Over the past weeks we have done a safari tour of the foundations of evolution, which is precisely the long term interaction of these two.

For the remainder of the course we will be focused mainly on what is called population genetics. Population genetics is concerned with variation within a single species or groups composed of the same species (in our case Homo sapiens ). Population genetics framed in an evolutionary framework is called microevolution , as opposed to macroevolution, which is concerned with evolutionary processes which lead to the formation of different species. We will be framing this within examinations of human adaption, variation due to adapting to local environmental circumstances.

This week we will look at basic features of human variability, and try to examine how these are framed in genetic and adaptive terms.

I. Body size. Heritability estimates for most body size measurements imply that
about 80% of the variability in body size is due to genetic factors (.8 heritiability), and about 20% is due to environmental factors. 
A. Sexual dimorphism in body size is typical of humans, with females being
90-95% the size of males in almost all populations. 
B. The secular trend. It seems that most of the world's populations are
becoming larger through time. This is part of what is called the secular trend. 
  1. Few people think that this is actually a genetic evolutionary trend, and place the cause somewhere in the environment, mostly to diet.
  2. Some biologists have suggested that the secular trend is, in part, the result of selection due to mate choice (called sexual selection).
  3. However, in 1900 I would have served nicely as an 'average' height male in either the US or UK. Now most males are grotesquely taller. In Japan in 1949 the average height of males was not quite 5'4''. Now the average is equal to European males (5'10''). In Pakistan in 1972 the average height of males was just over 5'4'', and is now (for males 18-35) just less than the average for European males. Selection can simply not operate this quickly.
  4. Changes in public sanitation, particularly water supplies may also have an important impact. Most human populations maintain a fairly large disease and parasite load, which compete for the energy produced by the human body. Many of these are transmitted by human waste products which contaminate water, and through water (or lack of it) food.

C. Advantages of large body size. There are some advantages to large body
  1. Big people are stronger.
  2. Bigger people are better predators - a lion can kill a wider variety of prey than a house cat.
  3. Bergman's rule. Large bodies are beneficial in colder climates according to Bergman's rule:
    The larger the animal the better it is at retaining heat. That
    is why during the glacial times many lineages of animals developed giant forms. Humans follow this rule in a broad way. People living near the poles tend to be larger on the average than those living near the equator, but there are a lot of exceptions. Certainly this is true in the new world where the native americans living in the amazon basin are among the shortest of all native americans.
  4. Larger people are generally faster runners. The fact that their stride length is longer and that they can apply more force with each stride due to larger muscles gives this advantage.

E. Advantages of small body size.

  1. The most important of all selective pressures on body size is that small people require less food and can better survive when food is limited. Famines kill people in size order from largest first to smallest last.
  2. Smaller people are generally quicker and more agile. This is due to the principle of inertia from physics. A larger body takes more force to get moving and more force to change direction than a smaller body does.

F. Distribution of body size.

  1. Europeans have the largest average body size. It is in Europe where Bergman's rule most clearly applies. The largest Europeans are from the far north, and the farther south you go in europe, the smaller the people.
  2. Africans include both the worlds tallest (if not exactly the largest) people and the world's smallest people. The Nuer, Masai, Watusi, and similar peoples of East Central Africa are the world's tallest and among the worlds largest. The Pygmies of West Central Africa and the Khoisan of Southern Africa are among smallest.
  3. Asians and Native Americans usually fall in the middle ranges. Only a few populations could be considered large, maybe the Samoans are one. Many populations could be described as small.

II. Body build. Most of the variation in body build in humans can be reduced
to linear build vs lateral build. Let's contrast an extreme linear build with an extreme lateral build. 
A. The extreme linear stereotype would be found in the previously
mentioned tall peoples of East Central Africa. These people are very tall and slender. The chests, shoulders, and hips are very narrow - the narrowest in the world for their height. The limbs are extremely long, especially the legs.

B. The extreme lateral stereotype would be found in some Asian and
Native Americans. Eskimos, Japanese, Samoans, Apache, and many South American Indians exhibit lateral build. A few Caucasoid groups also approach lateral build, especially the peoples of northern Europe. Laterally built people tend to have long and broad trunks, with wider chests, shoulders and hips. The widest hips of all can be found in Europeans. The limb bones tend to be short and the legs make less of a contribution to overall height.

C. Allen's rule. One primary selective force acting on body build
is Allen's rule: 
Animals living in colder climates should have shorter appendages
and be more spherical than those living in warmer climates. This says that laterally built people should be found in colder climates and linearly built people in warm climates. This is true for humans on the average. The traditional comparison is between the Inuit and the Masai. The Inuit of the far north tend to be stocky with short arms and legs. The Masai of east africa tend to be very tall and slender, with long arms and legs.

D. These contrasting body builds have definite advantages for
certain tasks. The linear builds seem to have a definite advantage in overall health, especially in that they experience much less heart disease and diabetes than laterally built people. Linear builds have the advantage in running speed - they make great sprinters. The mechanics of long slender legs, as explained in terms of levers and forces, are well designed for fast running. A non-human example is the contrast between cows and antelopes. Cows and antelopes are closely related, but cows are extreme laterals in build and antelopes are more linear. Obviously, antelopes can run faster than cows.

Narrow hips are another advantage in fast running. This is the reason
that most men can run faster than most women. Women have broader hips as an adaptation for childbearing. The muscular arrangements that accompany narrow hips are much more efficient in moving the legs rapidly and powerfully. Since most Europeans have very broad hips you would expect them to be among the world's slowest runners, and this is pretty much what is observed.

E. Lateral builds have an advantage in endurance running, and any
task that requires endurance. This is because the larger rib cage allows more room for a larger heart and lungs. The endurance feats that have been recorded for some Indians are legendary.

III. Hair. Several things about hair are variable.

A. Color. In general, dark hair goes with dark skin and light hair
goes with light skin. You can have dark hair with light skin, but it is rare to have light hair with dark skin. People can have different colored hair on different regions of their body. Beard color and pubic hair color are often lighter than head hair color - rarely darker. Eyebrow color can be different as well (usually darker). People's hair color changes with age too - most people's hair becomes darker as they grow from childhood into adulthood, then gets gray as they age.

Blond hair has little melanin and black hair (actually very dark
brown) has a lot. In contrast to skin melanin, which is always brown, hair melanin can be either brown or red. There is a wide range of variation in the amount of red melanin vs brown melanin in the hair from a slight reddish tint to strawberry red. In general, brown melanin is stronger in color than red melanin and can mask it.

The distribution of hair color is almost entirely a European phenomenon,
with most of the rest of the world's people having dark brown hair. The farther north you go in Europe the more likely the hair is to be blond. The farther west you go, the more likely the hair is to be red.

B. Hair curvature. The degree of curvature of the hair is broken
down into 3 categories: straight, wavy, and curly. The shape of the hair follicle is what determines the curvature of the hair produced by it. Round follicles produce straight hair, oval follicles produce wavy hair, and disk-shaped follicles produce curly hair.

Straight hair is found throughout most of the world, including the
Americas, Asia, and parts of Europe. Europe has the most variation in hair curvature, ranging from straight to very wavy. The Middle East and North Africa have a lot of wavy and a few curly heads. Subsaharan Africans range from curly to very curly. The most curly hair is found in the Khoisans whose hair is often so tightly curled that it is called peppercorn hair because it looks like pepper corns placed on their heads.

The predominance of straight hair on a world-wide basis suggests
that it is the original type of hair that the first humans had. If so, then we need to explain the development of curly hair in Africa. One explanation for curly hair has been proposed based on the fact that the Khoisan inhabit a very hot and dry environment. In this type of environment the body needs to cool itself by sweating, but when you sweat you lose water and run the risk of becoming dehydrated. It has been observed that the very curly peppercorn hair of the Khoisan does a good job of trapping the sweat on their heads and holding it in place as it evaporates and cools the body.

C. Hair length. Many people are surprised that there is genetic
variation in hair length. As in all mammals, each of us has a certain hair length beyond which the hair simply won't grow. Hair length is longest in people with round follicles, because round follicles seem to grip the hair better. So, people with straight hair have the potential to grow it longer. The shortest hair is associated with flat follicles.

Hair length is sexually dimorphic, not just because of cultural
rules - females really can grow longer hair than males in most cases. This has been measured for Europeans, where in males the maximum length for wavy hair is about shoulder length and the maximum for straight hair is about midback length. For female Europeans wavy hair can usually reach the waist, and straight hair can reach the buttocks or longer.

Nobody has proposed any selective advantage for either long or short
hair in terms of the environment. But, given the general worldwide cultural preference for longer hair in females, it is possible that there is some sexual selection going on.

IV. Eyes. Eye color and form presents a striking range of variation.

A. Color. The colored part of the eye is the iris, which surrounds
the pupil of the eye and contains muscles which dilate and contract the pupil. The iris has several layers, two of which contain melanin. All eye melanin is brown. 
The variation in eye color is caused by the nature of the pigmentation
of the external layer of the iris. The iris may or may not be pigmented. If pigmented, the distribution of pigment may not be even. There can be some contribution of other pigments as well.

If melanin is present in the external layer of the iris then the
eye will be brown. If melanin is lacking, the iris will be colorless but perceived as blue for the same reason that the sky seems blue and a lake seems blue. If melanin is present but unevenly distributed the eye is perceived as a brownish green and is called olive or hazel.

True green eyes are extremely rare, and only a few cases have been
documented. True green eye color is caused by presence of pigment in the external layer of the iris, carotene. The yellowish carotene combined with the blue of an unpigmented iris gives a green tint.

Most people in the world have brown eyes (uniformly distributed
pigmentation). The only place to find blue, hazel, or green eyes as anything other than an occasional mutation is in Europe. One proposed advantage of non-brown eyes that has been put forward is that it might be advantageous for hunters against a snowy or light-colored background.

B. Eyelid folding. The inner corner of the eyelid has a characteristic
fold in many Asian peoples, called an epicanthic fold. The fold makes the inner corner of the lid curve downward. Most Asians have this characteristic to some extent, as do many Europeans. The epicanthic fold does seem to offer some protection against snow blindness, caused by sunlight reflecting off snow.

V. Ears. Ears are extremely variable in size, size of the ear lobe, protrusion,
and whether the ear lobe is free or attached. 
The African ear stereotype is a relatively small, non-protruding
ears with small free lobes. The american indian stereotype is the other extreme in average size and protrusion, and asians stereotypically have the highest frequency of attached ear lobes. Europeans are the most variable and have ears that span the entire range of human variation.

No advantage or disadvantage to ear characteristics has been seriously
proposed, but the physics of sound tells us that large protruding ears should give an advantage in the ability to locate sounds.

VI Lips. Having distinct lips is a uniquely human phenomenon. When the
mouths of other mammals (including other primates) are closed all you see is the mouth slit with hairy skin going right down to the opening. All humans do have lips, which vary primarily in the extent to which they are rolled up to expose the pink membranous portion. This phenomenon is called lip eversion. Everted lips seem to have a slight ability to help cool the body because capillaries run very close to the surface of the lips, and the slight moistness of the lips helps in cooling by evaporation. The most everted lips are found on the faces of some Africans and the least everted lips on the faces of some Europeans. However, lip eversion is extremely variable by local population.
VII. The ABO blood group system. The antigens for the ABO system are a set of glycoproteins. Directly attached to the red cell membrane is a protein. At a certain section of the protein is attached a type of 5-carbon sugar, fucose. This fucose sugar molecules is referred to as the H antigen, and reacts with an antiserum called anti-H. The production of antigen H is controlled by a separate locus from that of the ABO blood group, but antigen H is intimately associated with the ABO system.

A. Most people who have an allele for blood type O have antigen H, and should more correctly be classified as blood type H. Therefore, the most correct way to refer to this blood group system is the ABH system. There are a few individuals, who lack the H antigen and just have a naked protein chain hanging off their red cells. This is also called the Bombay blood type, which I mentioned earlier in the course. The frequency of the Bombay allele is somewhere around .0066, so homozygotes are very rare. This allele is usually designated as h, and the normal H allele is dominant to it.

B. The allele for blood group A causes another sugar to be attached to the antigen H, fucose, sugar molecule. This attached sugar is N-acetylgalactosamine (NAG), and is the A antigen. The allele for blood group B causes a molecule of simple galactose sugar to be attached to the fucose molecule. This is the B antigen. The O allele causes the H antigen to remain unmodified.

C. People who are AA homozygotes or AO heterozygotes have mostly the A antigen, with usually a little free H antigen. Actually there are 4 different A alleles, the differences between which are poorly understood, but which seem to differ primarily in the amount of H antigen that gets converted to A antigen. So, they cause differences in the strengths of the antigen-antiserum reactions. However, specific antisera can be made to at least some of the 4 different blood group a subtypes, so there must be some differences in the actual antigen as well.

Most non-African populations have only A1 and A2 alleles, but Africans can also have Aint and Abantu alleles.

D. People who are BB or BO have mostly the B antigen, with a little free H antigen. There is no major variability in the B blood type.

E. People who are AB heterozygotes have both the A and B antigens. Each cell will have approximately half of its H antigens modified into A antigens and about half modified into B antigens. This illustrates the phenomenon of codominance. Neither A nor B is dominant to the other, so the products of both alleles can exist in a heterozygote. Both A and B are dominant to O.

F. An individual does not normally make antibodies to any antigen which he or she personally possesses. The galactose and fucose sugars are common enough in nature, particularly in disease carrying organisms, so people make antibodies to these sugars if they are not part of their personal antigen system. Therefore, everyone who is not blood group A will make anti-A antibodies. Everyone who is not blood group B will make anti-B antibodies. Almost nobody makes anti-H antibodies, but you can extract an anti-H antiserum from the seeds of the common (in England) gorse plant. Let's summarize the antigens and antibodies produced by each genotype.

The main importance of the ABH blood group system is in blood matching for transfusions. If the donor and recipient are not matched in terms of their ABH blood types then the antibodies in the recipient's plasma may cause an agglutination reaction of the red cells from the donor. This is not fun for the patient. Note, however, that people with blood type AB have no antibodies in their plasma, so they can actually receive blood from anybody. For this reason they are referred to as universal recipients.

Usually, the small amount of antibodies introduced with the plasma from the donor's blood doesn't cause a severe enough reaction in the recipient to cause any problems, although it is still preferable to match blood types exactly.

Note also that blood type O individuals have no A or B antigen. This means that nobody's antibodies can agglutinate their cells. For this reason type O people are often called universal donors, although you may occasionally have problems with the antibodies in the plasma of type O blood.

 |                | blood |antigens|                        |
 |    genotypes   | type  |present |   antibodies present   |
 |AOhh, AAhh, BOhh|Bombay | none   | anti-H, anti-A, anti-B |
 |BBhh, ABhh      |       |        |                        |
 | OOHH, OOHh     | O (H) |   H    | anti-A, anti-B         |
 | AOHH, AAHH,    |   A   | A, H   | anti-B                 |
 | AOHh, AAHh     |       |        |                        |
 | BOHH, BBHH,    |   B   | B, H   | anti-A                 |
 | BOHh, BBHh     |       |        |                        |
 | ABHH, ABHh     |  AB   | A, B, H| none                   |

Many other serum proteins, red cell proteins, blood groups, and antigen systems exist. we have just scratched the surface of the most well known ones. As we will see in a later lecture, many of these are very localized to certain racial or tribal groups.

No comments:

Post a Comment

Blog Archive