Biological Anthropology/Unit 1: Evolutionary Theory/Modern Human Variation

= Modern Human Variation: The population genetics of people =

One of the most striking things about humans is the huge range of biological variation we have as a species.

In the previous section, we explored the mechanisms of change involved in the evolution of populations (mutation, migration/gene flow, genetic drift, and natural selection).

In this section, we'll look at how those affect human populations to create human diversity.

Polymorphisms
A polymorphism is when a gene has at least two different alleles (and each allele is in more than 1% of the population).

ABO blood type is a perfect example of a polymorphism: there are three alleles (A, B, and O) prevalent in the world.

Clines
When alleles vary geographically, they have a clinal distribution.

ABO blood type varies geographically, with the O allele prevalent in North America and southern Africa, B is more common in Europe and Asia, and A is found in high numbers in Asia and central Africa. Here are some maps showing the global clinal distribution of blood types.

Using polymorphisms: clines and migration

If you know the clinal distribution of a polymorphism, you can use the movement of the alleles as a proxy for the movement of people.

For example, the map below shows the distribution of a group of genes called Haplogroup F (found on the Y chromosome) throughout the Middle East, Europe, and Asia. Presumably the distribution of the genes follow the movement of peoples as they dispersed from India outward.



Natural selection and polymorphisms

While some clinal distributions are due to migration (gene flow), others have been shaped by natural selection.

Lactose tolerance

The original human condition (found in most southern European, Asian, and African populations) is an inability for adults to digest lactose, the primary sugar found in dairy products. This is known as lactose intolerance and it has a genetic basis. Lactose intolerance can be a particularly unpleasant condition for those who consume dairy products and cannot digest them. Symptoms include diarrhea and intestinal gas.

However, some populations of people have developed an ability to digest lactose.

This is how that came about:


 * An individual (or individuals) in the population was born with a genetic mutation that allowed them to digest lactose.
 * During the same time span, the population was domesticating dairy-producing animals, collecting milk, and consuming it.
 * Those who could digest the lactose in the milk (and therefore had a good food source without the unpleasant side effects) had more children than those who could not digest the lactose.
 * Those children inherited the gene for lactose tolerance from their parents and passed it down to their children, etc.

This is biocultural coevolution between the domestication of dairy-producing animals in those populations and an increase in the prevalence of the lactose tolerance gene. In other words, the biology of the people was affected by their cultural behaviors and vice versa.

Sickle cell anemia

Sickle cell anemiais a very serious genetic disease that affects the hemoglobin in the blood.

There are two forms of the allele: HbA and HbB (Hb for hemoglobin).


 * The HbA allele is for normal functioning hemoglobin, that which allows red blood cells to carry oxygen efficiently.
 * The HbB allele(AKA HbS) is for sickle cell anemia (the hemoglobin and red blood cells do not function normally)

So, as we learned before about alleles and genotypes, when there are two alleles, there are three genotypes.

In this case, the genotypes and phenotypes are:


 * HbA/HbA = this person has totally normal hemoglobin and their blood transports oxygen perfectly
 * HbA/HbB = this person has "the sickle cell trait" and therefore is a genetic carrier, but usually has little to no associated medical problems due to it
 * HbB/HbB = this person has sickle cell anemia

Sickle cell anemia has a fascinating biological interaction with the infectious disease malaria. It has been found that those with one HbB allele are less likely to get malaria.

Learn all about malaria here

Learn about the relationship between malaria and the sickle cell allele here

So, basically:


 * those with HbA/HbA easily get malaria
 * those with HbB/HbB get sickle cell anemia
 * those with HbA/HbB have protection from malaria and not have sickle cell anemia

This is called the heterozygote advantage, because the heterozygotes are better off than either one of the homozygotes. It's this advantage that keeps the HbB allele present in certain populations.

Important note: Remember that this HbB advantage only exists in malarial environments. It is environmentally specific. In an area without malaria, there is no advantage to having the HbB allele and therefore, the frequency of that allele is lower in populations in non-malarial environments.

A little fun with malaria: Malaria the video game

Adaptations and adaptability
An adaptation is any variation that can increase one's biological fitness in a specific environment. It can be biological or cultural and is intended to return the body to homeostasis (a stable, neutral state). Biological adaptations vary in their length of time, anywhere from a few seconds for a reflex to a lifetime for developmental acclimatization or genetics.What type of adaptation is activated often depends on the severity and duration of stressors in the environment. This ability to adapt our biology to our environment means that we have biological plasticity.

Reflex
This is an adaptation to a short, usually acute stressor.

Accommodation
This is an adaptation to a short term, less acute stressor. This is also known as habituation.

A good example of accomodation is when you ignore the "normal noises" (refrigerator running, furnace on, etc.) in your home.

Acclimatization
This adaptation can take days to weeks.

Famous examples of acclimatization include adaptations to heat and altitude. The flip side of this is an inability to acclimatize appropriately or fast enough to compensate for the stressor. That's when one suffers from heat sickness and altitude sickness.

Developmental acclimatization
This type of adaptation occurs during the individual's growth and development. It's also called ontological acclimatization. Note that these cannot take place once the individual is fully grown. There is usually a "magic time window" of when the acclimatization can occur. This adaptation can take months to years to acquire.

A famous example of this is those who have grown up at high altitude vs. those who have moved to high altitude as adults. Those who were born at high altitude tend to develop larger lung capacities than do those who were not born at high altitude, but moved there later in life.

Genetics
This type of adaptation is for the lifespan of the individual because it is part of their genetic makeup. Remember that genetic adaptations are environmentally specific. In other words, while a particular gene may be advantageous to have in one environment (AKA a genetic adaptation), it may be detrimental to have in another environment.

Skin color
Click on this linkto watch a fantastic video explaining the interplay of skin color, UV, and vitamin D.

Body size and shape
There are two ecological rules, known as Bergmann's rule and Allen's rule, that explain the variation in size and shape of bodies and extremities using latitude and temperature.

Bergmann's rule: Warm-blooded animals tend to have increasing body size with increasing latitude (toward the poles) and decreasing average temperatures.

Allen's rule: A corollary of Bergmann's rule that applies to appendages. Warm-blooded animals tend to have shorter limbs with increasing latitude and decreasing average temperatures

When organisms are more compact, they tend to conserve heat (due to a high mass:surface area ratio). When organisms are more linear, they tend to lose more heat (due to a low mass:surface area ratio).

This has been applied to humans. The idea is that populations toward the pole tend to be shorter and have shorter limbs than do people on the equator.

For example, the Inuit people of Canada (pictured above) tend to be shorter than the Maasai people of Kenya (pictured below):





Race
Technically, a race is a biologically classifiable subspecies. So, when we are asking, "Do human races exist?", what we're really asking is, "Are there biologically classifiable subspecies in humans?".

Here's the American Anthropological Association's statement on race

and the American Association of Physical Anthropologists statement on race

What are they saying?

Basically:


 * race is an arbitrary categorization, races are not biologically distinct groups (in other words, race is a cultural construct, not a biological one)
 * while groups of people who have lived together for a long time may have some alleles in common (for example, those that code for skin color or hair color), there is more genetic variation within races than there is between races
 * the concept of race has historically been a tool that some people use to subjugate others

Further explorethe concept of race, its history, and human variation