5: Human Variation
and Population Ecology.
Humans are highly
variable in body proportions, coloration and body chemistry.
This can lead to
different susceptibility to disease and other forces of natural selection.
Much of human
variation is due to geography.
People of Japan differ from those in eastern Africa who differ from
those of northern Europe.
Evolution has changed populations from different areas of the world
to help them fit into their environment.
variation occurs within and between human populations.
variation starts with different alleles at the same gene locus.
be modified by the environment.
of these allele variations follow patterns we have previously described.
Homozygous, heterozygous, dominant, recessive, autosomal, sex-linked and the combinations.
when multiple genes play a role in determining a phenotype we often see
small variations among many individuals.
values are possible.
Example: height and
Can be affected by environment for example height and weight
may be affected by nutrition.
Results in a
You can calculate an
average but that conceals information about the volume of variation.
Any discussion of averages in the human population hides that variation.
variation traits that
are either present or absent with no intermediate phenotypes.
Includes many physiological conditions or diseases including
albinism and Tay-Sachs.
It is possible to figure out the frequency of these cases in a
population as well as allele frequency of the affected gene in the
Tells us how frequent an allele for a gene is in the entire
The study of variation in allele frequencies of populations is
called population genetics.
Then you can discern the genetics behind the phenotype. How?
Only species that
have genetic variation can evolve. Why?
We know that species
evolve and that they do it in response to their environment.
We define groups
within an individual species by populations.
Populations are consists of members of a species in a certain
These individuals will be a mating population of that species.
Human populations in
different geographical locations differ from one another in many physical
characteristics like coloration, height and weight.
However these are
mostly cosmetic more interesting and more important are those differences
in diseases and health.
Ex: Cystic fibrosis and skin cancer are more prevalent in people of
northern European descent, while people of African descent are more
susceptible to sickle-cell anemia and frostbite (due to extreme temps.).
It is relatively
easy to find physical differences between populations yet it is nearly
impossible to find sharp boundaries between them.
Meaning all traits in the human population are continuous even
if the trait itself is discontinuous.
Humans have developed several ways of describing themselves and others.
Anthropologists have assisted in this by studying traits prevalent in populations.
There are many ways human variation can be used one of the worst has been to separate people into different races.
Races based on cultural characteristics.
Most people have a word to describe themselves often it is the word
Bantu languages of Africa, people = Bantu.
Inuit word for people is Inuit.
Most groups also have words to describe other people, however,
often value judgments are included as well.
Often these value judgments have been used as excuses in conquering
and/or abusing a population.
Slavery and other atrocities were used to secure the labor of the
Even though these groups were often very similar and were nothing
more than a different language or cultural group.
This is recognized now as racism.
scientific studies were conducted to prove that certain races
the 1940s Nazis exterminated millions of Jews, Gypsies and Slavs after
they were found to be inferior races.
was a set of morphological features that someone of a particular race had.
we know this these things are not invariant.
a pure race concept where each race was originally pure.
Population geneticists realize that boundaries between groups are
arbitrary at best and that all transitions are gradual.
A gradual rise or fall of the average allele or phenotype frequency
of one trait is called a cline.
Clines are accurate yet time consuming way to describe trait
changes in a population.
Maps are often drawn including the information from clines.
Example: The frequency of blood group alleles differ depending on the area of the
The following maps are based on native populations.
After WWII the United Nations felt a need to refute the Nazi claims
about race and a statement on race by prominent anthropologists and
Under the new definition race is a geographic subdivision of a
species distinguished from one another by a allele frequencies.
Makes things like blood groups allele frequencies more important in
deciding race than coloration or religion.
Due to inequalities brought about by race there is also a no races
Especially in our age of immigration, emigration and world travel.
of human variation.
Despite the problems
and misinterpretations by racists the study of human variation is
Studying the patterns of disease in populations could help with
Also taking into consideration human averages into making vehicles
Understanding how people have changed over time, evolved.
Genetics can Help us Understand Human Variation.
blood groups and geography.
not always been the simple and safe affairs they are today.
Often people died
from transfusions without the doctors knowing why.
During the Crimean
War (1854-1856) A British army surgeon found that there were three types
of soldiers A, B and O.
A could give blood to A and B to B while O was the universal donor
and could donate blood to all.
A German doctor
(Karl Landsteiner) figured out that type A individuals made a type A
carbohydrate that was found on their blood cells, a type B
carbohydrate was found for blood type B and type O had neither
of these carbohydrates.
Transfusion with the wrong blood type led to clumping of the cells
in the veins of the patient due to the immune system binding the foreign
Blood type is
controlled by a gene that has three alleles: A, B and O.
Allele A and B are dominant while allele O is recessive.
What if you have an
A and B allele?
AB blood type is the universal recipient.
blood group system is the Rh system.
This system results from three genes (C or c, D or d and E or e)
found on the same chromosome.
All the cde alleles are recessive to CDE.
There are eight phenotypic possibilities.
The genotype ccddee is homozygous recessive for all and considered Rh-negative,
while all others are considered Rh-positive.
Rh-positive genotypes are the most common.
Rh-negative genotype is the second most common genotype in Europe
The problem arises when an Rh-negative mother gives birth to an
variation of blood type.
visualized the cline maps of A, B and O allele frequencies of native
group is unique to any population.
be variations even among small populations especially in rural populations
that do not move from town to town.
Example rural Italy,
Wales and India.
Populations that are more mobile see less microgeographic variation.
populations and genetic drift.
Principle- Mathematics of Evolution; Genotype proportions remain constant
from generation to generation.
Allele frequencies remain constant.
Presumes the following assumptions:
Size of the population is very large.
There are equal reproductive outputs (no natural selection).
No input of new genes from an extraneous source.
Equation p= dominant allele
(more common) and q= recessive (less common).
Equation is (p+q)2
when there are two alternative alleles.
= p2 + 2pq + q2
q2= homozygous recessive.
Very rarely is
correct because of the assumptions necessary, especially in human
equilibrium needs large populations because in small populations genotype
frequencies can be erratic.
is a change that occurs in small populations due to chance alone.
Small populations are also subject to the founder effect in
which if a small population is due to a small number of founder
individuals the whole population will reflect their allele frequencies.
A large population that for some reason became small then became
large again would reflect the genotypes of the small group or genotypes
due to genetic drift that occurred when they where small , bottleneck.
studying coat color in a population of 100 black and white mice 22 of
which are white. You know that white is the recessive phenotype (w). Using
the Hardy-Weinberg equilibrium equation (show your work) give me all the
allele and genotype frequencies.
can become the agents of Natural Selection.
populations must adapt in order to survive epidemics.
2,000,000 people a year and causes 110,000,000 illnesses.
Mosquitoes carry Plasmodium
falciparum the organism responsible.
parasite reproduces asexually in the red blood cells.
parasite digests oxygen carrying hemoglobin and also ruptures the blood
cells leading to anemia.
anemia and malaria resistance.
Another type of anemia is due to deformed blood cells and is called sickle-cell anemia (due to the cells shape).
is a hereditary disease affecting hemoglobin molecules in the blood.
single nucleotide change in the hemoglobin gene causes an amino acid
change (EΰV) that causes the hemoglobin molecules to
Persons heterozygous have enough functional hemoglobin to make keep
their red blood cells healthy.
9% of African Americans are heterozygous carriers and 0.2% are
homozygous recessive and have the disorder.
Because sickle-cell gives resistance to Malaria.
Two other diseases
that confer resistance to malaria are thalassemia and G6PD
a defect in the number of chains that make up hemoglobin and results in
G6PD deficiency is an enzyme deficiency have hemolytic anemia.
People heterozygous for sickle-cell allele are resistant to malaria one of the leading causes of death in central Africa.
Although some die from being homozygous for sickle-cell the death toll is less than it would be from malaria without the sickle-cell allele.
Another example northern Europeans, CFTR and typhoid fever.
Variation Due to Physical Factors.
variation can also cause natural selection.
variation in physiology.
During the Korean War troops had to survive a harsh winter many of the Euro-American soldiers responded to frost-bite treatments very well where are African American soldiers did not and many lost fingers and toes.
The Army did more tests looking at the effect of climate on troops and found that African, Asian and Native Americans fared much better in heat and humidity than the Euro-Americans.
Others found that only the Inuit were able to keep their arms in
ice water indefinitely with no loss of heat or shivering.
differences based on climate are the basis for a number of rules.
states that body sizes tend to be larger in colder parts of a range and
smaller in warmer parts.
Peoples with the largest body masses tend to live in the coldest
places like Siberia.
Allens rule states that arms, legs and tails tend to be longer in warmer regions
and shorter and thicker in colder regions.
selection, skin color and disease resistance.
Skin is the largest organ of the body and varies widely in color.
Skin is the most visible human characteristic.
By examining populations that have lived in one place since before 1500 A.D. we may be able to see adaptation of skin coloration.
If we look at these populations we see that people with the darkest
skin colorations have lived in tropical climates for millennia (Sri Lanka
and Africa) and people with light skin, eyes and hair have lived in colder
climates for millennia (Scotland and Scandinavia).
Sunlight as an Agent
Takes into account sunlight and humidity.
Camouflage has been suggested as one cause while Vitamin D
synthesis may also play a role.
Vitamin D is needed for proper bone formation.
Vitamin D can be found in many founds (eggs and whole milk) but it
is biologically inactive.
Needs sunlight to be activated.
Europeans live where they get less light so they need lighter skin
for maximum light penetration.
Closer to the equator there is so much light they must worry about
UV radiation and cancer.
Same wavelength of the light needed to activate Vitamin D.
These individuals have developed more melanin (protects against UV
rays) in their skin in order to survive the risks posed by skin cancer in
The Inuit of the far north have darker skin color.
The reason is they do not need or want to expose themselves to the
sun to get their vitamin D they get it from the cold water fish they eat.
type O is correlated with duodenal ulcers, does this correlation
demonstrate a cause? Does it imply a mechanism?
What would be a hypothesis? How might we answer these questions?