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9.8 Option - The Human Story: 4. Human
migration
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4. How humans have adapted to their
environment
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Students learn to: |
Students: |
Extract from Biology Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit 16 June 09]
Prior learning: Recall statements in HSC
module 9.3.3.
Science Stages 4–5 syllabus: Outcome 5.8 (content
5.8.3d,e).
outline the mechanisms that led to human diversity
- As humans migrated throughout the world small groups of people would have become isolated by barriers such as mountain ranges or seas. Within these groups the individuals shared a genetic variation that increased their chances of surviving and reproducing. This favourable factor would therefore become common in this group.
- Geographical isolation can lead to differences in gene frequencies. During their migrations across the Earth, humans experienced different climactic conditions which led to the selection of regional differences.
explain the differences between polymorphism and clinal gradation using at least one appropriate example from human phenotypes
- Polymorphism is the presence of two or more distinct forms of a genetically determined character. Humans are polymorphic for skin colour, body stature, sickle-cell anaemia, blood groups and the epicanthic eye-fold. Polymorphic differences occur as selective adaptations to different environments.
- Clinal gradation occurs as a result of interbreeding and intermediate environments. Clinal gradation is seen in skin colour, eye colour and body stature. A cline is a gradual change in gene frequency in a population. This produces a gradual change in the phenotypic characteristics of a species over a geographical range.
- An example of polymorphism and clinal gradation in humans is the gradual increase in the frequency of blonde haired blue-eyed people as you move from the Mediterranean to the countries of Scandinavia. Eye colour and skin colour are both polymorphic. The clinal gradation that occurs is because of the existence of intermediate environments where different genetic factors have an advantage.
- The presence of these clines makes it difficult to maintain the concept of race. A race is a separate breeding population within a species. As the amount of trade, migration and air travel increases the concept of race is breaking down as people from around the world interbreed and produce offspring with intermediate features. In fact it is social and cultural differences rather than geographic barriers that keep populations apart.
gather and process
information from secondary sources to identify two
examples of polymorphism in humans and analyse the
evolutionary significance of the phenotypes
displayed.
Background information
It is very difficult to analyse the evolutionary significance of polymorphic features. Many features are trivial when considered as survival adaptations. Curly hair is an example of a polymorphic feature in humans but it would not necessarily affect the ability of the individual to reproduce and pass on genes to the next generation. Sickle-cell anaemia is a good example of the environment influencing the gene pool. Malaria is a disease that kills millions of people each year. It is a major selective agent in regions where the malaria parasite is present. The sickle-cell anaemia gene is deleterious to the individuals that possess it but it gives protection from malaria in the heterozygous form.
- Gather information from a range of resources including popular scientific journals, CD ROMs and the Internet. Choose two polymorphic characteristics.
- Process the information by assessing the accuracy of scientific media by comparison to similar information presented in scientific journals.
- Identify two examples of polumorphism.
- Analyse the evolutionary significance of the two examples.
Example 1 Skin colour
Skin colour in humans is controlled by a number of gene
pairs. The pigment melanin absorbs ultraviolet light. People
with dark skin who live in areas where there is a high level
of solar radiation have a selective advantage over those with
paler skin. The melanin in the skin prevents the sun’s
rays from penetrating to the lower layers of the skin and
protects against skin cancer. In areas where the levels of
solar radiation are low then light skin had a selective
advantage. This is because of Vitamin D production. Vitamin D
is required by the body for calcium absorption. Without
Vitamin D a disease called rickets can occur. Lighter skin
absorbs more ultraviolet radiation and this increases the
production of Vitamin D. White skin is better at producing
Vitamin D in low levels of solar radiation while dark skin
prevents skin cancer in areas that have high levels of solar
radiation.
Example 2 Body stature
Body stature in humans shows polymorphism.
Tall thin body stature is common in hot tropical areas to
allow a greater surface area to volume ratio for heat loss. A
stocky short body is found in groups such as the Inuits
(Eskimos). The stocky body with a smaller surface area to
volume ratio allows better heat retention.
Example 3 Sickle-cell anaemia
Sickle-cell anaemia is a human genetic disease. It produces
red blood cells that are folded and have a sickle shape.
These cells are less efficient at carrying oxygen around the
body; they block the capillaries causing intense pain and
death of the surrounding cells. Few people with the disease
live past ten years of age. Those who are homozygous for the
disease die before they can pass the gene on but those that
only carry one gene for sickle-cell anaemia (heterozygous)
have a resistance to the disease malaria. Therefore the
heterozygous form has an adaptive advantage in areas that
have malaria present. In parts of Africa that have malaria
the population is 40% heterozygous for sickle-cell
anaemia.
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