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Wednesday, March 11, 2009


Shedding the weight of exams, my mind is once again allowed to wander and I am thus
reminded of one great example of evolution in progress.

Evolution comprises of a few concepts

1. Random mutation occurs in a species (occuring at a genetic level- unobservable)
2. the mutation gives rise to observable traits, features and characteristics
3. Some traits are beneficial to life, others have no effect whatsoever, some however are downright detrimental
4. Species adapt to the environment
5. Those that are suitably adapt to a particular environment by virtue of their traits survive, are able to produce more offspring and in the process pass their genes on to the next generation (which will inherit the traits)

Examples of the above are plenty
Including adaptive radiation among the Finches of the Galapagos, the change in the dominant colour pattern of moths living in industrialized areas

but what about humans?

This can be observed in the inheritance of the Sickle Cell Trait.
Sickle cell Disease

Sickle cell Disease affects the red blood cells where a genetic defect causes the formation of defective haemoglobin that causes red blood cells to be deformed (sickle shaped) (defective polymerisation)

sickle cell

There are homozygotes - having both defective alleles
And heterozygotes - having a single defective allele

Excerpts from the article in wikipedia:

"In people homozygous for HgbS, the presence of long-chain polymers of HbS distort the shape of the red blood cell, from a smooth donut-like shape to ragged and full of spikes, making it fragile and susceptible to breaking within capillaries"
This condition predisposes the patient to disease states called crises.

Carriers have symptoms only if they are deprived of oxygen (for example, while climbing a mountain) or while severely dehydrated.

One-third of all aboriginal inhabitants of Sub-Saharan Africa carry the gene because in areas where malaria is common, there is a survival value in carrying only a single sickle-cell gene (sickle cell trait). Those with only one of the two alleles of the sickle-cell disease are more resistant to malaria, since the infestation of the malaria plasmodium is halted by the sickling of the cells which it infests.

Thus the mutation causes the characteristic change in the property of red blood cells in the affected person.

This property while homozygous causes a marked reduction in life-expectancy and substantial morbidity

Whereas when heterozygous causes minimal ill effects while conferring a survival advantage in malaria endemic areas in Africa.

Thus those people with sicke cell trait are able to survive longer, produce nmore offspring and will successfully pass the gene on to their progeny...

Resulting in

One-third of all aboriginal inhabitants of Sub-Saharan Africa carry the gene

distribution of sickle cell trait
-distribution of sickle cell trait

-distribution of malaria

Simple and elegant example of the theory of evolution
applied onto humans!

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