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Line 38: − Here are good examples of natural selection in action.+ Line 49:
Line 49: − + − − ===Life on Earth=== − All life on Earth shows that favorable traits are more likely to survive. Some organisms − survive and some don't. Those that survive are the ones with the favorable traits; the − traits are considered favorable if they increase the likelihood of survival. It is hard − to see how life could be any other way.
Fixed sickle cell anemia nonfactual info and removed unnecessary "life on earth" section.
== Examples of natural selection ==
== Examples of natural selection ==
Following are good examples of natural selection in action.
==='''[[Antibiotic]] resistance'''===
==='''[[Antibiotic]] resistance'''===
==='''Sickle cell anemia'''===
==='''Sickle cell anemia'''===
This is a condition where red blood blood cells have a sickle shape caused by faulty hemoglobin genes. Suffers with full sickle cell anemia have two alleles for the condition and none of their red blood cells are formed correctly. These cells get stuck in blood capillaries and cannot carry oxygen properly. This is clearly not advantageous and many sufferers die early in life. Even though death occurs in some parts of East Africa up to 50% of babies carry the genes for sickle cell anaemia and 14% have anemia<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>. This can be explained as an individual with one normal allele and one sickle cell allele will have half normal red blood cells and half sickle red blood cells. While this condition is not desirable they will ''not'' suffer from full anaemia and can live a normal life. However, the protoctist ''plasmodium'' that causes malaria cannot survive in sickle red blood cells and the sickle cell allele is selected for. People with two normal alleles for hemoglobin do not have anemia but are much more susceptible to malaria and in a study of 100 children who died from sickle cell anemia all but one child had two normal alleles<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>.
This is a condition where red blood blood cells have a sickle shape caused by faulty hemoglobin genes. Suffers with full sickle cell anemia have two alleles for the condition and none of their red blood cells are formed correctly. These cells get stuck in blood capillaries and cannot carry oxygen properly. This is clearly not advantageous and many sufferers die early in life. Even though death occurs in some parts of East Africa up to 50% of babies carry the genes for sickle cell anaemia and 14% have anemia<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>. This can be explained as an individual with one normal allele and one sickle cell allele will have mostly normal red blood cells and a few sickle red blood cells. This condition is not harmful and the carriers can live a normal life. However, the protoctist ''plasmodium'' that causes malaria cannot survive in sickle red blood cells, thus giving an advantage to carriers in a high-risk malaria area.
== References ==
== References ==