Changes

'''Natural selection''' is a mechanism,

'''Natural selection''' is a mechanism,

first proposed by [[Charles Darwin]],

first proposed by [[Charles Darwin]] and Alfred Russel Wallace, by which organisms with favorable traits are more likely to survive and reproduce than those lacking those traits or possessing unfavorable traits. Over time this mechanism might allow those favorable traits which are inheritable to become more common.

by which organisms with favorable traits are more likely to survive and reproduce than those lacking those traits or possessing unfavorable traits. Over time this mechanism might allow those favorable traits which are inheritable to become more common.

According to the [[Theory of Evolution]], it is through natural selection that biological complexity and adaptation arises. This view is disputed by advocates of [[Intelligent Design]]. See also [[Creationism]].   

According to the [[Theory of Evolution]], it is mainly through natural selection that biological complexity and adaptation arise from earlier generations of life. This view is disputed by advocates of [[Intelligent Design]]. See also [[Creationism]].   

== Variation ==

== Variation ==

During fertilisation, the genetic material ([[DNA]]) of a male gamete combines with the genetic material from a female gamete.  The exact gametes which combine is also a random process.  By these two processes gametes with unique [[genomes]] combine randomly to produce a unique offspring, and hence variation.

During fertilisation, the genetic material ([[DNA]]) of a male gamete combines with the genetic material from a female gamete.  The exact gametes which combine is also a random process.  By these two processes gametes with unique [[genomes]] combine randomly to produce a unique offspring, and hence variation.

Continuing the rabbit example, it is unknown whether the gamete coding for the albino polypeptide will fertilise another gamete containing the albino allele or not.  As the albino allele is recessive, there must be two copies present in the cells for it to be expressed.

Continuing the rabbit example, it is unknown whether the gamete coding for the albino polypeptide will fertilize another gamete containing the albino allele or not.  As the albino allele is recessive, there must be two copies present in the cells for it to be expressed.

Variation produces individuals within a population that have unique characteristics, such as different coat colours, blood groups or [[genetic disorders]].

Variation produces individuals within a population that have unique characteristics, such as different coat colors, blood groups or [[genetic disorders]].

== Selection pressures ==

== Selection pressures ==

*Using two or more antibiotics at once can help ensure that those bacteria that are resistant to one of the drugs is killed by the other.

*Using two or more antibiotics at once can help ensure that those bacteria that are resistant to one of the drugs is killed by the other.

==='''Industrial melanism'''<ref>[http://www.darwinismrefuted.com/mechanisms04.html| Darwinism Refuted] accessed on 24 March 2007</ref>===

==='''Industrial melanism'''<ref>[http://www.darwinismrefuted.com/mechanisms04.html| Darwinism Refuted] accessed on 24 March 2007</ref>===

In England during the industrial revolution pollution killed mosses and lichens on tree trunks, turning them from light colours to dark brown. Before this occurred most specimens of the peppered moth Biston betularia had white wings with black spots - giving a peppered appearance. During the period with dark tree trunks (1849 to circa 1970) the white version of betularia was easily predated on by birds, decreasing the frequency of the white-wing allele. Individuals with dark wings caused by high concentrations of melanin became much more frequent and the allele frequency for melanic moths increased. When pollution levels were reduced in the late 1960s the trees gained more moss and lichens, making the trunks lighter and predation selected against melanic forms of the peppered moth. It is now thought that the proportions of white to melanic moths in England have now returned to pree-1849 levels.

In England during the industrial revolution pollution killed mosses and lichens on tree trunks, turning them from light colors to dark brown. Before this occurred most specimens of the peppered moth Biston betularia had white wings with black spots - giving a peppered appearance. During the period with dark tree trunks (1849 to circa 1970) the white version of betularia was easily predated on by birds, decreasing the frequency of the white-wing allele. Individuals with dark wings caused by high concentrations of melanin became much more frequent and the allele frequency for melanic moths increased. When pollution levels were reduced in the late 1960s the trees gained more moss and lichens, making the trunks lighter and predation selected against melanic forms of the peppered moth. It is now thought that the proportions of white to melanic moths in England have now returned to pree-1849 levels.

==='''Sickle cell anaemia'''===

==='''Sickle cell anemia'''===

This is a condition where red blood blood cells have a sickle shape caused by faulty haemoglobin genes.  Suffers with full sickle cell anaemia 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 anaemia<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 heamoglobin do not have anaemia but are much more susceptible to malaria and in a study of 100 children who died from sickle cell anaemia 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 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>.

== References ==

== References ==