Changes

==='''Spontaneous''' [[genetic mutations]]===

==='''Spontaneous''' [[genetic mutations]]===

These mutations change the [[gene]]s within the [[chromosome]]s of the organism and can result in new [[polypeptides]] being produced.  These abnormal polypeptides can have significant affects on the biochemistry of the organism, for example [[sickle cell anaemia]] occurs because one [[base]] within the gene for beta-[[haemoglobin]].  Mutations may occur for several reasons such as [[chemical]] [[mutagens]] or exposure to [[ionising]] [[radiation]], or with no apparent reason.  Although most mutations are harmful due to their random nature (see the example regarding sickle cell anaemia), beneficial mutations may also occur.

These mutations change the [[gene]]s within the [[chromosome]]s of the organism and can result in new [[polypeptides]] being produced.  These abnormal polypeptides can have significant affects on the biochemistry of the organism, for example [[sickle cell anemia]] occurs because one [[base]] within the gene for beta-[[hemoglobin]].  Mutations may occur for several reasons such as [[chemical]] [[mutagens]] or exposure to [[ionizing]] [[radiation]], or with no apparent reason.  Although most mutations are harmful due to their random nature (see the example regarding sickle cell anemia), beneficial mutations may also occur.

For example, there may be a spontaneous mutation in a [[germline]] ([[gamete]] producing) cell of an agouti (brown) rabbit that changes the allele so that it produces a polypeptide that dyes fur white and produces an [[albino]] rabbit.  There are also alleles that produce chinchilla and Himalayan coats<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>

For example, there may be a spontaneous mutation in a [[germline]] ([[gamete]] producing) cell of an agouti (brown) rabbit that changes the allele so that it produces a polypeptide that dyes fur white and produces an [[albino]] rabbit.  There are also alleles that produce chinchilla and Himalayan coats<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>

==='''[[Meiosis]]''' and random [[fertilisation]] of gametes===

==='''[[Meiosis]]''' and random [[fertilization]] of gametes===

Meiosis is the division of diploid germline [[cell]]s in the [[gonads]] of an organism to produce [[haploid]] gametes for [[sexual reproduction]].  During this process sections of [[homologous]] chromosomes ([[bivalents]] can 'swap', in a process called crossing over, to form [[chiasmata]].  This process is random and produces unique gametes.  Secondly, during the formation of gametes the distribution of the maternal and paternal chromosomes is random, causing further variation.

Meiosis is the division of diploid germline [[cell]]s in the [[gonads]] of an organism to produce [[haploid]] gametes for [[sexual reproduction]].  During this process sections of [[homologous]] chromosomes ([[bivalents]] can 'swap', in a process called crossing over, to form [[chiasmata]].  This process is random and produces unique gametes.  Secondly, during the formation of gametes the distribution of the maternal and paternal chromosomes is random, causing further 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 fertilization, 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 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.

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.

*Competition for space.

*Competition for space.

These environmental pressures can account for apparent over production my many species, especially those commonly regarded as prey species, and have large parts to play in [[populatiton growth]].

These environmental pressures can account for apparent over production my many species, especially those commonly regarded as prey species, and have large parts to play in [[population growth]].

But what will determine which individual organisms will survive?  The key factor is the genome of the individual.  Some individuals may have variations of the species' genome that enables them to cope will with these selection pressures.  For example, if large butterflies were more visible to predating birds the allele for large butterflies will be slowly removed from the population as large insects are predated on.  Conversely, if small butterflies were seen as unattractive to the opposite sex they would be unable to mate and their allele for smallness would be removed from the gene pool.  Such pressures provide a mechanism that changes the ''frequency'' of a particular allele within a population.

But what will determine which individual organisms will survive?  The key factor is the genome of the individual.  Some individuals may have variations of the species' genome that enables them to cope will with these selection pressures.  For example, if large butterflies were more visible to predating birds the allele for large butterflies will be slowly removed from the population as large insects are predated on.  Conversely, if small butterflies were seen as unattractive to the opposite sex they would be unable to mate and their allele for smallness would be removed from the gene pool.  Such pressures provide a mechanism that changes the ''frequency'' of a particular allele within a population.

Another example carries on the theme of rabbit coat colour:  If the rabbits are living in a temperate region where there is plenty of foiliage and the predominant colour is green/brown the it is clear that the agouti rabbits will have better protection from predatory foxes than albino rabbits and would be selected ''against'', reducing the albino allele frequency.  However, if the climate were to change and become arctic the albino rabbits would have more of an advantage in camouflage and be selected ''for'', increasing their allele frequency.  This is, of course, assuming that the rabbits would be able to survive in arctic conditions.

Another example carries on the theme of rabbit coat color:  If the rabbits are living in a temperate region where there is plenty of foliage and the predominant color is green/brown the it is clear that the agouti rabbits will have better protection from predatory foxes than albino rabbits and would be selected ''against'', reducing the albino allele frequency.  However, if the climate were to change and become arctic the albino rabbits would have more of an advantage in camouflage and be selected ''for'', increasing their allele frequency.  This is, of course, assuming that the rabbits would be able to survive in arctic conditions.

== Examples of natural selection ==

== Examples of natural selection ==

==='''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 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.

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 pre-1849 levels.

==='''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 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.

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 anemia 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 ==