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 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 because of errors made when DNA is copied inside a cell at the first stage of [[mitosis]].  Although most mutations are harmful due to their random nature (see the example regarding sickle cell anemia), 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 because of errors made when DNA is copied inside a cell at the first stage of [[mitosis]].  Although most mutations are harmful due to their random nature (see the example regarding sickle cell anemia), beneficial mutations may also occur.

Mutations will not be inherited unless the affected cells are [[gamete]] producing cells or gametes themselves

Mutations will not be inherited unless the affected cells are [[gamete]] producing cells or gametes themselves.

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 [[fertilization]] 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 get entangled and '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 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.

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, whether the gamete coding for the albino polypeptide will fertilize another gamete containing the albino allele (this happens if both the sperm of the male and the egg of the female carry the albino allele) or not relies on coincidence.  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 colors, 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]].

*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 [[population growth]].

These environmental pressures can account for apparent over production by 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 and killed before they can reproduce. 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 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.

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.

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.

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 ==

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