Changes

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

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

first proposed by [[Charles Darwin]] and Alfred Russel Wallace, by which orgasms 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. This selection process is in response to forces in the natural world, as opposed to [[artificial selection]], whereby selection is made by a human being, such as a farmer selecting his breeding stock or variety of [[plant]].

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. This selection process is in response to forces in the natural world, as opposed to [[artificial selection]], whereby selection is made by a human being, such as a farmer selecting his breeding stock or variety of [[plant]].

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

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

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

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

These mutations change the [[gene]]s within the [[chromosome]]s of the orgasm and can result in new [[polypeptides]] being produced.  These abnormal polypeptides can have significant affects on the biochemistry of the orgasm, 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.

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 [[fertilization]] of gametes===

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

Meiosis is the division of diploid germline [[cell]]s in the [[gonads]] of an orgasm 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 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.

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 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 orgasms 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 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.