Difference between revisions of "Natural selection"
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| − | + | '''Natural selection''' is the mechanism by which [[allele]] frequency within a population changes over time due to [[genetic]] [[variation]] and selection pressures. Over long periods of time natural selection may lead to evolution under the [[Darwin-Wallace theory of evolution]] | |
| − | + | == Variation == | |
| + | Genetic variation occurs due to several factors. | ||
| − | == | + | ==='''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. | |
| + | 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 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. | ||
| + | |||
| + | 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. | ||
| + | |||
| + | Variation produces individuals within a population that have unique characteristics, such as different coat colours, blood groups or [[genetic disorders]]. | ||
| + | |||
| + | == Selection pressures == | ||
| + | Habitats place pressures upon their inhabitants. These can include, but are not limited to: | ||
| + | *Predation; | ||
| + | *Competition for food and water; | ||
| + | *Disease; | ||
| + | *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]]. | ||
| + | |||
| + | 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. | ||
| + | |||
| + | == Examples of natural selection == | ||
| + | Here are three good examples of natural selection in action. | ||
| + | |||
| + | ==='''[[Antibiotic]] resistance'''=== | ||
| + | As bacteria are exposed to increasing amounts of antibiotics only those that have or mutate genes for resistance survive as they are selected for. This is a problem in medicine as many antibiotic drugs are becoming less and less effective. The problem can be solved by: | ||
| + | *Ensuring that patients complete the course prescribed by them by their doctor. | ||
| + | *Only prescribing antibiotics when necessary - many people expect to be given a medicine when they visit their doctor even if one is not necessary. | ||
| + | *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'''=== | ||
| + | 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. | ||
| + | ==='''Sickle cell anaemia'''=== | ||
| + | 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>. | ||
| + | |||
| + | == References == | ||
| + | <references/> | ||
| + | |||
| + | [[Category:Science]] | ||
[[Category:Biology]] | [[Category:Biology]] | ||
| + | [[Category:Genetics]] | ||
Revision as of 22:48, March 22, 2007
Natural selection is the mechanism by which allele frequency within a population changes over time due to genetic variation and selection pressures. Over long periods of time natural selection may lead to evolution under the Darwin-Wallace theory of evolution
Variation
Genetic variation occurs due to several factors.
Spontaneous genetic mutations
These mutations change the genes within the chromosomes 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.
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[1]
Meiosis and random fertilisation of gametes
Meiosis is the division of diploid germline cells 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.
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.
Variation produces individuals within a population that have unique characteristics, such as different coat colours, blood groups or genetic disorders.
Selection pressures
Habitats place pressures upon their inhabitants. These can include, but are not limited to:
- Predation;
- Competition for food and water;
- Disease;
- 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.
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.
Examples of natural selection
Here are three good examples of natural selection in action.
Antibiotic resistance
As bacteria are exposed to increasing amounts of antibiotics only those that have or mutate genes for resistance survive as they are selected for. This is a problem in medicine as many antibiotic drugs are becoming less and less effective. The problem can be solved by:
- Ensuring that patients complete the course prescribed by them by their doctor.
- Only prescribing antibiotics when necessary - many people expect to be given a medicine when they visit their doctor even if one is not necessary.
- 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
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.
Sickle cell anaemia
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[2]. 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[3].