Biology [[Classification, Selection and Evolution]] Evolution

[[Evolution]]

Natural selection keeps things the way they are, and this is Stabilising Selection. But if a new environmental factor or new allele is introduced, allele frequencies can change. This is directional selection.

[[A new Environmental Factor]]

If the climate is plunged into a new ice age and snow covers the ground for 11 months out of 12, the white rabbits now have a selevtive advantage, as they are camoflauged agaisnt the white environment. So rabbits with white fur are more likley to survive in this new environment, and reproduce, passing on their alleles.

The allele for the white coat increases in frequency. Over many generations, almost all rabbits will have white coats (but the allele will still be recessive)

[[A New Allele]]

Most mutations are harmful as they produce organisms that are less well adapted to their environments than normal organism is. Some are neutral, which means that they produce neither advantageous or disadvantageous. Occasionally mutations may produce useful features.

Such changes in the allele frequency (rabbit mutation results in change in coat colour which is better camouflaged, so that allele survives and is passed on) are the basis of evolution.

Evolution: occurs when natural selection gives some alleles a better chance if survival than others

Some examples of Evolution.

[[Antibiotic Resistance]]
Antibodies: chemicals produced by living organisms, which inhibit or kill bacteria. They do not usually harm human tissue. Produced by fungi.
First antibiotic discovered = Penecillin
It stops cell wall production and prevents cell reproduction.

If someone takes penecillin to treat a bacterial infection, bacteria sensitive to it are killed. In most cases, this is the entire population. But there may be one or two that have an allele giving resistance to the penecillin. This occurs in Staphylococcus (some individual bacteria produce enzym penicillinase which inactivates penecillin).

Bacteria only have a single loop of DNA so they have one copy of each gene. The mutant allele has an immediate effect on the bacteria's phenotyope.

The bacteria without resistance will be killed, while those with the resistance survive and reproduce rapidly.

[[Industrial Melanism]]
Peppered Moth is a night flying moth which spends most if its time resting on the branches of trees and relies on camouflage to protect it from insect eating birds.

Until 1848, all peppered moths had pale wings with dark markings = speckled appearance
In 1848, a drak individual specimen was caught

Over 19th century, numbers of dark moths increased rapidly in some areas, while remained low in other areas.

The difference in colour is caused by a single gene. The normal speckled colour is recessive and the black is dominant. The frequency of the dominant allele increased near to industrial cities up to 1960s, while it remained the same in non-industrial areas.

The selection pressure which caused this change was predatation from birds. Lichens which cover the trees and make the light moths camouflage are sensitive to pollution levels and do not grow on trees in suc areas. This means that the trees are darker brown, so the lighter moths stand out and are eaten by the birds.

Pollution levels are being reduced, and it is predicted that there will be hardley any specimens left soon.

[[Sickle Cell Anaemia]]

The gene that codes for Beta Polypeptides has to alleles. Hn can produce normal cells while Hs can produce sickling of redblood cells. Homozygous Hs produces a phenotype of sickle cell anaemia (often lethal)

Until recently, almost everyone homozygous for Hs died before they reached reproductive age but their are still very high frequencies of the sickle cell allele is very high. This is due to the high incidence of Malaria in these parts of the world. People who are heterozgous are much less likley to suffer an attack of malaria than people who are homozygous for normal allele

Two selectron pressures:
* Selection agaisnt homozygous Hs people as they are seriously anaemic
* Selection agaisnt people homozygous Hn as they are more likley to suffer from Malaria