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natural selection/ definition, characters, industrial melanism, and example - science info
some genetic variability is always presence in a population in a population. such alleles make organism better adapted to the environment leading to their survival and continuous reproduction.
The frequency of such alleles in a population, thus increase gradually. this process in called selection. if the breeders selection for the desired characteristics it is Artificial selection, and if the environment condition operate operating the natural population it is said to be natural selection.
condition for natural selection :
for natural selection to occur and result in evolutionary charge, three condition must be met
(1) variation must exist among individual in a population. natural selection works by favouring individual with some traits over individual with alternative traits. if .
(2) variation among individual result in difference in the number of offspring surviving in the next generation. this is the essence of natural selection. it means that some individual are more successful than other in producing offspring and thus, passing their genes on the next generation because of their phenotype or behaviour.
(3) variation must be genetically inherited for natural selection to result in evolutionary changes . even genetically identical individual may be phenotypically quite distinctive, if they grow up in different environment.
therefore, when phenotypically difference individual do not differ genetically, then differences in the number of their offspring will not alter the genetic composition of the population in the next generation. hence, no evolutionary charge will occur.
types of natural selection :
there are following three kind of natural selection
(1) stabilising / normalising selection :
this occur when the environment does not change and favours the individual possessing the value of the mean characters. thus, natural selection produces value. thus , natural selection process change in favour of those individuals which posses an intermediate form of trait, while elimination extremes form the population. the stabilising selection pressure does not promoting evolutionary charge but tend to maintain phenotypic stabilising within the population from generation to generation. fossil evidences suggest that many species remain unchanged for long periods of geological time. for example : coelacanth (a fish species) was assumed to have been extinct for 70 million years until a living specimen was found in a trawler net of south Africa in 1938 . so, this species has not changed in all the time.
another example of stabilising selection can be be seen in the birth weights of human. the heaviest and lightest babies have the highest mortality and are less likely to survive to reproduce and pass on their alleles.
(2) directional selection
this occurs whenever the environment change in a particular direction. thus, natural selection production change in a fever of one extreme while elimination the extreme. therefore, thare is a selective pressure for species to change in response to that environment change, e.g. industrial melanism.
industrial melanism :
the term refers to the evolution of dark body colour in animals species that live in habitats blackened by industrial soot. the occurrence of industrial melanism is closely associate with the progress of the industrial revolution in a grate Britain during 19th century. it has occur in several species of moth. of this, peppered moth ( Biston betularia) is the most intensive study. thes on existed in 2 variety, i.e. light coloured (white) and dark coloured (black). before the emergence of industrialisation, the with moth were numerous than dark moth in Britain.
however, after industrialisation , i.e. in 1920 the condition was reversed. this was due to the fact that, white moth were easily spotted by predation (birds) on soon covers tree trunk, so there population decrease and the population of black moth increase. we can also say that black moths got naturally selected. Disruptive/ Diverging selection.
this occurs where an environment change may produce selection pressure that favours two extreme characteristics.
some of the examples of disruptive selection are given below grass plant of welsh copper mine grew in soil.
contaminated soil by copper which was lethal to the normal grass plant. however, a chance mutation allowed one plant to grow. this plant prospered and reproduced, but only on the contaminated soil. on normal soil, it grows more slowly than the normal plant and was easily out-competed. so, now there are two varieties of grass plant growing close together.
sickle- cell anaemia:
people who are homozygous for this recessive allele, usually die before reproduction. their red blood cell contains abnormal haemoglobin, which makes them become a sickle-shaped and stick in their capillaries. people heterozygous for the allele are at a disadvantage because their red cells becomes sickle-shaped during exercise due to lack of oxygen.
the allele should therefore, be selected again and rare, however, its frequency is a high is a part of the world, where malaria is common. people heterozygous for sickle cell anaemia are more resistant to malaria than people homozygous for a normal allele because the malarial parasite is not able to survive to sickle-shaped RBCs, hence, where malaria is found, people with heterozygous alleles are at advantage.
