Natural selection

   Modern synthetic theory of evolution

Natural selection

➢ It is the process by which those organisms which are physically, 

physiologically and behaviourally better adapted survive, reproduce and 

produce more number of surviving offsprings as compared to those who are 

not better adapted.

➢ Selection depends upon the existence of phenotypic variation within the 

population and is part of the mechanism by which a species adapts to its 

environment. 

➢ A population generally has three types of individuals on the basis of their trait:

○ Average-sized 

○ Large-sized

○ Small-sized.

Types of natural selection:

1.stebalishing selection

➢ When selection operates in such a way so that individuals with mean value 

of the trait are favoured at one time.

➢ This type of selection favours average sized individuals while eliminates small 

sized individuals.

➢ If we draw a graphical curve of population, it is bell-shaped.

Examples of stebalishing selection:

➢ There is an optimum wing length for a hawk of a particular size with a certain 

mode of life in a given environment. 

➢ Stabilising selection, operating through differences in breeding potential, will 

eliminate those hawks with wing spans larger or smaller than this optimum 

length.

➢ Human baby birth weight.

➢ Babies with birth weight less than five pounds / more than ten pounds have 

less chances of survival as compared to babies with mean birth weight.

2.Directional selection

➢ When selection operates in such a way so that individuals with extreme 

value of the trait are favoured at one time.

➢ In this selection, the population changes towards one particular direction. 

➢ It means this type of selection favours small or large-sized individuals and 

more individuals of that type will be present in next generation.

Example of directional selection:

➢ Evolution of DDT resistant mosquitoes. 

➢ Industrial melanism in peppered moth.

3.Disruptive selection

➢ When selection operates in such a way so that individuals at both extreme 

values of the trait are favoured at one time.

➢ It favours both small-sized and large-sized individuals. 

➢ It eliminates most of members with mean expression, so produces two peaks 

in the distribution of the trait that may lead to development of two different 

populations.

Examples of disruptive selection

➢ Shell colour in marine limpet (mollusc).

➢ Stebbins and his coworkers studied population of sunflowers in the 

Sacramento Valley of California over a period of 12 years.

➢ In the beginning the genetically variable population of these sunflowers was a 

hybrid between two species. 

➢ After five years this population had split into two subpopulations separated by 

a grassy area. 

➢ One of these subpopulations occupied a relative dry site and other occupied 

comparatively wet site. 

➢ During the next seven years the size of the population fluctuated greatly in 

response to differences in rainfall, but the differences between the two 

subpopulations were maintained.

Industrial malanism

➢ Industrial melanism in moths has been originally studied by R.A. Fischer and 

E.B. Ford; and in recent times, by H.B.D. Kettlewell.

➢ The occurrence of industrial melanism is closely associated with the progress 

of the industrial revolution in Great Britain, during the nineteenth century. 

➢ Peppered moth (Biston betularia) is the most intensely studied. 

➢ Peppered moth exists in two strains (forms):light coloured (white) and 

melanic (black).

Industrial Melanism

Examples of Natural Selection

➢ Industrial melanism can be explained as follows: 

○ In the past, bark of trees was covered by whitish lichens, so white moths 

escaped unnoticed from predatory birds. 

○ After industrialisation barks got covered by smoke, so the white moths 

were selectively picked up by birds. 

○ Black moths escaped unnoticed so they managed to survive resulting in 

more population of black moths and less population of white moths.

Resistance of insect to pesticides

➢ DDT, which came to use in later 1945, was thought to be an effective 

insecticide against household pests, such as mosquitoes, houseflies, body lice, 

etc. 

➢ Within two to three years of the introduction of this insecticide, new DDT 

resistant mosquitoes appeared in the population. 

➢ These mutant strains, which are resistant to DDT, soon became well 

established in the population.

Antibiotics restance of bacteria

➢ When a bacterial population encounters a particular antibiotic, those 

sensitive to it die. 

➢ Some bacteria having mutations are resistant to the antibiotic. 

➢ Such resistant bacteria survive and multiply quickly as the competing bacteria 

have died. 

➢ Soon the resistance providing genes become widespread and entire bacterial 

population becomes resistant.

Sickel cell animea

➢ One of the best examples has been discovered in the human populations, 

inhabiting in tropical and subtropical Africa. 

➢ Sickle cell anaemia is caused by the substitution of glutamic acid by valine at 

sixth position of beta chain of haemoglobin.

➢ In people, homozygous for this abnormal haemoglobin, the red blood cells 

become sickle-shaped, and this condition is described as sickle cell anaemia

➢ Inspite of its disadvantageous nature, the gene has a high frequency in some 

parts of Africa, where malaria is also in high frequency. 

➢ It has been discovered that the heterozygotes for the sickle cell trait are 

exceptionally resistant to malaria