Natural Selection

The greater chance of passing genes by the best adapted organisms is known as natural selection.

The term ‘selection’ simply means that nature selects one characteristic of an organism out of several characteristics as that characteristic is bound to give it a better chance to survive in the wild. (It’s like a teacher selecting the best student to participate in a competition!)

Darwin’s theory of natural selection

Charles Darwin- the famous naturalist has submitted the following points in his book ‘the origin of species’

Stage Notes
Variation Organisms in a population vary slightly amongst each other.
Over production Most organisms reproduce to give birth to offsprings that would survive to adulthood.
Struggle for existence There is considerable competition for survival between the organisms in non-favourable environmental conditions (such as famine).
Survival of the fittest Organisms who are well adapted to the environment will survive
Advantageous characteristics are passed on to the offsprings The organisms who survive reproduce till adulthood. The advantageous characteristics that helped them to survive are passed on to their offsprings
Gradual change over population for a period of time This continues over generations and soon the well adapted organism has a greater population than the other.

 

Let’s take an example of the cacti:

Stage Notes
Variation In a population of cacti,

·       Some have longer roots.

·       Some have roots spread over a large area.

Over production In the wet season, they flower and produce large numbers of offsprings.
Struggle for existence During dry season, there is competition for water.
Survival of the fittest The cacti with the longest roots are able to go deep and obtain water; so they survive

The cacti with roots spread over a large area die with dehydration.

Advantageous characteristics are passed on to the offsprings The long rooted cacti reproduce; producing offsprings more likely to be long rooted themselves!
Gradual change over population for a period of time The population of cacti with long roots increases; the population of cacti with roots spread over a large area decrease.

For better understanding, here is another example:

Antibiotic resistance in bacteria:

  • Penicillin works by stopping bacterial cells from forming cell walls.
  • When a person gets affected with a bacterial disease, he/she takes a dose of an antibiotic such as penicillin to kill the bacteria by stopping bacterial cells from forming cell walls.
  • This exerts a selection pressure if the same antibiotic is used again and again.
  • The probability that a mutant bacterium will be produced is low; however it is possible.
  • When a mutant bacterium (a bacteria with resistance against the antibiotic) forms, it has tremendous advantage.
  • The antibiotic kills all the bacteria except the mutant one.
  • The mutant (resistant) one multiplies and forms a population of resistant bacteria just like itself.
  • This causes the antibiotic to be ineffective against the resistant bacteria.

Stabilising selection

Many times, natural selection tends to keep the characteristics of organisms the same for generations to generations.

This is called as stabilising selection.

If

  • An organism is well adapted to it’s environment
  • The environment around it remains the same

Stabilising selection will begin to show its effects.

Coelacanths for example have remained unchanged for 350 million years. They live in a very stable environment in the depths of the Indian Ocean.

Sickle cell anaemia

Sickle cell anaemia is a genetic disease.

Some people have mutations in a gene that result in the production of haemoglobin

  • People with the normal allele HbA code for the production of haemoglobin
  • People with the mutant allele HbS code for the production of a faulty type of haemoglobin

The faulty type of hemoglobin can produce fibers inside RBCs when oxygen concentrations in the blood drop. These causes the RBC to become sickle shaped and get stuck in blood capillaries.

When this happens, the person is said to be suffering from sickle cell crisis as a blockage in the capillary forms. This can cause intense pain over hours and weeks! (Ouch!)

Damage caused by sickle cell crisis over a long time:

  • Kidneys get damaged
  • Liver gets damaged
  • Eyes get damaged
  • Heart gets damaged

A person having sickle cell anemia will always have breathlessness even when they don’t experience sickle cell crisis. This is because the faulty hemoglobin decreases the efficiency of the red blood cells in delivering oxygen to the cells in the body.
 

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