Variation and Selection

Variation and Selection

Variation is all the difference (in terms of physical characteristics) that exists between the members of the same species.

Genetic Variation, like the term suggests, is variation determined solely by Genes.

For example, skin colour is solely determined by Genetic Variation

Phenotypic Variation, is variation determined either by Genetic Variation, Environmental influence, or a combination of both Genetic Variation and Environmental Influences.

For example, a Sun Tan is an acquired characteristic, and depending on your skin colour, your skin may respond differently to the Tan. Classic Phenotypic Variation at work!

Phenotypic Variation can be divided into two types according to how the variation is being measured.

  1. Continuous Variation (affected by genes and the environment)
  2. Discontinuous Variation (affected only by genes)

Continuous variation

about 3000 people across the shibuya crossing at a time
they look like army of ants from this point

Continuous Variation results in differences in the features of a group of organisms in which there are no definite categories; each individual’s characteristics can lie anywhere between two extremes.


  1. Height πŸ“
  2. Weight πŸ‹οΈβ€β™€οΈ
  3. Finger length πŸ‘†
  4. Leaf length 🍁
  5. Heart beat rate πŸ’“

Discontinuous variation

Discontinuous Variation results in differences in the features of a group of organisms, where each individual fits into one of a few clearly defined categories.

For example, blood group. Every person has a specific blood group and there are no exceptions to it; Blood groups always have specific categories such as A, B, O, and AB.


Mutation is genetic change. New Alleles are formed as a result of Mutation.

Gene Mutation is the genetic change in the base sequence in DNA.

Causes of Mutation

Ionising radiation and some Chemicals (Such as Lead and Mercury) are main causes of Mutation.

Mutagens are defined as chemicals causing Mutation
  1. Ionising Radiation- exposure to Ultraviolet Rays, Gamma Rays, and X rays
  2. Chemical Mutagens- regular consumption of drinking water with heavy metal concentrations of Lead and Mercury

Sickle Cell Anaemia

Sickle Cell Anaemia is a genetic disease. It is caused when a person's gene that is responsible for the production of Haemoglobin undergoes Mutation.

The normal Allele HbA represents normal Haemoglobin
The mutant Allele HbS represents an allele that produces faulty Haemoglobin

  • The difference between the two alleles has to do with the change in amino acid sequence of the Haemoglobin produced, which can prevent it from working like as it should.
  • The faulty haemoglobin tends to produce fibres inside Red Blood Cells when Oxygen concentrations in blood are low.
  • The fibres cause Red Blood Cells to turn into a sickle shape, and get stuck in blood capillaries, restricting blood flow, and causing pain in the person body.

Symptoms of Sickle Cell Anaemia

A person with Sickle Cell Anaemia may frequently experience tiredness and short of breath; the faulty Haemoglobin is very inefficient when it comes to delivering oxygen, and reduces the oxygen carrying capacity of the lungs.

Inheritance of Sickle Cell Anaemia

The following genetic diagram demonstrates one case by which Sickle Cell Anaemia is inherited.

Parent 1
Parent 2
(Offspring with Normal Haemoglobin)
(Offspring with recessive sickle
cell anaemia allele)
(Offspring with recessive sickle
cell anaemia allele)
(Offspring with sickle cell anaemia)

In the case above, both the parents have recessive genes for Sickle Cell Anaemia, which brings the probability that their offspring will inherit Sickle Cell Anaemia down to 25%.

Sickle Cell Anaemia and Malaria

Malaria is a serious disease caused when a single celled parasite enters the blood through an infected mosquito's bite. It is the cause of death for millions around the world annually.

People who are heterozygous for the sickle cell allele (HBA HBS have a resistance to Malaria. This is because it seems that the parasite causing malaria cannot reproduce in cells with the sickle cell allele.

The presence of both Malaria and Sickle Cell Anaemia are present in common parts of the world. Those who were homozygous (with HBA HBA or HBS HBS) either died of Malaria, or Sickle Cell Anaemia. But those with who were heterozygous (with HBA HBS) were the ones who were most likely to reproduce. This continued from generation to generation.

Adaptive Features

Core: An Adaptive feature is an inherited characteristic that helps an organism to survive and reproduce in its environment.
Extended: An Adaptive feature is the inherited functional feature of an organism which increases its fitness.
Extended: Fitness is the probability of an organism surviving and reproducing in the environment in which it is found

Adaptive Features of Xerophytes

Plants living in hot arid conditions are known as Xerophytes. Xerophytes have adaptive features that supplement their survival in their environment.

Adaptation Function in increasing Fitness
Closing Stomata - Plants loose most water through stomata by transpiration β˜€οΈ
- Stomata close when it is very hot and arid, or when
photosynthesis is not possible (at night!) πŸ₯΅
- This helps conserve water by restricting plant growth
Waxy cuticle - The wax makes the leaf waterproof β˜”οΈ
- It cuts down transpiration rates πŸ“‰
Hairy Leaves - Some plants have thick growing hair present on their leaves.
- These help trap a layer of moist air next to the leaf. πŸ’¨
Stomata on
underside of
- Lower part of the leaf is usually cooler than the upper part🌑
- Less stoma means less transpiration, hence less growth. ⬇️
Leaves with
lower surface
- Cuts down transpiration rate (with a compromise on
photosynthetic rates), conserving water πŸ’§
Have deep or
spreading roots
- Deep roots can access water underground
- Spreading roots can access water over a large area

Adaptive Features of Hydrophytes

Plants living in water, and wet places are known as Hydrophytes. Like Xerophytes, Hydrophytes also have adaptive features that help them survive in their environment.

Adaptation Function in increasing Fitness
Roots not attached
to riverbed
- Help the plant to float freely without the need of anchorage β›“
Leaf stalks and stem
have hollow air spaces
- Increases buoyancy 🎈
- Helps the plant float above water
- Enables plant to get plenty of light for photosynthesis as
they float above water
Stomata on both
Helps absorb carbon dioxide for photosynthesis ⚑️
Thin or no cuticle As there is no need of conserving water, a cuticle is not needed 🌊


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

The term β€˜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!)

Process of Adaptation

The process, resulting from natural selection, by which populations become more suited to their environment over many generations is defined as the process of adaptation.

Selective Breeding (Artificial Selection)

The choice by a farmer or grower of only the β€˜best’ parents to breed, generation after generation is defined as Selective Breeding.

For example, if a dairy farmer wants to increase the overall milk yield of his cows,

  1. He will first select a cow which gives the highest volume of milk πŸ₯› in the herd and then breed it with the strongest bull he owns πŸ’ͺ.
  2. This is what the term β€˜best’ means in the definition.
  3. Thus the offsprings produced will have abilities to produce vast volumes of milk if they are female and great strength to plough the land if they are male.
  4. This will continue for generations until he has a herd of the highest milk yielding cows and the strongest bulls πŸ“ˆ!

This is how artificial selection has been utilised since generations to breed the best cattle and the best plants.

Antibiotic Resistance in Bacteria

  • Penicillin works by stopping bacterial cells from forming cell walls.
  • When a person gets affected with a bacterial disease, they take 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.

Difference between Natural Selection and Artificial Selection

  • Natural Selection takes place over a larger time frame than artificial selection.
  • This is because advantageous characteristics in Natural Selection are those that support a species long term survival in its environment.
  • In comparison, in Artificial selection, advantageous characteristics are selected by humans breeding species that might increase their economic value.
  • Artificial selection can occur over a faster period of time depending on the species being selectively bred.
  • The downside to Artificial Selection is that it eliminates variation, making species more vulnerable to new diseases, and changes in the environment.

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