12.1 - Types and Factors of Variation

Definition of Variation

Variation refers to the different characteristics present in organisms from a population or the same species. The effects of structural, physiological and biochemical changes can result in individual variations. No same individuals on this Earth are identical, not even identical twins.

The Necessity of Variation for the Survival of Species

• Variation is the basis of natural selection and important part in evolution of organisms.
• Natural selection is an evolutionary force that selects beneficial genes and eliminate non-beneficial genes.
• Natural selection enables species with certain phenotypes to adapt to the surrounding environment in order to live and breed for the survival of the species.

1. Allows good and beneficial characteristics be selected naturally and continue to be passed down. Non-suitable and non-beneficial characteristics will be eliminated after several generations.
2. Ensure the survival of the species when the surrounding condition changes.
3. Allows cross breeding between species to form new species.

Figure 1 - Biston betularia with light color body is easily detected by predators in a darker area compared to dark color Biston betularia. On the other hand, Biston betularia with a dark color body is easily detected in a brighter area compared to light color Biston betularia.

Types of Variation

There are 2 types of variation present in a natural population which are

• Continuous variation
• Discontinuous variation

Continuous variation

• Indistinct differences in the characteristic.
• When the data is plotted on a graph, a normal distribution or bell-shaped curve is formed.
• Quantitative , where it can be measured from one end of extreme to another end.
• Examples of characteristics that show continuous variation are height, body weight and skin color.

Figure 2 - Parameters such as height and body weight will give a bell-shaped graph.

Discontinuous variable

• Distinct differences in the characteristics.
• When the data is plotted in a graph, a discrete distribution or bar chart with separate bars are formed.
• Qualitative , where it cannot be measured due to genetic factors. Therefore, these variations can be inherited.
• Not influenced by environmental factors.
• Controlled by a single gene which consists of two or three alleles.
• Examples of characteristics that show discontinuous variation are ability to roll tongue, eye color and fingerprint pattern.

Figure 3 - Graph with separate bars.

Comparison between Continuous and Discontinuous Variation

• Similarities

  • Both show characteristics differences among organisms of the same species.

• Differences

  • Continuous variation

    • Characteristics differences: Not distinct
    • Intermediate characteristics: Present
    • Graph shape: Normal distribution
    • Characteristics controlled by: Many genes
    • Influence of environmental factors: Yes
    • Quantitative/Qualitative: Quantitative

  • Discontinuous variation

    • Characteristics differences: Distinct
    • Intermediate characteristics: Absent
    • Graph shape: Taburan diskrit
    • Characteristics controlled by: A single gene
    • Influence of environmental factors: No
    • Quantitative/Qualitative: Qualitative

Causes of Variation

Differences in morphology, physiology and genetic of organisms are caused by internal as well as external factors.

• Continuous variation

  • Environmental factors
  • Examples are temperature, light, pH value

• Discontinuous variation

  • Genetic factor
  • For instance, crossing over, independent assortment of chromosomes, random fertilization, mutation

Genetic factor

• During prophase I of meiosis, the process of crossing-over occurs at the non-sister chromatids of a homologous chromosome.
• This process results in a new combination of genes in the gametes.

• The random arrangement of the homologous chromosomes during metaphase I of meiosis will also produce a different genetic content in the gametes formed.

• The fertilization process of sperms and oocytes also happens at random.
• The product from the combination of genetics during the process of crossing-over and random arrangement of the homologous chromosomes during meiosis form gametes with different genetic content.
• Therefore, the zygote formed will also have different genetic combination.

• Mutation can also result in new genetic content. Mutation is the permanent changes on the gene or chromosome that happen spontaneously.
• A new genotype will be formed.
• If mutation occurs in the gametes, the characteristics formed will be inherited.
• Mutation in the somatic cells can lead to characteristics variation, but they are not inheritable.

Environmental factors

• Variations that can be affected by the environment are called environmental variations.

• Examples of environmental factors include abiotic components such as temperature, light and pH.

• These factors can only have an effect on the phenotypic changes.

• Environmental variation is not inheritable because it does not involve genotypic changes.

• pH value of soil

  - The flowers of *Hydrangea* sp. will turn blue when the soil is acidic (pH value less than 5.5)
  - The flowers will turn pink when the soil becomes alkaline.
  

• Temperature

  - Dark pigment enzymes in Siamese cat will only function when the temperature is lower than its body temperature.
  - Therefore, only parts of the body such as the ears, face, tail and paws will turn dark due to lower temperature.
  

• Light

• Sunlight can turn a person's skin dark. For example, an athlete.

• The ultraviolet rays from the sun destroy the melanin pigments on the skin exposed.

• Therefore, more new melanin pigments will be produced which make the skin darker.

Interactions between Genetic and Environmental Factors

Genetic and environmental factors can interact with one another and cause variation. One of the examples where the environmental factor acts on genetic factor is the case of an identical twins.

Figure 4 - The effect of environmental factor on the phenotypes of an identical twins. This shows that different diet and environment can result in varying phenotype.