12.1 - Types and Factors of Variation

Definition of Variation

• Variation: Differences in characteristics found within the same population or species.
• No two organisms are the same, even organisms of the same species and identical twins.
• Variation normally refers to physical characteristics observed in phenotypic differences caused by changes in structure, physiology and biochemistry.
• Variation enables individuals within a population to be identified.

Necessity of Variation for the Survival of Species

• Variation is important in evolution and forms the basis of natural selection.
• Natural selection is an evolutionary force that selects beneficial genes and removes non-beneficial genes from the natural environment.
• Species with phenotypes that enable adaptation to their surroundings survive and breed.
• Variation:
  • enables the natural environment to continue selecting beneficial characteristics and remove non-suitable ones;
  • ensures survival of the species when the environment changes; and
  • allows cross-breeding among species to form new species.

Types of Variation

• The two types of variation are continuous variation and discontinuous variation.

Continuous Variation

• Differences in a characteristic are not distinct.
• Individuals show gradual differences from one extreme to the other; a spectrum of phenotypes is observed.
• A plotted data set produces a normal distribution or bell-shaped curve.
• Most individuals have intermediate phenotypes.
• Characteristics are quantitative: they can be measured and graded.
• Characteristics are influenced by environmental factors.
• Examples: height, body weight and skin colour.

Discontinuous Variation

• Differences in a characteristic are distinct.
• A plotted data set produces a discrete distribution or bar chart with separate bars.
• There are no intermediate characteristics.
• Characteristics are qualitative: they cannot be measured or graded.
• Characteristics are determined by genetic factors, are not influenced by environmental factors and can be inherited.
• A characteristic is controlled by a single gene with two or three alleles.
• Examples: ability to roll tongue, eye colour and fingerprint pattern.

Comparison between Continuous and Discontinuous Variation

Similarities

• Both show differences in characteristics among individuals of the same species.

Differences

Characteristic	Continuous variation	Discontinuous variation
Differences	Not distinct	Distinct
Intermediate characteristics	Present	Absent
Graph	Normal distribution	Discrete bars
Genetic control	Many genes	One single gene
Environmental influence	Influenced	Not influenced
Data	Quantitative	Qualitative

Sources and Factors of Variation

• Continuous variation is caused by environmental factors.
• Discontinuous variation is caused by genetic factors.
• Genetic factors: crossing over, independent assortment of chromosomes, random fertilisation and mutation.
• Environmental factors: temperature, light and pH.

Genetic Factors

• Crossing over:
  • occurs between non-sister chromatids of homologous chromosomes during prophase I of meiosis;
  • occurs at the chiasma, where non-sister chromatid segments are exchanged;
  • recombination produces new combinations of genes; and
  • sister chromatids separate during anaphase II, forming gametes with different genetic materials.
• Independent assortment of chromosomes:
  • one maternal and one paternal chromosome in each homologous pair arrange randomly on the equatorial plane during metaphase I; and
  • meiosis produces gametes with different combinations of paternal and maternal chromosomes.
• Random fertilisation:
  • fertilisation between a sperm and a secondary oocyte is random;
  • crossing over and random arrangement of homologous chromosomes produce gametes with genetic contents different from their parents; and
  • fertilisation produces a diploid zygote with a new genetic combination.
• Mutation:
  • is a permanent change that occurs spontaneously in genes or chromosomes;
  • creates new genotypes;
  • a germ-cell mutation in a gamete can be inherited; and
  • a somatic-cell mutation causes variation but cannot be inherited by the next generation.

Environmental Factors

• Variation caused by environmental factors is environmental variation.
• Abiotic factors that cause variation include temperature, light and pH.
• Environmental variation involves phenotypic differences, not genotypic differences, and cannot be inherited.
• Soil pH: Hydrangea sp. produces blue flowers in acidic soil (pH less than 5.5) and pink flowers in alkaline soil.
• Temperature: A Siamese cat inherits a gene that produces the dark pigment enzyme for fur colour. The enzyme functions only below body temperature, making the cooler ears, face, tail and paws dark.
• Light: Long exposure to sunlight produces a darker skin tone. Ultraviolet rays damage melanin in exposed skin; more melanin is then produced, causing the skin to darken.

Interaction between Genetic and Environmental Factors

• Environmental factors interact with genetic factors to cause variation and determine phenotypes.
• Inherited characteristics such as height, intelligence and skin colour are greatly influenced by environmental factors.
• Identical twins J and K have a similar genotype for obesity because they originated from the same embryo.
• J follows a low-fat diet and exercises; K follows a high-fat diet and does not exercise.
• Different eating habits and environments while growing up produce different body-weight phenotypes.