12.1 Types and Factors of Variation
Variation refers to the differences in characteristics observed among individuals of the same species or population. It enables the identification of individuals and plays a fundamental role in evolution and survival through natural selection.
Learning Objectives
- Define variation and explain its necessity for the survival of species.
- Differentiate between continuous variation and discontinuous variation based on their characteristics, graphs, and examples.
- Explain the genetic factors that cause variation, including crossing over, independent assortment of chromosomes, random fertilisation, and mutation.
- Describe the environmental factors that cause variation, including temperature, light, and soil pH.
- Describe how interactions between genetic and environmental factors affect phenotypes, such as in identical twins.
Comparison of Continuous and Discontinuous Variation Curves and Influencing Factors
Continuous variation forms a bell-shaped normal distribution curve, while discontinuous variation forms separate discrete bars. Each type is connected to different genetic and environmental causes.
Continuous
Quantitative, measurable, intermediate phenotypes present, normal distribution curve.
Discontinuous
Qualitative, distinct categories, no intermediates, discrete bar chart.
Genetic + Environmental
Genetic factors cause discontinuous variation; environmental factors influence continuous variation.
Short Explanation
Definition and Necessity of Variation
Variation refers to the differences in characteristics found within the same population or species. It is observed in physical characteristics resulting from changes in structure, physiology, and biochemistry. Variation plays an important role in evolution and forms the basis of natural selection, which selects beneficial genes and eliminates non-beneficial ones. This ensures the survival of a species when environmental conditions change.
Continuous Variation
Differences in characteristics are not distinct; individuals display gradual differences from one extreme to the other. It is quantitative, controlled by many genes, greatly influenced by environmental factors, and forms a normal distribution curve. Examples: height, body weight, skin colour, intelligence.
Discontinuous Variation
Shows distinct, clear-cut differences without intermediate phenotypes. It is qualitative, controlled by a single gene with two or three alleles, not influenced by environmental factors, and forms a discrete distribution bar chart. Examples: ABO blood group, ability to roll the tongue, fingerprint pattern, eye colour.
Genetic Factors
Genetic factors produce variations that can be inherited across generations. Crossing over occurs between non-sister chromatids during prophase I of meiosis. Independent assortment occurs during metaphase I. Random fertilisation fuses two genetically unique haploid gametes. Mutation is a spontaneous, permanent change in the structure of a gene or chromosome. Germ cell mutations can be inherited; somatic cell mutations cannot.
Environmental Factors
Variations caused solely by the environment are termed environmental variations. They are influenced by abiotic factors such as temperature, light, and soil pH. They affect only phenotypes without altering the genotype and cannot be inherited. Examples: Hydrangea sp. flower colour changes with soil pH; Siamese cat dark fur develops on cooler extremities; UV radiation causes tanned skin.
Interactions between Genetic and Environmental Factors
Environmental factors interact with genetic factors to determine final phenotypes. Identical twins share the same genotype because they develop from a single embryo. When separated and exposed to different environments, differences in lifestyle and environment produce different phenotypes despite identical genetic composition.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What type of distribution graph represents discontinuous variation, and why?
Why can a mutation in a germ cell be passed on to the next generation, whereas a mutation in a somatic cell cannot?
Explain how soil pH affects the flower colour of Hydrangea sp.
Common Mistakes
- Confusing the graphs: drawing or identifying a bell-shaped continuous line for discontinuous traits like blood groups or tongue rolling.
- Classifying body weight or skin colour as discontinuous variation simply because they can be categorized into groups; these are continuous because they form a spectrum and are quantitative.
- Assuming that all mutations are inherited; only germ cell mutations can be inherited, while somatic cell mutations are not passed to offspring.
- Stating that environmental factors alter an individual's genotype; environmental factors change only phenotypes and allele frequencies in populations, not an individual's underlying genetic code.
Concept Misunderstandings
Identical twins are completely identical in every physical characteristic throughout their lives.
Although identical twins share identical genotypes, their phenotypes can diverge significantly over time due to differences in environmental influences such as diet, physical activity, and sun exposure.
Discontinuous variation can be altered by environmental factors if exposure is prolonged.
Discontinuous variation (such as blood group or fingerprint pattern) is strictly controlled by genetics (a single gene) and cannot be changed by environmental conditions.
Summary
- Variation represents differences in characteristics among individuals of the same species.
- It provides the basis for natural selection, enabling species to adapt and survive environmental changes.
- Continuous variation is quantitative, shows intermediate phenotypes, is controlled by many genes, is influenced by the environment, and forms a normal distribution (e.g., height, weight).
- Discontinuous variation is qualitative, shows distinct classes with no intermediates, is controlled by a single gene, is independent of the environment, and forms a discrete distribution (e.g., ABO blood groups, tongue rolling).
- Genetic causes of variation include crossing over (prophase I), independent assortment (metaphase I), random fertilisation, and mutation.
- Environmental causes include temperature, light, and soil pH, which interact with genetic factors to determine an individual's final phenotype.
Short Activity
Objective: identify types of variation, genetic mechanisms, and environmental influences.
A. Objective Quiz
Which of the following characteristics exhibits continuous variation in humans?
At which stage of meiosis does crossing over occur between non-sister chromatids of homologous chromosomes?
B. Fill in the Blanks
When continuous variation data is plotted on a graph, it forms a bell-shaped curve known as a distribution.
The plant Hydrangea sp. produces blue flowers when grown in soil with a pH of less than .
C. Matching / Drag and Drop
Drag each factor or characteristic to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Factor or Characteristic to Its Description
Write the correct factor or characteristic next to each description.
| Description | Factor / Characteristic |
|---|---|
| 1. Random arrangement of maternal and paternal homologous chromosome pairs at the equatorial plane during metaphase I | |
| 2. Qualitative variation showing distinct categories with no intermediate phenotypes and controlled by a single gene | |
| 3. A permanent change in the genetic material of gametes that can be inherited by the next generation | |
| 4. Quantitative variation displaying a spectrum of phenotypes influenced by both polygenes and environmental factors |
Answer choices: Independent assortment • Discontinuous variation • Germ cell mutation • Continuous variation
