Variation refers to the differences in traits (such as size, color, shape) among individuals within a population. These variations can be classified into two broad categories: continuous variation and discontinuous variation. These two types of variation differ in how the traits are expressed and how they are inherited.
1. Continuous Variation:
Continuous variation refers to differences in traits that show a range of small differences within a population. The variations are usually quantitative and can be measured along a spectrum or scale. These traits are typically influenced by multiple genes (polygenic inheritance) and can also be affected by environmental factors.
Key Characteristics of Continuous Variation:
- Traits show a range of values (e.g., height, weight, skin color).
- These traits do not fall into distinct categories but rather vary gradually from one extreme to the other.
- Polygenic inheritance (involvement of multiple genes) and environmental factors play significant roles in producing continuous variation.
- The frequency of individuals exhibiting a specific trait tends to form a normal distribution (bell curve) when plotted on a graph.
Examples of Continuous Variation:
- Height in humans: Human height varies from very short to very tall, and there are many intermediate heights in between. It is determined by multiple genes (polygenic inheritance) as well as environmental factors like nutrition.
- Skin color: Human skin color shows a range of shades from very light to very dark. This is also controlled by multiple genes and influenced by environmental factors like exposure to sunlight.
- Seed weight in plants: In a population of plants, the weight of seeds might vary gradually, with some plants producing heavier seeds and others producing lighter ones.
Graphical Representation:
If you were to graph the height of individuals in a population, the results would form a bell-shaped curve, with the average height in the middle and fewer individuals at the extremes (very short or very tall).
2. Discontinuous Variation:
Discontinuous variation refers to differences in traits that fall into distinct categories or groups with no intermediate forms. These traits are typically controlled by single genes (monogenic inheritance) and do not overlap. Discontinuous traits are qualitative, meaning they are expressed as either one form or another, with no gradual variation between them.
Key Characteristics of Discontinuous Variation:
- Traits fall into distinct categories (e.g., blood type, flower color).
- The variation is typically controlled by a single gene or a few genes.
- These traits do not show a range of intermediate forms, and individuals will exhibit one of the distinct categories.
- Environmental factors typically have little to no effect on the trait.
Examples of Discontinuous Variation:
- Blood group in humans: Human blood groups (A, B, AB, and O) fall into distinct categories. Individuals have one of these blood types and cannot have an intermediate or blending of two types.
- Pea plant flower color: In Mendel’s pea plant experiments, flower color was an example of discontinuous variation, with the plants either having purple or white flowers, with no intermediate color.
- Gender: In most species, gender is determined by distinct categories: male or female, with no intermediate forms.
Graphical Representation:
If you were to graph a trait like blood type, you would get distinct, separate bars representing each blood group. There would be no overlap between the categories, unlike in continuous variation.
Comparison between Continuous and Discontinuous Variation:
| Feature | Continuous Variation | Discontinuous Variation |
|---|---|---|
| Trait Expression | Shows a range of values (e.g., height, skin color). | Shows distinct categories (e.g., blood group, flower color). |
| Genetic Control | Controlled by multiple genes (polygenic inheritance). | Controlled by one or a few genes (monogenic inheritance). |
| Effect of Environment | Environment can influence the expression of the trait. | Environment typically has little effect on the trait. |
| Intermediate Forms | Intermediate forms exist (e.g., varying heights). | No intermediate forms (e.g., either A or B blood type, not a mix). |
| Graphical Distribution | Forms a bell-shaped curve (normal distribution). | Results in distinct categories with no blending. |
| Example | Height, weight, skin color, seed size. | Blood type, flower color, gender. |