Variation

Introduction

Variation refers to the differences between individuals of the same species. This variation is crucial for evolution, as it provides the raw material upon which natural selection can act. Variation can be broadly classified into two main types: discontinuous variation and continuous variation. The genetic basis of these variations differs significantly.

Discontinuous Variation

Definition

Discontinuous variation is characterized by distinct, non-overlapping categories. Individuals fall into one category or another, with no intermediate phenotypes.

Genetic Basis

Discontinuous variation is typically determined by **single gene** inheritance patterns. These genes often show **multiple alleles**, leading to distinct phenotypes. The inheritance patterns are often Mendelian.

Examples

• Blood groups (A, B, AB, O)
• Sex (Male, Female)
• Blood type (e.g., AB, A, B, O)
• Presence or absence of a particular trait (e.g., widow's peak)

Diagram

Table: Genetic Inheritance of Discontinuous Variation

Trait	Gene	Alleles	Inheritance Pattern
Blood Group	ABO gene	$I^A$, $I^B$, $i$	Multiple alleles, Mendelian inheritance
Sex	Sex chromosome gene	$X$, $Y$	Sex-linked inheritance

Continuous Variation

Definition

Continuous variation displays a range of phenotypes, often forming a bell-shaped curve when plotted on a graph. Individuals can have intermediate phenotypes.

Genetic Basis

Continuous variation is typically influenced by **multiple genes** (polygenic inheritance) and environmental factors. The effect of each gene is often small, and the interaction between genes contributes to the range of phenotypes.

Examples

• Height
• Weight
• Skin colour
• Reaction time

Diagram

Table: Genetic Inheritance of Continuous Variation

Trait	Genes Involved	Inheritance Pattern
Height	Multiple genes (polygenic)	Polygenic inheritance, influenced by environmental factors (nutrition)
Skin Colour	Multiple genes (polygenic)	Polygenic inheritance, influenced by environmental factors (sunlight)

Summary Table

Feature	Discontinuous Variation	Continuous Variation
Categories	Distinct, non-overlapping	Range of phenotypes, intermediate values
Number of Genes Involved	Typically single gene	Typically multiple genes (polygenic)
Inheritance Pattern	Often Mendelian	Polygenic inheritance, environmental influence
Examples	Blood groups, sex	Height, weight, skin colour