Study Notes: Variation (Biology 9201)

Hello Biologists! Welcome to the fascinating world of Variation. This chapter is super important because it answers a fundamental question: Why are we all different? Understanding variation is the foundation for understanding evolution, so let's get started. Don't worry if this seems tricky at first; we will break it down step-by-step!

1. What is Variation?

Variation simply means the differences that exist between individuals or groups of organisms. If you look around your classroom, everyone is unique! That uniqueness is variation in action.

Key Definitions
  • Variation: Differences in characteristics shown by individuals within a species or population.
  • Species: A group of organisms that can reproduce with one another to produce fertile offspring.

Why is Variation Important?
Variation provides the raw material for evolution. If everyone was exactly the same, a sudden change in the environment (like a new disease) could wipe out the entire species. Because there is variation, some individuals will have characteristics that help them survive and reproduce.

Quick Review: Variation = Differences. These differences help species survive changing conditions.

2. Types of Variation

We classify variation into two main types based on how the characteristics can be measured or grouped.

2.1. Discontinuous Variation (Clear Categories)

Discontinuous variation refers to characteristics that fall into distinct, clear-cut categories, with no in-between values. Think of it like a light switch—it’s either ON or OFF.

  • Features: The data can be placed into distinct groups.
  • Control: Usually controlled by one or a few genes (little to no environmental influence).
  • Graph: When plotted, you use a bar chart (separate bars).

Examples of Discontinuous Variation:

  1. Human Blood Groups (A, B, AB, O): You cannot have a blood group that is halfway between A and O.
  2. Eye Colour: Blue, brown, green, etc.
  3. Ability to Roll Your Tongue: You either can or you can’t.
2.2. Continuous Variation (A Range of Values)

Continuous variation refers to characteristics that can take any value within a range. Think of it like a dimmer switch—it can be slightly dim, moderately bright, or fully bright.

  • Features: The data changes gradually from one extreme to the other. There are many intermediate values.
  • Control: Usually controlled by many genes (called polygenic inheritance) and is often strongly influenced by the environment.
  • Graph: When plotted for a large population, it typically forms a smooth, bell-shaped curve called a normal distribution. You use a histogram (bars touching).

Examples of Continuous Variation:

  1. Height in Humans: People can be 1.5m, 1.51m, 1.515m, and so on.
  2. Weight/Mass: Varies along a scale.
  3. Leaf Length: In plants, leaves can be slightly longer or slightly shorter.

Memory Tip:
Continuous = Curve (bell shape)
Discontinuous = Distinct categories (separate bars)

3. The Sources of Variation

Where do these differences come from? Variation is caused by two main factors: the genes you inherit, and the environment you live in.

3.1. Genetic Variation (Inherited)

Genetic variation is the difference in characteristics caused by the genes inherited from your parents. These are the differences you are born with.

  • Genes: Units of inheritance passed from parents to offspring.
  • Genotype: The specific set of genes an individual possesses (the genetic code).

Sources of Genetic Variation:

  1. Inheritance: Receiving a mix of genes from two parents (sexual reproduction). This creates new combinations of traits.
  2. Mutation: This is the ultimate source of new variation. A mutation is a random change to the genetic material (DNA). Mutations can be neutral, harmful, or occasionally beneficial.

Did you know? All the genetic differences between humans and chimpanzees began with mutations millions of years ago!

3.2. Environmental Variation (Acquired)

Environmental variation is the difference in characteristics caused by the surroundings, diet, or lifestyle of the organism. These are traits that are acquired (developed) during life.

  • Features: These characteristics are generally not passed on to the next generation.
  • Examples:
    • A scar from an injury.
    • Changes in height due to poor childhood nutrition.
    • A person developing muscles from lifting weights.
    • The colour of flowers changing due to the acidity of the soil (e.g., hydrangea flowers).

Common Mistake to Avoid: Don't confuse acquired traits with inherited traits. If you dye your hair purple, your children won't automatically inherit purple hair! That trait was acquired through the environment (hair dye).

4. How Genes and Environment Interact

Most characteristics are not purely genetic or purely environmental. They are usually a result of both factors working together.

  • Phenotype: This is the characteristic you actually observe (what the organism looks like or how it behaves).
  • Phenotype = Genotype + Environment

Analogy: The Cake Recipe
Imagine baking a cake.
The recipe (Genotype) tells you the ingredients and potential size.
The oven temperature and quality of ingredients (Environment) determine how good the cake (Phenotype) actually turns out.

If you have genes for being tall (a good recipe), but you suffer from severe malnutrition as a child (poor ingredients/oven), your final height (Phenotype) might be shorter than your genetic potential.

Example: Identical Twins
Identical twins have the exact same genes (same genotype). However, if they live in different countries or lead different lifestyles, they will show differences in weight, fitness, or even disease patterns. These differences are due to environmental variation.

4.1. The Role of Variation in Survival

Variation is essential because it is the driving force behind Natural Selection (which you will cover in detail in the next chapter on Evolution).

The Survival Advantage:

  1. Within a population, there is variation (differences).
  2. If the environment changes (e.g., gets colder, food becomes scarce), some individuals will have characteristics that make them better suited to survive.
  3. These better-suited individuals are more likely to live long enough to reproduce and pass their beneficial traits (genes) on to the next generation.

Example: In a population of rabbits, some have slightly thicker fur (variation). If a long, cold winter hits, the rabbits with thicker fur are more likely to survive and produce offspring. This leads to the population gradually changing over many generations.

Key Takeaway for Evolution: Variation means that organisms are not equally likely to survive and reproduce, which leads to species changing over time.


Chapter Summary: Quick Review

The Big Picture:

  • Variation is the differences between individuals.
  • Discontinuous Variation: Traits in distinct categories (e.g., blood group). Controlled mainly by genes.
  • Continuous Variation: Traits showing a range of values (e.g., height). Controlled by many genes and the environment.
  • Genetic Variation (Inherited) comes from genes passed down and mutation.
  • Environmental Variation (Acquired) comes from diet, surroundings, or lifestyle.
  • The observed trait (Phenotype) is usually a combination of both genes and environment.