Variation & Natural Selection
Cambridge IGCSE Biology
The big idea: All organisms show variation because of genetic differences and environmental factors. Those best suited to their environment survive longer and reproduce more, passing on their genes to the next generation. This is natural selection—the mechanism that drives evolution.
Chapter Overview
- Variation in Biology: Differences between individuals (genetic vs environmental, continuous vs discontinuous)
- Mutations: Random genetic changes that increase variation in populations
- Adaptive Features: Inherited traits that help organisms survive and reproduce in their environment
- Natural Selection: The process by which organisms with beneficial traits are more likely to survive, reproduce, and pass on their genes
- Evolution: Change in the characteristics of a population over time as a result of natural selection
- Artificial Selection: Selective breeding by humans to develop desired traits in plants and animals
Variation in Biology
What Is Variation?
Variation is simply this: if you look at any group of organisms of the same species, they're not all identical. Two humans are never exactly alike. Two daisies in a field differ in size, leaf shape, and flowering time. This is variation—the reality of biological diversity.
Variation: Differences between individuals of the same species. These can be caused by differences in their genes (genetic variation) or by the environment they grow up in (environmental variation)—or usually, a combination of both.
Two Types of Phenotypic Variation
Phenotypic variation is the difference in visible features or characteristics between individuals. Scientists classify phenotypic variation into two major categories, and understanding the difference is absolutely crucial for exams.
Continuous Variation
Imagine measuring the height of everyone in your year group. You'd get a spread: someone is 152 cm, another 155 cm, another 158 cm, and so on, with many values in between. There are no distinct categories—height is a sliding scale. This is continuous variation.
Continuous variation: When a characteristic shows a wide range of small differences across a population. Individuals can have any value within a range. When you plot it on a graph, you get a smooth bell-shaped (normal) curve.
Examples: Height, mass, skin tone, intelligence, hand span.
Why? Continuous traits are usually controlled by many genes (polygenic inheritance) and heavily influenced by the environment. A person might have genes for tallness, but poor nutrition will limit their height. Both nature and nurture matter.
Height in humans: If you measure 100 Year 11 students, you won't get 3 or 4 different heights—you'll get a spectrum. Plot them, and you see a bell curve peaking around the average. Some outliers at both ends, but most cluster in the middle.
Discontinuous Variation
Now imagine sorting people by blood type: you get A, B, AB, or O. There's no "in-between" blood type. You either have free earlobes or attached earlobes—there's no partially-attached option. This is discontinuous variation.
Discontinuous variation:
Examples: Blood group (A, B, AB, O), sex (male or female), ability to roll your tongue (yes or no), presence or absence of horns.
Why?
Earlobe attachment: