Survival of the Fittest
Darwin's big idea, step by step: how random variation plus a struggle to survive slowly reshapes a whole species.
Revise this, the fun way
Play it interactively, earn XP and build a streak — free.
Start revising freeWhat you'll cover
Survival of the Fittest 🦎
Why does a species change over time? Charles Darwin worked out the answer — **natural selection** — and it is one of the biggest ideas in all of science. This module builds it up piece by piece, so you can lay out the whole chain of reasoning an exam rewards.
Everyone's different 🎲
The individuals in any population show **variation** — differences in their characteristics. This variation has three possible causes: • **Genetic** — the alleles they inherited. • **Environmental** — conditions during their life. • **Both** together — which is the case for most characteristics.
Where variation comes from
Pick the TWO genuine causes of variation between individuals.
- Genetic differences (the alleles they inherited)
- Environmental factors during their life
- The organism choosing to change itself
- The time of day it was born
The source of the new 🧬
Where do brand-new alleles come from? **Mutations** — random changes to the DNA. They happen **continually**. Most mutations have **no effect** on the phenotype; some influence it; very few determine it. But **rarely**, a mutation gives an advantage — and that is the raw material natural selection works on.
New alleles
What is the ultimate source of new alleles in a population?
- Mutations — random changes to the DNA
- The environment changing the DNA on purpose
- Organisms choosing to make new alleles
- Using a feature a lot during life
Darwin's theory 🌍
**Evolution** is a **change in the inherited characteristics of a population over time**, through natural selection — which may lead to the formation of **new species**. Darwin's theory of evolution by natural selection states that all species alive today evolved from simple life forms that first appeared over **three billion years** ago.
How natural selection works 🔗
Here is the chain of reasoning — learn every link: 1. A population shows **variation**. 2. Organisms face a **selection pressure** (competition, predators, a changing environment). 3. Those with **advantageous** characteristics **survive** and **reproduce**. 4. They pass on the advantageous **alleles**. 5. Over many **generations**, those alleles become more common — the species changes.
Order the process
An interactive activity.
Say the key line
In natural selection, individuals with _____ characteristics are more likely to survive and _____, passing their _____ to the next generation.
Match the terms
- Variation
- Selection pressure
- Evolution
- A new species
- Differences between individuals in a population
- Something that makes survival harder, e.g. predators
- A change in a population's inherited characteristics over time
- What can form after very long timescales of evolution
Getting it right
Which statement about natural selection is **correct**?
- Variation arises randomly (e.g. by mutation); organisms do not choose to adapt
- Animals grow the features they want in order to survive
- Using a body part more makes offspring inherit it stronger
- A whole species changes within a single generation
The long game
Over a very long time, what can evolution by natural selection eventually produce?
- A new species
- No lasting change at all
- Exact copies of the original organisms
- Organisms that never die
In the exam 🎓
Chain mastered. Grade-9 habits for evolution: • Give the **full chain**: variation → selection pressure → survival & reproduction of the advantaged → alleles passed on → frequency rises over generations. • **Mutations** are the source of new alleles; **variation is random** — never say organisms "want" or "try" to adapt. • Over very long timescales, natural selection can produce **new species**.