# Natural Selection and Evolution

> Darwin and Wallace's theory of evolution by natural selection, its steps, how new species form, and patterns of divergent and convergent evolution.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/natural-selection-and-evolution/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Cane toads with longer legs
In 1935, about 100 cane toads were released in Queensland to eat beetles in sugar cane fields. They ate everything except the beetles and spread across northern Australia.

Scientists measuring the toads at the invasion front found something odd. The toads leading the advance had **longer legs** than toads back in Queensland. Each year the front was moving faster: about 10 km a year at first, over 50 km a year by the 2000s. The fastest toads reached new land first, bred with each other, and passed on their long legs. Evolution was happening in front of our eyes, in less than a century.

**In short:** Evolution is the change in the inherited features of a population over many generations, driven mainly by natural selection. Individuals vary, those with features that suit the environment survive and reproduce more, and they pass those features on, so the population changes.

## Starting point: what you already know

- Offspring inherit features from their parents through DNA.
- Individuals of the same species are not all the same: they show **variation**.
- Adaptations help organisms survive (see the previous chapter).

## Darwin and Wallace

In the 1830s, Charles Darwin sailed around the world on HMS Beagle, including a visit to Sydney. On the Galápagos Islands he noticed that finches on different islands had different beaks suited to their food. In the 1850s, Alfred Russel Wallace came to the same idea in South-East Asia. In 1858 their ideas were presented together, and in 1859 Darwin published *On the Origin of Species*.

**Evolution** is the change in the inherited features of a population over many generations.

## Natural selection, step by step

### Key idea: The steps of natural selection
1. **Variation**: individuals in a population differ, and some differences are inherited.
2. **Overproduction**: more offspring are born than can survive.
3. **Selection pressure**: there is a struggle for survival (predators, food shortage, disease).
4. **Survival of the fittest**: individuals with helpful variations are more likely to survive and **reproduce**.
5. **Inheritance**: they pass the helpful features to their offspring.
6. **Change over time**: over many generations, the helpful feature becomes more common in the population.

"Fittest" does not mean strongest. It means best suited to the environment, with the most offspring.

### Worked example: Antibiotic resistant bacteria
A person with an infection takes an antibiotic. Most bacteria die, but a few have a mutation that lets them survive. They multiply. If the course is stopped early, the resistant ones remain and spread. After many cycles, a population of mostly **resistant** bacteria exists. This is natural selection, and it is why golden staph resistant to many antibiotics is now a serious problem in hospitals.

### Watch out: Individuals do not evolve
A single giraffe does not stretch its neck and pass the longer neck to its babies. **Populations** evolve, because individuals with helpful inherited variations leave more offspring. Changes made during an animal's life are not inherited.

## How new species form

A **species** is a group of organisms that can breed together to produce **fertile** offspring. A horse and a donkey can breed, but the mule they produce is infertile, so they are different species.

**Allopatric speciation** (speciation in separate places):

1. A population is split by a barrier: a mountain range, a river, rising sea.
2. The two groups cannot breed with each other (**isolation**).
3. Different selection pressures act on each group, and different mutations arise.
4. Over a long time they become so different that they can no longer interbreed even if they meet. They are now **two species**.

When Bass Strait flooded about 12 000 years ago, populations in Tasmania were cut off from the mainland and began to change separately.

## Patterns of evolution

| Pattern | What happens | Example |
|---|---|---|
| **Divergent** | One ancestor gives rise to many species suited to different ways of life (**adaptive radiation**) | Marsupials radiated into kangaroos, wombats, koalas and possums |
| **Convergent** | Unrelated species in similar environments evolve similar features | The extinct thylacine (a marsupial) and the wolf (a placental mammal) look very alike |

**Mind map**

```text
Natural selection
  People
    Darwin: Galápagos finches
    Wallace: South-East Asia
  Steps
    Variation
    Overproduction
    Struggle to survive
    Fittest reproduce
    Inheritance
    Change over generations
  Speciation
    Barrier splits population
    Different pressures
    Cannot interbreed
  Patterns
    Divergent: adaptive radiation
    Convergent: unrelated look alike
```

### Year 11: Punctuated equilibrium, microevolution and macroevolution
**Gradualism** says species change slowly and steadily. **Punctuated equilibrium** says species stay much the same for long periods, then change quickly (in geological terms) when the environment changes. The fossil record shows examples of both.

**Microevolution** is a small change within a species, like longer legs in cane toads. **Macroevolution** is large change over long times that forms new species and groups.

Other modern examples of natural selection you can use in exams:

- Rabbits becoming resistant to myxoma virus.
- Insects becoming resistant to pesticides.
- The Australian **sea snakes** that evolved from land snakes and now have paddle tails.

### Year 12: The modern view
Darwin did not know about genes. The **modern synthesis** joins natural selection with genetics: mutations make new **alleles**, meiosis shuffles them, and natural selection changes **allele frequencies** in a population's gene pool. In Module 6, **genetic drift** (chance changes in small populations) and **gene flow** (migration) also change allele frequencies.

## Quick summary

- Evolution is the change in inherited features of a population over generations.
- Natural selection: variation, overproduction, struggle, survival and reproduction of the fittest, inheritance, change over time.
- Populations evolve, not individuals. Features gained during life are not inherited.
- Allopatric speciation: a barrier isolates groups, they change separately and can no longer interbreed.
- Divergent evolution makes many species from one ancestor. Convergent evolution makes unrelated species look alike.

## Quiz

Try these questions from the chapter quiz.

1. Who proposed the theory of evolution by natural selection at the same time as Darwin?
   - A. Mendel
   - B. Wallace
   - C. Pasteur
   - D. Hooke
   - Answer: B. Wallace. Alfred Russel Wallace reached the same idea independently.

2. What does "fittest" mean in "survival of the fittest"?
   - A. Strongest
   - B. Fastest
   - C. Best suited to the environment, leaving the most offspring
   - D. Largest
   - Answer: C. Best suited to the environment, leaving the most offspring. Fitness is about survival and reproduction.

3. Which must exist for natural selection to happen?
   - A. Inherited variation in a population
   - B. All individuals being identical
   - C. No competition
   - D. Unlimited food
   - Answer: A. Inherited variation in a population. Without variation there is nothing to select.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/natural-selection-and-evolution/#quiz