Survival of the Fittest

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.

Where difference comes from

Every population shows **variation**: differences in the characteristics of its individuals. Two things cause it, and only one thing creates anything genuinely new.

Where variation comes from

A litter of puppies from the same two parents differ in several ways. Pick the TWO differences that have a GENETIC cause, at least in part.

• The colour of their coats
• How much each one weighs
• A scar one of them got in an accident
• The name each puppy is given
• The order in which they were born

New alleles

A bacterium in a colony turns out to carry an allele that none of its ancestors had. How did that allele arise?

• By a random mutation, a change to the DNA of the kind that happens continually
• The environment altered the bacterium's DNA deliberately to help it survive
• The bacterium produced the allele because the colony needed one
• The bacterium used one of its features heavily and passed the improvement on

Evolution, defined

**Evolution** is a change in the inherited characteristics of a population over time, through the process of **natural selection**, which may result in the formation of a **new species**.

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

Put the steps of evolution by natural selection into the correct order.

• Individuals in the population show variation
• A selection pressure makes survival a struggle
• Those with advantageous features survive and reproduce
• They pass on the advantageous alleles to their offspring
• Over many generations, those alleles become more common

Say the key line

In natural selection, individuals with _____ characteristics are more likely to survive and _____, passing their _____ to the next generation.

Rabbits and a new predator

A fast-running predator arrives on an island where rabbits live. Over many generations the rabbits become faster. Work out how, one link at a time.

• Before the predator arrived, were all the rabbits equally fast?
• What does the arrival of the predator represent in the process?
• Which rabbits are now more likely to survive and reproduce?
• What becomes of the alleles for greater speed over many generations?

Mark the error

This student answer about giraffes contains ONE statement examiners mark wrong. Tap it.

• The giraffes in the herd
• already varied in neck length
• . When the low leaves ran out,
• the shorter giraffes stretched their necks to reach higher
• , and
• the longer-necked ones survived and reproduced
• , so
• the alleles for long necks became more common

Match the terms

The long game

Two populations of the same beetle are separated by a new river for a very long time, and each is shaped by different selection pressures. What can eventually happen?

• They can become so different from each other that they form two separate species
• They stay identical to one another, because they started from one population
• They both evolve back into the ancestral beetle they originally came from
• They become two separate species within a single generation of the split

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**.

Write the six-marker

A species of moth living on pale tree bark is mostly pale, with a few dark individuals. Soot from nearby factories blackens the bark. Within fifty years almost every moth is dark. Explain how that happened, by natural selection.

• Start with the variation that was already there before the bark changed.
• Name the selection pressure, and say which moths it now favours.
• Say which moths survive and reproduce, and why.
• Say what is passed on: alleles, never an acquired feature.
• Finish with the change in allele frequency over many generations.