GCSE · Biology · AQA · Spec 8461

Evolution by natural selection

Here's a question that catches almost everyone out: can a single animal evolve? The answer is no. Once you see why, the rest of this topic falls into place.

Biology · Evolution

Rebuild natural selection, then break it

2 stages are missing. Use the rest of the loop to work out which goes where.

Stage 1: Variation
Individuals in the population already differ in their inherited characteristics. New variants come from mutations, which happen all the time, at random.
Stage 3: Survival and reproduction
The better-suited individuals are more likely to survive and breed, so they leave more offspring than the rest.
Stage 5: A changed population
The next generation has a higher proportion of the well-suited variant. It still varies, so the loop runs again, generation after generation.

…and stage 5 leads back to stage 1.

What do you really think?

How did the population get its thick fur?

Many generations ago, most animals in a population had short fur. Then the climate became colder. Today, most of the population has long, thick fur.

Which of these is closest to what you think happened?

Each animal grew thicker fur during its life to keep warm, then passed the thicker fur on to its young.The cold made the animals mutate, which gave them the long-fur gene they needed.Some animals already had longer fur, so they survived and bred more, and their young inherited it.In one cold winter every short-furred animal died, so from then on they all had long fur.

How sure are you?

I'm sureI think soI'm guessing

Each animal grew thicker fur during its life to keep warm, then passed the thicker fur on to its young.

This is a common misconception

This is the most common idea about evolution, and it's the one to let go of. A change an animal picks up during its life isn't written into its genes, so its young don't inherit it. Evolution doesn't happen inside one lifetime. It happens because some animals already had longer fur, coped better with the cold and left more offspring.

The cold made the animals mutate, which gave them the long-fur gene they needed.

This is a common misconception

You're right that mutations are where new variants come from. But the cold doesn't order them up. Mutations occur continuously and at random, whether or not they'd help. The environment doesn't create the long-fur variant. It only decides which of the variants already present do best.

Some animals already had longer fur, so they survived and bred more, and their young inherited it.

This matches the evidence

Yes. The long-fur variant was in the population before the climate changed. The cold made it the better-suited phenotype, so those animals were more likely to survive and breed. Their offspring inherited long fur, and over many generations it became the most common kind.

In one cold winter every short-furred animal died, so from then on they all had long fur.

This is a common misconception

Natural selection is about being more likely to survive and breed, not a guarantee. Some short-furred animals still survive and have young; they just leave fewer offspring on average. That's why the change builds up over many generations, as the proportion of long-furred animals rises.

Individuals don't evolve. Populations do. The variation has to be there first, and the environment only decides which variants leave more offspring.

Predict, then check

Think about who survives and breeds once the conditions have changed.

A population's environment changes. By chance, a mutation has already given a few individuals a new phenotype, and it happens to suit the new conditions. What do you predict?

One species becomes two

Reason it through

How can one species become two new species?

Link 1 of 4

First link · your turn

Two populations of the same species end up living in different conditions. What does natural selection do in each place?

Locked — reveal the link above first

Your turn

Write the whole chain

Explain how natural selection can change the characteristics of a population over many generations. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Individuals in the population show variation (new variants arise from mutations). (1 mark)
Some variants have a phenotype better suited to the environment. (1 mark)
These individuals are more likely to survive and reproduce. (1 mark)
They pass on the characteristic (their genes) to their offspring. (1 mark)
Over many generations the characteristic becomes more common in the population. (1 mark)

Model answer

Individuals in a population show variation, and new variants arise from mutations. Some variants have a phenotype that is better suited to the environment. These individuals are more likely to survive and reproduce. They pass on the genes for this characteristic to their offspring. Over many generations, the characteristic becomes more common, so the population's inherited characteristics change.

If your answer talks about one animal changing, rewrite it about the population. If it jumps from 'survive' straight to 'the population changes', add the breeding and the inheriting.

Exam line: Every link is a separate mark, so a missing link is a missing mark. Before you finish, check you've said the characteristic is passed on to the offspring.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Populations change because some of the variety already in them leaves more offspring, one generation after another.

What you need to know

Evolution is a change in the inherited characteristics of a population over time, through natural selection. It may result in a new species.
Natural selection acts on variation that's already there. Variants with phenotypes best suited to the environment survive and reproduce more, and pass on their characteristics.
Mutations occur continuously. Very rarely, a mutation leads to a new phenotype. If that phenotype suits a change in the environment, the species can change relatively rapidly.
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.
The theory of evolution by natural selection states that all species of living things evolved from simple life forms that first developed more than three billion years ago.

The big picture

Evolution is a change in the inherited characteristics of a population over time. It happens by natural selection: individuals already vary, those whose phenotypes suit the environment best are more likely to survive and reproduce, and they pass their characteristics on, so the population shifts generation by generation. New variants come from mutations, which occur continuously but very rarely produce a new phenotype. When two populations of one species become so different that they can no longer interbreed to produce fertile offspring, they have formed two new species.

Key points

1Individuals don't evolve. Populations do, as the mix of inherited characteristics shifts from one generation to the next.

2Natural selection needs three things: variation, a difference in who survives and breeds, and inheritance. Remove one and the population stops changing.

3The environment doesn't create new variants. It only decides which of the variants already present leave more offspring.

4The test for a separate species is whether two populations can interbreed to produce fertile offspring, not how different they look.

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Worked example

Problem

A population of plants grows in an area that has become much drier over many years. Some of the plants have deeper roots than others. Explain how the population is likely to change.

⚠ Watch out

Writing that an organism changed, or mutated, because it needed to survive. Mutations happen continuously and at random, and a change gained during an organism's life isn't inherited. Write about variants that were already there and about how the population changes.

🧠

Memory hook

Vary, suit, survive, breed, pass it on. The population moves on; the individual doesn't.

✓

Check yourself

Cover the page. Name the three things natural selection can't work without, and for each one say why the population would stop changing if it were missing.

Flashcards

(12)

What is evolution?

A change in the inherited characteristics of a population over time, through natural selection.

Can a single organism evolve?

No. Evolution is a change in a population across generations, not in one organism's lifetime.

Where do new variants in a population come from?

Mutations.

How often do mutations occur?

Continuously. They're happening all the time, at random.

How often does a mutation lead to a new phenotype?

Very rarely.

When can a mutation lead to a relatively rapid change in a species?

When the new phenotype it produces is suited to a change in the environment.

Does the environment cause the mutations that natural selection acts on?

No. It only affects which of the existing variants survive and reproduce more.

Why does a characteristic have to be inherited for natural selection to change a population?

Only inherited characteristics reach the offspring, so only they can become more common in the next generation.

Why might two populations of one species living in different conditions become different?

Natural selection favours different phenotypes in each environment, and the differences build up over many generations.

What is the test for whether two populations are still the same species?

Whether they can interbreed to produce fertile offspring.

What are fertile offspring?

Offspring that can themselves reproduce.

According to the theory of evolution by natural selection, where did all species come from?

Simple life forms that first developed more than three billion years ago.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Rebuild natural selection, then break it

How did the population get its thick fur?

Write the whole chain

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