GCSE · Biology · Edexcel · Spec 1BI0

Darwin's theory of evolution by natural selection

Two islands. One ancestor. Two completely different shell shapes. Nobody designed them and no tortoise chose them, so how did it happen?

Biology · Speciation

Follow the tortoises

Pick an island and follow what happened to the tortoises that ended up there.

Island conditions → Shell that became more common → Where it ends up

Some of this species end up geographically separated, living on different islands. Pick an island and see what natural selection does there.

2 island story.

On One species of tortoise on mainland South America. 2 branches to choose from.

Pick an island. Walk the route natural selection took there, then see where both routes end up.

Watch out: Nobody changed their shell. A shape became more common in the population because the tortoises that had it were better adapted to that island.

Biology · The steps

Put natural selection in order

Put the steps of natural selection in order, from first to last

1 · Happens first5 · Happens last

Survival: the advantaged are more likely to survive to reproductive age and have offspring
Spread: over generations the trait becomes more common in the population
Variation: by random chance, individuals have different traits
Inheritance: those individuals pass the trait on to their offspring
Advantage: some traits help individuals compete for finite resources

Watch out: Variation comes first and it is down to random chance. The environment does not ask for a trait and the organisms do not go and get it.

Biology · Tempting but wrong

Which explanation sounds like you?

A population lives in a place where one trait helps individuals survive. Three students explain why the population ends up better adapted.

Which explanation is closest to what you think right now?

The animals need the trait, so they adapt themselves to get it.Individuals vary by chance. The better-adapted ones are more likely to survive and reproduce, so each generation is better adapted.Each animal changes during its life to suit the place, then passes that change on.

How sure are you?

I'm sureI think soI'm guessing

The animals need the trait, so they adapt themselves to get it.

This is a common misconception

This is the big trap. Organisms do not choose to adapt themselves, and needing something does not produce it. Individuals already vary by random chance, and the better-adapted ones are simply more likely to survive and reproduce.

Individuals vary by chance. The better-adapted ones are more likely to survive and reproduce, so each generation is better adapted.

This matches the evidence

Yes. Chance variation, then the better-adapted individuals being more likely to survive and reproduce. Nothing has to choose anything, which is why this explains adaptation without any planning.

Each animal changes during its life to suit the place, then passes that change on.

This is a common misconception

Evolution is change in a species over many generations, and natural selection explains it through who survives and reproduces. What shifts is which traits are common in the population: the better-adapted individuals are more likely to survive and reproduce, so each generation becomes better adapted.

Not 'they needed to'. Chance variation first; then the better-adapted individuals are more likely to survive, reproduce and pass their traits on.

Two terms, easily blurred

EvolutionvsNatural selection

One is the change. The other is the cause.

Focus

What is it?

Evolution

The process in which the characteristics of species change over many generations

Natural selection

The theory that explains how and why evolution occurs

The insight

One is the change, the other is the cause.

What does it say?

Evolution

Species change over many generations, sometimes becoming new species

Natural selection

Better-adapted organisms are more likely to survive, reproduce and pass on their genes to their offspring

Where do new species fit in?

Evolution

Sometimes the change produces a new species

Natural selection

Speciation is when natural selection causes a new species to form

Words to use in an answer

Evolution

change, characteristics, many generations, new species

Natural selection

better adapted, survive, reproduce, pass on genes

Biology · Your turn

Write the explanation

Imagine a field where sunlight is limited. Some of the plants have a trait that helps them win more of it.

Explain how natural selection could make the plants in this field better adapted over generations. [5 marks]

Your answer

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

The mark scheme

Plants differ in their traits by random chance (variation) (1 mark)
The trait is an adaptation: an advantage in competing for a finite resource (1 mark)
Plants with the trait are more likely to survive to reproductive age and have offspring (1 mark)
They pass the trait on to their offspring (1 mark)
Over generations the trait becomes more common in the field (1 mark)

Model answer

Plants in the field vary by random chance, and the trait that wins more sunlight is an adaptation because it gives an advantage in competing for a finite resource. Plants with it are more likely to survive to reproductive age and have offspring, so they pass the trait on. Over the generations the trait becomes more common in the field, so the plants become better adapted. No plant chose to adapt.

Five links, in order: chance variation, advantage, survive and reproduce, pass on, more common. Miss one and the explanation has a gap.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a species changes over generations, and how one species can become two.

What you need to know

Evolution is the process in which a species' characteristics change over many generations, sometimes producing new species.
Natural selection explains how and why: better-adapted organisms are more likely to survive, reproduce and pass on their genes.
An adaptation is a feature that helps an organism live in a particular place and survive.
Variation is down to random chance: individuals differ, like eye colour in humans or numbers of seeds in plants.
Have a goYour friend says the plant that made more seeds 'just tried harder'. Is that variation down to effort or to random chance?
Random chance.
Individuals differ by random chance, such as different numbers of seeds in plants, not because they tried or needed to.
Some traits give an advantage in competing for finite resources, like sunlight for plants or mates for animals.
Advantaged individuals are more likely to survive to reproductive age and have offspring, so their traits become more common over generations.
Have a goA helpful trait is carried by only a few individuals today. Predict: is it more likely to be more common or rarer many generations from now?
More common.
Advantaged individuals are more likely to survive to reproductive age and have offspring, so the trait tends to spread.
Organisms do not choose to adapt themselves; the better-adapted individuals are simply more likely to survive and reproduce.
A species is a group of similar organisms that can reproduce and produce fertile offspring.
Speciation is when natural selection changes a population so much that it can no longer interbreed to produce fertile offspring with the original group.
Have a goTo decide whether two populations are separate species, which is the better check: how different they look, or whether they can still produce fertile offspring together?
Whether they can still produce fertile offspring together.
A species is defined by being able to reproduce and produce fertile offspring, not by appearance.
Speciation often happens when a geographical barrier, such as a mountain range, a river or the sea, separates populations.

The big picture

Evolution is change in a species over many generations, and natural selection explains it: chance variation, an advantage in competing for finite resources, then better-adapted individuals surviving, reproducing and passing their traits on. If populations are separated and meet different conditions, the same process can eventually produce new species.

Key points

1Natural selection works in a chain: random variation, an advantage in competing for finite resources, survival and reproduction, traits passed on, traits more common over generations.

2Each generation becomes better adapted, but organisms do not choose to adapt themselves.

3A species is a group of similar organisms that can reproduce and produce fertile offspring.

4Speciation often follows geographical separation: different conditions favour different traits until the populations can no longer interbreed.

5Galapagos tortoises: saddle-shaped shells on drier islands with little food on the ground; dome-shaped shells on more humid islands with plenty of food on the ground.

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Worked example

Problem

Two populations of one original species have been separated by a river for many generations. Different traits have been favoured on each side. When members of the two populations are brought together, they cannot interbreed to produce fertile offspring. Are they still one species? Explain.

⚠ Watch out

Saying organisms adapt because they need to. They don't choose anything: individuals vary by chance, and the better-adapted ones are simply more likely to survive and reproduce.

🧠

Memory hook

Chance varies, competition selects, offspring carry it on. And on the tortoise islands: saddle to stretch up, dome to shield the neck.

✓

Check yourself

A classmate writes: 'The dome-shelled tortoises grew domes because they needed protection.' Fix their sentence using the words variation, survive and reproduce.

Flashcards

(13)

What is evolution?

The process in which the characteristics of species change over many generations, sometimes becoming new species.

What does the theory of evolution by natural selection explain?

How and why evolution occurs. It suggests that all modern species evolved from earlier species.

What is natural selection?

Better-adapted organisms are more likely to survive, reproduce and pass on their genes to their offspring.

What is an adaptation?

A feature that helps an organism live in a particular place and survive.

What causes variation between individuals?

Random chance. For example, eye colour in humans or numbers of seeds in plants.

Which finite resources do plants and animals compete for?

Plants: sunlight, water, minerals and space. Animals: mates and territory.

Why does an advantageous trait become more common?

Individuals with it are more likely to survive to reproductive age and have offspring, so they pass the trait on over generations.

Do organisms choose to adapt?

No. The better-adapted individuals are simply more likely to survive and reproduce.

What is a species?

A group of similar organisms that can reproduce and produce fertile offspring.

What is speciation?

When natural selection changes a population so much that it can no longer interbreed to produce fertile offspring with the original group, so it is a new species.

How does a geographical barrier lead to speciation?

It separates populations (reproductive isolation). Natural selection favours different traits in the different conditions until they can no longer interbreed.

Which Galapagos tortoises have saddle-shaped shells, and why?

Those on drier islands with little food on the ground: the shape lets them stretch their necks up to reach food.

Which Galapagos tortoises have dome-shaped shells, and why?

Those on more humid islands with plenty of food on the ground: the shape protects the neck and head.

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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Follow the tortoises

Put natural selection in order

Which explanation sounds like you?

Write the explanation

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