Natural selection

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Lesson notes

What is natural selection?

Natural selection explains how species change over time.
It is driven by variation, competition, and survival of the fittest.
Individuals with traits best suited to their environment are more likely to survive and reproduce.

Natural selection in a peppered moth population after pollution darkens the bark: the variation already exists, birds eat the moths that now stand out, and the dark form becomes more common.

Variation and competition

Variation means individuals in a species are not identical; differences come from their genes.
Resources like food, water, and shelter are limited, leading to competition.
Organisms with advantageous traits are more likely to survive and reproduce.

Organisms compete when resources are limited. Plants compete for light, water, space and mineral ions; animals compete for food, territory and mates.

Survival of the fittest

'Survival of the fittest' means the best-adapted individuals survive.
'Fittest' does not mean strongest or fastest; it means best matched to the environment.
Surviving longer increases the chance of reproducing and passing on genes.

Polar bears, cacti and camels have features suited to their environments. Being well adapted helps an organism survive and reproduce.

Examples of natural selection

Beetle colour: In a forest with brown leaves, brown beetles are less visible to predators than green beetles, so they survive and reproduce more.
Giraffe neck: Originally shorter necks; those with slightly longer necks could reach food others couldn't, leading to longer necks over generations.

How a resistance mutation spreads through a bacterial population under antibiotic exposure.

Outcome of natural selection

Advantageous traits become more common in the population over generations.
Over long periods, these changes can lead to the development of new species.

On dark ground, darker beetles are less visible to predators. More survive and reproduce, so their inherited colour becomes more common over generations.

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Practice questions

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1.All individuals within the same species are exactly the same.
Easy
- AFalse
- BTrue
- CSometimes true
- DOnly for plants
2.Individuals that survive for longer are more likely to reproduce.
Easy
- ATrue
- BFalse
- COnly if they are male
- DOnly if they are female
3.Which of the following best describes 'survival of the fittest'?
Easy
- AIndividuals with traits best matched to their environment are most likely to survive and reproduce
- BThe strongest and fastest individuals always survive
- COnly the largest individuals survive
- DIndividuals that compete the most aggressively survive
4.Variation within a species leads to competition because:
Medium
- AResources such as food, water, and shelter are limited
- BIndividuals want to be different from each other
- CPredators always eat the weakest individuals
- DAll individuals have the same traits
5.Giraffes evolved long necks because:
Medium
- AGiraffes with slightly longer necks could reach food that others couldn't, so they survived and reproduced more
- BAll giraffes stretched their necks and passed on the stretched length to offspring
- CGiraffes needed longer necks to see predators from far away
- DA mutation suddenly gave all giraffes long necks
6.In a forest with brown leaves, brown beetles survive longer than green beetles because:
Medium
- ABrown beetles are less likely to be seen and eaten by predators
- BBrown beetles are faster than green beetles
- CGreen beetles are poisonous
- DBrown beetles can fly higher
7.In a population of birds, food becomes available only in deep crevices. Which birds are more likely to survive?
Medium
- ABirds with longer beaks
- BBirds with shorter beaks
- CBirds with wider beaks
- DBirds with stronger wings
8.Natural selection can lead to the development of new species because:
Medium
- AOver many generations, advantageous traits become more common and changes accumulate
- BIndividuals change their traits during their lifetime to adapt
- CAll organisms eventually become the same species
- DNew species appear suddenly without any gradual change

More questions

9.What is the main source of variation within a species?
Easy
- ADifferences in their genes inherited from parents
- BDifferences in their diet
- CDifferences in their environment
- DDifferences in their age
10.Which of the following is an example of natural selection in action?
Medium
- ABrown beetles becoming more common in a forest with brown leaves over many generations
- BA rabbit learning to run faster after being chased
- CA plant growing taller because it receives more sunlight
- DA bird changing its beak shape after eating hard seeds
11.In the context of natural selection, what does 'fittest' mean?
Easy
- ABest adapted to the environment
- BStrongest and most powerful
- CFastest runner
- DLargest in size
12.A population of rabbits lives in a cold climate. Which trait would be most advantageous for survival?
Easy
- AThicker fur
- BLonger legs
- CLarger ears
- DShorter tail
13.In a population of rabbits, some have thicker fur and some have thinner fur. If the climate becomes colder, which rabbits are most likely to survive and reproduce?
Medium
- ARabbits with thicker fur
- BRabbits with thinner fur
- CAll rabbits equally
- DRabbits with the fastest running speed
14.A population of beetles lives in a forest with both brown and green leaves. After several generations, the beetle population becomes mostly green. Which statement best explains this change?
Medium
- AGreen beetles are better camouflaged on green leaves, so they survive longer and reproduce more
- BBrown beetles cannot survive on green leaves
- CGreen beetles produce more offspring than brown beetles
- DThe environment caused the beetles to change colour
15.
A scientist measures the beak depth of finches on an island before and after a drought. The table shows the average beak depth. What conclusion can be drawn?
Time Average beak depth (mm)
Before drought 8.5
After drought 10.2
Medium
- AFinches with deeper beaks survived better during the drought
- BBeak depth increased because of the drought
- CThe drought caused finches to grow deeper beaks
- DThere is no significant change in beak depth
16.A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive and reproduce. After several generations, the entire population is resistant to the antibiotic. Which statement explains this?
Medium
- AThe antibiotic caused mutations that made bacteria resistant
- BResistant bacteria survived and reproduced, passing on resistance genes
- CThe bacteria learned to resist the antibiotic
- DThe antibiotic became weaker over time
17.In a species of moth, there are two colour forms: dark and light. In a polluted area where trees are covered in soot, dark moths are more common. Which graph best shows the change in moth population over time?
Hard
- AA line graph with dark moth frequency increasing from 20% to 80% over 50 years
- BA bar chart showing equal numbers of dark and light moths
- CA line graph with light moth frequency increasing from 50% to 90%
- DA pie chart showing dark moths at 10% and light moths at 90%
18.In an experiment, a scientist places equal numbers of brown and green beetles in a tray with brown soil. After one hour, a bird is allowed to eat beetles for five minutes. The scientist then counts the remaining beetles. Which result would support the theory of natural selection?
Medium
- AMore brown beetles remain than green beetles.
- BMore green beetles remain than brown beetles.
- CEqual numbers of brown and green beetles remain.
- DAll beetles are eaten.
19.A population of bacteria is treated with an antibiotic. Most bacteria die, but a few survive and reproduce. After several generations, the entire population is resistant to the antibiotic. Which statement best explains this?
Medium
- AThe antibiotic caused mutations that made bacteria resistant.
- BSome bacteria were already resistant due to natural variation, and they survived and reproduced.
- CThe bacteria learned to become resistant and passed on this learning.
- DThe antibiotic triggered an immune response in the bacteria.
20.In a population of rabbits, some have thick fur and some have thin fur. If the climate becomes colder over many years, which outcome is most likely?
Medium
- ARabbits with thin fur will grow thicker fur to adapt.
- BThe proportion of rabbits with thick fur will increase over generations.
- CAll rabbits will develop thick fur within one generation.
- DThe rabbit population will become extinct.
21.A scientist measures the beak depth of finches on an island before and after a drought. The data shows that average beak depth increased after the drought. Which conclusion is best supported?
Medium
- AThe drought caused finches to grow deeper beaks.
- BFinches with deeper beaks were more likely to survive the drought and reproduce.
- CBeak depth is not inherited.
- DThe drought had no effect on beak depth.
22.In a species of moth, there are two colour forms: dark and light. In a polluted area where trees are covered in soot, dark moths are more common. Which graph best shows how the proportion of dark moths would change over time if pollution continues?
Hard
- AA straight horizontal line at 50%
- BA line increasing from 50% to near 100%
- CA line decreasing from 50% to near 0%
- DA line that goes up and down randomly
23.A student investigates whether a particular antibiotic is effective against a strain of bacteria. They grow bacteria on agar plates, add antibiotic discs, and measure the zone of inhibition. After three days, they observe bacteria growing inside the zone. What is the most likely explanation?
Hard
- AThe antibiotic has degraded over time.
- BSome bacteria were resistant and survived, then multiplied.
- CThe bacteria mutated after being exposed to the antibiotic.
- DThe agar plate was contaminated.
24.Which of the following is required for natural selection to occur?
Medium
- AA constant environment
- BNo variation within the population
- CVariation, competition, and differential survival
- DArtificial selection by humans
25.What is the original source of the variation that natural selection acts upon?
Medium
- AMutations in DNA
- BChanges in behaviour
- CLearning from parents
- DChanges in diet
26.In a population of plants, some individuals produce a toxin that deters herbivores. A new species of herbivore arrives that is unaffected by the toxin. What is the most likely long-term outcome for the plant population?
Medium
- AAll plants will evolve to produce a different toxin.
- BPlants that produce the toxin will be at a disadvantage, and the frequency of the toxin allele will decrease.
- CThe plants will stop producing the toxin to save energy.
- DThe herbivore will evolve to be affected by the toxin.
27.A farmer sprays a field with pesticide to kill insects. Most insects die, but a few survive. After several generations, the insect population is no longer affected by the pesticide. This is an example of:
Medium
- AArtificial selection
- BNatural selection
- CGenetic engineering
- DSelective breeding
28.Which of the following best describes natural selection?
Easy
- AA process by which species change over time due to survival and reproduction of individuals best suited to their environment
- BThe idea that all individuals in a population are identical
- CThe process by which organisms become stronger over time
- DThe idea that only the fastest organisms survive
29.In natural selection, what does the term 'fittest' mean?
Easy
- AIndividuals whose traits are best matched to their environment
- BThe strongest individuals in the population
- CThe fastest individuals in the population
- DThe largest individuals in the population
30.A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive and reproduce. After several generations, the entire population is resistant to the antibiotic. This is an example of:
Medium
- ANatural selection
- BArtificial selection
- CGenetic engineering
- DMutation
31.A scientist measures the beak depth of finches on an island before and after a drought. The data shows that average beak depth increased after the drought. Which conclusion can be drawn?
Medium
- ABirds with deeper beaks were more likely to survive the drought
- BBirds with shallower beaks were more likely to survive the drought
- CThe drought caused beaks to grow deeper
- DBeak depth is not affected by natural selection
32.In an experiment, a student places equal numbers of brown and green beetles in a tray with brown soil. After one hour, a bird is allowed to eat beetles for five minutes. The student then counts the remaining beetles. Which result would support natural selection?
Medium
- AMore brown beetles remain than green beetles
- BMore green beetles remain than brown beetles
- CEqual numbers of both remain
- DAll beetles are eaten
33.A farmer sprays a field with pesticide to kill insects. Most insects die, but a few survive. After several generations, the insect population is no longer affected by the pesticide. This is because:
Medium
- AThe surviving insects passed on their resistance genes to their offspring
- BThe pesticide caused the insects to become resistant
- CThe insects learned to avoid the pesticide
- DThe pesticide lost its effectiveness over time
34.Which of the following best explains how giraffes evolved long necks according to natural selection?
Medium
- AGiraffes with slightly longer necks could reach more food and were more likely to survive and reproduce
- BGiraffes stretched their necks to reach high leaves and passed on the stretched trait
- CAll giraffes gradually grew longer necks over time
- DA mutation caused all giraffes to have long necks at once
35.In a population of beetles, some are brown and some are green. They live in a forest with brown leaves. A bird predator is introduced. Which outcome is most likely after many generations?
Medium
- AThe population will become mostly brown.
- BThe population will become mostly green.
- CThe population will become equal numbers of brown and green.
- DAll beetles will become a new colour.
36.A farmer sprays a field with pesticide. Most of the insects die, but a few survive and reproduce. After several generations, the insect population is no longer affected by the pesticide. Which statement best explains this?
Medium
- AThe pesticide caused the insects to mutate and become resistant.
- BThe insects that were already resistant survived and passed on the resistance gene.
- CThe insects learned to avoid the pesticide.
- DThe pesticide became weaker over time.
37.
A scientist measures the beak depth of finches on an island before and after a drought. The table shows the average beak depth. Before drought: 9.5 mm. After drought: 11.2 mm. What conclusion can be drawn?
Time Average beak depth (mm)
Before drought 9.5
After drought 11.2
Medium
- AThe drought caused finches to grow larger beaks.
- BFinches with larger beaks were more likely to survive the drought and reproduce.
- CThe finches' beaks changed because they needed to crack harder seeds.
- DThe drought had no effect on beak size.
38.In an experiment, a student places 50 brown beetles and 50 green beetles in a tray with brown soil. A bird is allowed to eat beetles for 5 minutes. The student counts the remaining beetles. Which result would support natural selection?
Medium
- AMore brown beetles remain than green beetles.
- BMore green beetles remain than brown beetles.
- CEqual numbers of brown and green beetles remain.
- DAll beetles are eaten.
39.A population of bacteria is treated with an antibiotic. Most bacteria die, but a few survive and reproduce. After several generations, the entire population is resistant to the antibiotic. What is the original source of the resistance?
Medium
- AThe antibiotic caused mutations that made the bacteria resistant.
- BThe bacteria learned to resist the antibiotic.
- CA random mutation in the bacteria's DNA gave some individuals resistance.
- DThe antibiotic became less effective over time.
40.In a species of moth, there are two colour forms: dark and light. In a polluted area where trees are covered in soot, dark moths are more common. Which graph best shows the change in moth population over time after pollution increases?
Hard
- AGraph A: dark moth population increases, light decreases.
- BGraph B: light moth population increases, dark decreases.
- CGraph C: both populations remain constant.
- DGraph D: both populations fluctuate randomly.
41.Natural selection explains how species change over time.
Easy
True or false?
42.All individuals within a species are exactly the same.
Easy
True or false?
43.What is the main reason for variation within a species?
Easy
- ADifferences in their environment
- BDifferences in their genes inherited from parents
- CDifferences in their diet
- DDifferences in their age
44.In a forest with brown leaves, why are brown beetles more likely to survive than green beetles?
Medium
- ABrown beetles are stronger than green beetles
- BBrown beetles are less likely to be seen and eaten by predators
- CBrown beetles can run faster
- DBrown beetles produce more offspring
45.Place the following steps of natural selection in the correct order:
Medium
- Individuals with advantageous traits survive and reproduce
- Variation exists within a population
- Advantageous traits become more common over generations
- Competition for limited resources occurs
46.Which of the following are required for natural selection to occur? (Select all that apply)
Hard
- AVariation within the population
- BCompetition for resources
- CEqual survival of all individuals
- DInheritance of traits
47.Match each term to its correct description.
Medium
- Variation
- Competition
- Survival of the fittest
- Individuals best suited to environment survive
- Differences in genes between individuals
- Struggle for limited resources
48.Natural selection can lead to the development of new species over many generations.
Medium
True or false?
49.Match each term on the left with its correct description on the right.
Medium
- Variation
- Competition
- Survival of the fittest
- Natural selection
- The process by which species change over time
- Differences between individuals of the same species
- The struggle for limited resources
- Individuals best suited to their environment survive and reproduce
50.Put the following steps of natural selection in the correct order, starting with the first step.
Hard
- Individuals with advantageous traits survive longer
- Variation exists within a population
- Over many generations, the population changes
- These individuals reproduce and pass on their genes
- Limited resources lead to competition
51.Complete the sentence using the correct term.
Easy
Natural selection is often summarised as 'survival of the ____'.
52.In a population of 100 beetles, 20 are brown and 80 are green. The environment has brown soil. If brown beetles have a 90% chance of surviving to reproduce and green beetles have a 50% chance, how many brown beetles are expected to survive?
Medium
53.Describe what is meant by 'survival of the fittest'.
Medium
54.Describe how giraffes evolved to have such long necks according to natural selection.
Hard
55.In a population of 100 beetles, 20 are brown and 80 are green. The environment has brown soil. If brown beetles have a 90% chance of surviving to reproduce and green beetles have a 50% chance, how many brown beetles are expected to survive?
Medium
- A20
- B17
- C18
- D19

Revision notes

## Natural Selection Natural selection explains how species change over time. Key steps: - **Variation**: Individuals in a species are not identical due to genetic differences. - **Competition**: Limited resources (food, water, shelter) mean not all survive. - **Survival of the fittest**: Individuals with traits best suited to the environment are more likely to survive and reproduce. - **Inheritance**: Advantageous traits are passed to offspring, becoming more common over generations. **Examples**: - **Beetles**: Brown beetles survive better on brown leaves than green beetles, so brown becomes more common. - **Giraffes**: Longer necks allowed access to food during scarcity, leading to longer necks over generations. **Key point**: "Fittest" means best adapted to the environment, not necessarily strongest or fastest.

Natural selection: A process that explains how species change over time through variation, competition, and survival of the fittest.
Variation: Differences between individuals of the same species, often due to genetic inheritance.
Competition: The struggle between organisms for limited resources such as food, water, and shelter.
Survival of the fittest: The idea that individuals best suited to their environment are most likely to survive and reproduce.
Adaptation: A trait that helps an organism survive in its environment, becoming more common over time.

Flashcards

What is natural selection?

Natural selection is a process that explains how species change over time.

What is variation?

Variation means that not all individuals in a species are exactly the same; they differ in traits like height, speed, or strength.

What causes variation within a species?

Variation is due to differences in genes inherited from parents.

Why do organisms compete for resources?

Resources like food, water, and shelter are limited, so not all individuals can survive, leading to competition.

What does 'survival of the fittest' mean?

It means that individuals best suited to their environment are most likely to survive and reproduce.

Does 'fittest' always mean strongest or fastest?

No, 'fittest' means having traits best matched to the environment, not necessarily being strongest or fastest.

How does a beneficial trait become more common in a population?

Individuals with the beneficial trait are more likely to survive, reproduce, and pass on the trait to offspring, making it more common over generations.

What happens to a species over many generations of natural selection?

Significant changes can accumulate, potentially leading to the development of new species.

In a forest with brown leaves, why might brown beetles survive longer than green beetles?

Brown beetles are less likely to be seen and eaten by predators because they blend in with the brown leaves.

What happens to the beetle population over many generations if brown beetles survive better?

The population becomes mostly brown because brown beetles survive and reproduce more, passing on their color.

How did giraffes evolve long necks according to natural selection?

Originally shorter-necked giraffes with slightly longer necks could reach leaves others couldn't during food scarcity, survived better, reproduced, and passed on longer necks over generations.

True or false: All individuals within the same species are exactly the same.

False.

True or false: Individuals that survive for longer are more likely to reproduce.

True.

Describe how variation within a species can lead to competition.

Variation means individuals have different traits; those with advantageous traits are better at getting limited resources, leading to competition where some survive and reproduce more.

In a bird population with long and short beaks, if food is only in deep crevices, which birds survive better?

Birds with longer beaks survive better because they can reach food in deep crevices.

What is the source of variation in a species?

Differences in genes inherited from parents.

What role does reproduction play in natural selection?

Individuals that survive longer are more likely to reproduce and pass on their advantageous genes.

Give an example of a trait that helps rabbits survive in cold climates.

Thicker fur helps rabbits survive in cold climates.

What does natural selection lead to over time?

It leads to changes in species and can result in new species.

Why is competition important in natural selection?

Competition ensures that only individuals with traits best suited to the environment survive and reproduce.

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Time	Average beak depth (mm)
Before drought	8.5
After drought	10.2

Time	Average beak depth (mm)
Before drought	9.5
After drought	11.2