5.2.2Genes, Inheritance and Selection
Evolution Through Natural Selection — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 8 of 9
Section 1 of 9Evolution and natural selection
Evolution is a change in the inherited characteristics of a population over many generations, which may eventually result in the formation of a new species. The theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace. The steps can be set out in a logical chain:
Variation
Individuals in a population show variation, caused by differences in their alleles.
Struggle for survival
More offspring are produced than can survive, so there is a struggle for survival: competition for food, space, mates, and pressure from predators and disease.
Reproduce
Individuals with characteristics that suit the environment are more likely to survive and reproduce. This is sometimes called survival of the fittest.
They pass the advantageous alleles to their offspring.
Over many generations, the proportion of individuals with these alleles increases, so the population changes.
Individual organisms do not evolve; populations do. An organism does not change its genes because it needs to.
Section 2 of 9Examples of natural selection
Peppered moths. Before the Industrial Revolution, most peppered moths in Britain were pale, which camouflaged them against lichen-covered tree bark.
1
A few were dark through a mutation.
2
Soot from factories killed the lichen and blackened the bark, so the pale moths were now easily seen and eaten by birds, while dark moths were camouflaged.
3
The dark form became common.
4
After pollution controls were introduced and the bark became lighter again, the pale form increased again.
Antibiotic resistance in bacteria. Bacteria divide rapidly and mutations occur by chance.
A mutation can make a bacterium resistant to an antibiotic.
When the antibiotic is used, non-resistant bacteria are killed but the resistant ones survive and reproduce, passing on the resistance allele.
MRSA
The population becomes mostly resistant. MRSA is a well-known example of a bacterium resistant to several antibiotics.
To slow this, doctors avoid prescribing antibiotics for viral infections, and patients should complete the full course.
Few new antibiotics are being developed.
Section 3 of 9Speciation
A species is a group of organisms that can interbreed to produce fertile offspring.
If two populations of a species become separated, for example by a mountain range, a river or an ocean, they can no longer interbreed.
Each experiences different environmental conditions, so natural selection favours different features in each.
Over a long time, mutations and selection cause the two populations to diverge so much that, if they were brought together, they could no longer interbreed to produce fertile offspring.
They have become two new species.
Speciation
This is speciation.
Wallace's work on island species and Darwin's study of finches on the Galapagos islands were key in developing the theory.
Section 4 of 9Evidence for evolution
Evidence for evolution
Fossils
Fossils are the remains or traces of organisms preserved in rock.
- A series of fossils, such as the horse or the whale, shows gradual change.
- Fossils record how organisms have changed over time and extinct species,
- but the record is incomplete,
- because many organisms rotted away or the rocks have not been found.
Antibiotic resistance
, observed within a human lifetime, shows evolution happening.
DNA comparisons
DNA comparisons the more recently two species shared an ancestor, the more similar their DNA sequences. Human and chimpanzee DNA is very close.
Similar structures
, such as the same bone pattern in a human arm, a bat wing and a whale flipper, suggest common ancestry.
Section 5 of 9Extinction
A species becomes extinct when no individuals of it remain.
Extinction
Causes include a change in the environment (such as climate change), a new predator, a new disease, competition from another species, habitat destruction by humans and a catastrophic event such as a meteor impact.
Species that cannot adapt quickly enough, or that have little variation, are most at risk.
Understanding natural selection explains why biodiversity is valuable: more variation gives a better chance that life continues when conditions change.
Section 6 of 9Using data and graphs in selection questions
Questions on selection often supply numbers, so practise a standard approach.
Using data and graphs in selection questions
For the moth example, suppose a survey in a polluted city finds 90% dark moths while in the countryside only 10% are dark.
Say what the data show, explain it by selective predation and camouflage, and note that the population has changed because the frequency of one allele has risen.
For bacteria, a graph of the percentage of resistant cells against the number of antibiotic treatments will rise over time; the explanation uses mutation, survival and reproduction.
When asked to 'evaluate' evidence, give a strength and a limitation: for example, DNA comparisons are objective and quantitative, but they cannot show the details of extinct organisms, whereas fossils show extinct forms but are incomplete.
Section 7 of 9Exam-style questions
Explain how bacteria in a hospital can become resistant to an antibiotic. [4]4 marks
11. A random mutation produces an allele giving resistance in a bacterium. [1]22. The antibiotic kills non-resistant bacteria, but resistant ones survive. [1]33. The survivors reproduce and pass on the resistance allele. [1]44. Over time the proportion of resistant bacteria in the population increases. [1]Explain how natural selection could cause a rise in the number of dark peppered moths in a polluted area. [3]3 marks
Exam-style questions1. Dark moths are better camouflaged on dark, sooty bark so are eaten less by birds. [1]2. They survive and reproduce more, passing on the allele for dark colour. [1]3. The frequency of the dark allele increases over generations. [1]Suggest why the fossil record is incomplete. [2]2 marks
1. Many organisms were soft-bodied and decayed before fossils could form. [1]
2. Many fossils have been destroyed by geological activity or are not yet found. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Evolution | Change in the inherited characteristics of a population over many generations. |
| Natural selection | The process by which individuals with advantageous features are more likely to survive and reproduce. |
| Speciation | The formation of a new species. |
| Species | A group of organisms able to interbreed to produce fertile offspring. |
| Extinction | The complete loss of a species. |
| Fossil | The preserved remains or traces of an ancient organism. |
| Antibiotic resistance | The ability of bacteria to survive an antibiotic. |
| Camouflage | Coloration or patterns that make an organism hard to see. |
Section 9 of 9Exam technique
1
Saying that organisms change because they 'need' to. Variation exists first and selection acts on it.
2
Writing that individuals evolve. Populations evolve over generations.
3
Saying that resistant bacteria are made by the antibiotic. The mutation happens by chance before exposure.
4
Thinking 'fittest' means strongest. It means best suited to survive and reproduce in that environment.
5
Forgetting that selected alleles must be passed on to the next generation for evolution to occur.
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