Natural Selection and Classification Review

This lesson brings together all the key concepts from Topic 4: Natural Selection and Genetic Modification. Use it to consolidate your knowledge, practise extended-response questions and check your understanding of how the different topics link together.

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Linking Evolution to Classification

Evolutionary Trees (Phylogenetic Trees)

An evolutionary tree (also called a phylogenetic tree) is a diagram that shows the evolutionary relationships between organisms. It represents how species have diverged from common ancestors over time.

                    Common ancestor
                         │
              ┌──────────┴──────────┐
              │                     │
         Ancestor A            Ancestor B
           │                       │
      ┌────┴────┐            ┌────┴────┐
      │         │            │         │
  Species 1  Species 2  Species 3  Species 4

How to Read an Evolutionary Tree

Feature	What It Means
Branch point (node)	A common ancestor where two lineages diverged
Branch length	Sometimes represents time or amount of genetic change
Tips (ends)	Modern-day species (or extinct species if labelled)
Closer branches	Species that share a more recent common ancestor are more closely related

How New Evidence Changes Classification

Classification is not static — it is constantly updated as new evidence emerges:

Type of Evidence	How It Has Changed Classification
DNA sequencing	Revealed that whales are most closely related to hippos (not pigs, as previously thought)
rRNA analysis	Led Woese to propose the three-domain system, splitting prokaryotes into Archaea and Bacteria
Protein comparisons	Confirmed that fungi are more closely related to animals than plants
Fossil discoveries	New transitional fossils (e.g., Tiktaalik, a fish-tetrapod intermediate) fill gaps in our understanding

Exam tip: If asked "How has new evidence changed classification?", give a specific example. The best example for Edexcel is the three-domain system: rRNA analysis showed archaea and bacteria are fundamentally different, leading to reclassification.

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Key Concept Summaries

Evidence for Evolution

Evidence	Key Detail
Fossil record	Shows organisms have changed over time; oldest fossils are simplest
Pentadactyl limb	Same bone structure in different vertebrates → common ancestor
Antibiotic-resistant bacteria	Natural selection observed in real time
DNA/molecular evidence	More similar DNA = more closely related

Natural Selection — The Mechanism

Always describe in this order:

1. Variation exists within a population (caused by mutations and sexual reproduction)
2. A selection pressure acts (predation, disease, competition, environmental change)
3. Individuals with advantageous characteristics are more likely to survive
4. Survivors reproduce and pass on their advantageous alleles
5. Over many generations, the allele frequency changes — the population evolves

Selective Breeding vs Natural Selection vs Genetic Engineering

Feature	Natural Selection	Selective Breeding	Genetic Engineering
Who/what selects	Environment	Humans	Scientists
Speed	Very slow (many generations, often millions of years)	Moderate (many generations but faster than nature)	Very fast (one generation)
Precision	Not directed; depends on random variation	Moderate — choose parents but whole genomes mix	Very precise — specific gene transferred
Species barrier	Within populations of a species	Same species or very close relatives	Any species (genes can cross species barriers)
Effect on genetic variation	Maintains or increases	Reduces (narrows gene pool)	Introduces new genes from other species
Example	Peppered moths adapting to pollution	Holstein-Friesian cattle for high milk yield	Bt crops with bacterial insecticide gene

Exam tip: This comparison table is one of the most commonly tested topics in the exam. Make sure you can explain at least three differences between any two of these processes.

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Common Exam Questions and Model Answers

Question Type 1: "Explain how antibiotic-resistant bacteria develop" (6 marks)

Model answer structure:

1. Within a population of bacteria, there is genetic variation due to random mutations.
2. Some bacteria have a mutation that gives them resistance to an antibiotic.
3. When the antibiotic is used, it acts as a selection pressure — non-resistant bacteria are killed.
4. The resistant bacteria survive because the antibiotic does not affect them.
5. With no competition, the resistant bacteria reproduce rapidly by binary fission, passing on the resistance allele.
6. Over time, the frequency of the resistance allele increases in the population. The population is now resistant to the antibiotic.

Key points to remember:

• The mutation occurs randomly and before the antibiotic is applied — the antibiotic does NOT cause the mutation
• This is natural selection: variation → selection pressure → survival → reproduction → allele frequency change

Question Type 2: "Explain how a new species can evolve" (6 marks)

Model answer structure:

1. A population of one species becomes geographically isolated — separated by a physical barrier (e.g., mountain range, river, ocean).
2. The two isolated populations experience different environmental conditions and selection pressures.
3. In each population, different characteristics are advantageous, so natural selection acts differently.
4. Over many generations, the two populations accumulate different genetic changes (different alleles become common in each population).
5. Eventually, the populations are so genetically different that they can no longer interbreed to produce fertile offspring.
6. They are now two separate species — this process is called speciation.

Named example: Darwin's finches on the Galapagos Islands — different islands had different food sources, leading to different beak shapes evolving in isolated populations.

Question Type 3: "Describe the process of genetic engineering" (6 marks)

Model answer structure:

1. The desired gene is identified in the donor organism.
2. The gene is cut out using restriction enzymes.
3. A plasmid (vector) is cut open using the same restriction enzyme, creating complementary sticky ends.
4. The gene is inserted into the plasmid using DNA ligase.
5. The recombinant plasmid is inserted into the target organism (e.g., bacterium).
6. The target organism expresses the gene and produces the desired protein.

Question Type 4: "Compare selective breeding and genetic engineering" (4 marks)

Point	Selective Breeding	Genetic Engineering
Method	Choosing parents with desired traits to breed	Transferring a specific gene between organisms
Speed	Takes many generations	Can be done in one generation
Species	Same species only	Genes can be transferred between different species
Precision	Whole genomes combine; other unwanted genes may be included	Very precise; only the desired gene is transferred

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Extended Response Practice

Practice Question

"Evolution by natural selection and selective breeding both lead to changes in populations. Describe the similarities and differences between these two processes." (6 marks)

Planning your answer:

Similarities (at least 2):

• Both involve variation in a population
• Both result in certain organisms reproducing more than others
• Both lead to changes in the characteristics of a population over time
• Both depend on inheritance — advantageous traits must be passed to offspring

Differences (at least 2):

• Natural selection: environment selects; Selective breeding: humans select
• Natural selection: maintains genetic variation; Selective breeding: reduces genetic variation
• Natural selection: very slow (many generations); Selective breeding: relatively fast
• Natural selection: can lead to new species; Selective breeding: does not typically produce new species
• Natural selection: no predetermined goal; Selective breeding: aimed at a specific desired trait

Exam tip: For 6-mark extended response questions, aim for 6 clear, distinct points. Use scientific terminology. Write in full sentences, not bullet points. Structure your answer logically (similarities first, then differences, or vice versa).

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Classification Summary

System	Proposed By	Basis	Structure
Linnaean hierarchy	Carl Linnaeus	Physical characteristics	Kingdom → Phylum → Class → Order → Family → Genus → Species
Five kingdoms	Robert Whittaker (refined by Margulis)	Cell type, nutrition, body plan	Animalia, Plantae, Fungi, Protista, Prokaryotae
Three domains	Carl Woese	rRNA and biochemical analysis	Archaea, Bacteria, Eukarya
Binomial naming	Carl Linnaeus	Standardised Latin names	Genus species (e.g., Homo sapiens)

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Key Vocabulary Checklist

Use this checklist to make sure you can define and use every key term from Topic 4: