All organisms are related through their evolutionary history — Eduqas A-Level Biology

This topic explores the fundamental principle that all life on Earth shares a common ancestry. The theory of evolution by natural selection, first proposed by Charles Darwin, explains how species change over time and how new species arise. By studying the evidence for evolution—from the fossil record to comparative anatomy and DNA analysis—you will understand how organisms are related through branching patterns of descent, forming a 'tree of life'. This concept is central to modern biology, linking genetics, ecology, and biodiversity.

Understanding evolutionary relationships allows biologists to classify organisms into a hierarchical system (taxonomy) that reflects their evolutionary history. For example, humans and chimpanzees share a common ancestor that lived about 6–7 million years ago, which is why we share 98–99% of our DNA. This topic also explains why antibiotic resistance evolves in bacteria—a key example of natural selection in action. Mastering these ideas is essential for A-Level Biology, as they underpin topics like speciation, adaptation, and genetic variation.

In the WJEC A-Level specification, this topic appears in Unit 2 (Biodiversity and Physiology of Body Systems) and Unit 4 (Variation, Inheritance and Evolution). You will need to interpret phylogenetic trees, evaluate evidence for evolution, and explain how natural selection drives change. A solid grasp of this material will also help you in exam questions that ask you to apply evolutionary principles to unfamiliar scenarios, such as the evolution of pesticide resistance in insects.

• →Natural selection: Individuals with advantageous traits are more likely to survive and reproduce, passing on those traits to the next generation. This requires variation, inheritance, and differential survival.
• →Common ancestry: All organisms share a common ancestor; the more recent the common ancestor, the more closely related the species. Evidence includes homologous structures (e.g., pentadactyl limb) and molecular similarities (e.g., cytochrome c).
• →Phylogenetic trees: Branching diagrams that show evolutionary relationships. The nodes represent common ancestors, and the length of branches can indicate time or genetic change.
• →Speciation: The formation of new species, often due to reproductive isolation (e.g., geographical isolation leading to allopatric speciation).
• →Fossil evidence: Fossils show a progression of forms over time, with transitional fossils (e.g., Archaeopteryx) linking major groups.

• Classification is based on evolutionary relationships
• Classification systems are hierarchical and discrete
• Comparison of three-domain vs five-kingdom systems
• Characteristic features of the five kingdoms: Prokaryotae, Protoctista, Plantae, Fungi, Animalia
• Use of physical features and biochemical methods (DNA fingerprinting, enzyme studies) to assess relatedness
• Avoidance of morphological convergence in classification
• Definition of species and use of the binomial system
• Definition of biodiversity (number and variety of organisms in a region)
• Assessment of biodiversity using Simpson’s Diversity Index
• Assessment of genetic biodiversity via proportion of polymorphic loci
• Assessment of molecular biodiversity via DNA fingerprinting and sequencing
• Natural selection as the generator of biodiversity
• Types of adaptations: anatomical, physiological, and behavioural

• 💡Ensure you can explain why biochemical methods are superior to morphological ones for determining evolutionary relationships
• 💡Be prepared to interpret data regarding biodiversity indices
• 💡Understand the difference between the three-domain and five-kingdom systems
• 💡Use correct terminology when describing adaptations (anatomical, physiological, behavioural)
• 💡When answering questions on natural selection, always mention the three key components: variation (due to mutations), selection pressure (e.g., predation, antibiotics), and differential survival/reproduction. Use specific examples like antibiotic resistance in bacteria or peppered moths.
• 💡For phylogenetic tree questions, remember that the branching pattern shows relationships, not necessarily time. The length of branches may or may not be proportional to time—check the axis labels. Also, the node represents the most recent common ancestor.
• 💡In evaluation questions (e.g., 'Evaluate the evidence for evolution'), you must discuss multiple lines of evidence: fossil record, comparative anatomy, DNA sequences, and direct observation (e.g., Darwin's finches). Conclude by stating that the evidence is overwhelming and consistent.

• Confusing morphological convergence with true evolutionary relatedness
• Failing to distinguish between habitat biodiversity and genetic biodiversity
• Misinterpreting the tentative nature of classification systems
• Incorrect application of Simpson’s Diversity Index formula
• Misconception: 'Evolution is just a theory, so it's not proven.' Correction: In science, a theory is a well-substantiated explanation supported by extensive evidence. Evolution is a fact (observed changes in populations) and a theory (the mechanism of natural selection).
• Misconception: 'Humans evolved from monkeys.' Correction: Humans and modern monkeys share a common ancestor that lived millions of years ago. We did not evolve from any living monkey species; rather, we share a common ancestor with them.
• Misconception: 'Natural selection creates perfect organisms.' Correction: Natural selection acts on existing variation; it does not produce perfection. Traits are limited by genetic constraints and trade-offs (e.g., the human eye has a blind spot).