CIE AS/A Level Biology -17.3 Evolution- Study Notes- New Syllabus
CIE AS/A Level Biology -Link- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -Link- Study Notes- New Syllabus
Key Concepts:
- outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
- discuss how DNA sequence data can show evolutionary relationships between species
- explain how speciation may occur as a result of genetic isolation by:
- geographical separation (allopatric speciation)
- ecological and behavioural separation (sympatric speciation)
Theory of Evolution
🌱 Key Concept
- Evolution is the gradual change in gene pools of populations over generations.
- Leads to the formation of new species from pre-existing species over time.
🔹 How Evolution Occurs
Variation in Populations
- Individuals in a population show genetic differences due to mutations, gene shuffling, and sexual reproduction.
Changes in Gene Pools
- Certain alleles may increase or decrease in frequency over generations due to:
- Natural selection (survival of the fittest)
- Genetic drift (random changes)
- Founder effect or bottleneck effect (population size changes)
Adaptation
- Alleles that improve survival and reproduction become more common.
- Population becomes better suited to its environment.
Speciation
- Over long periods accumulated genetic changes can make populations so different that they can no longer interbreed.
- This results in the formation of a new species.
🔹 Example
- Darwin’s finches in the Galápagos:
- Different islands → birds adapted to different food sources.
- Over generations, populations evolved distinct beak shapes → new species formed.
📌 Key Points
- Evolution is gradual and occurs over many generations.
- Driven by changes in allele frequencies in populations.
- Results in adaptation and potentially speciation.
DNA Evidence for Evolutionary Relationships
🌱 Key Concept
- DNA sequence data can be used to compare genes or genomes between different species.
- The more similar the DNA sequences, the more closely related the species are likely to be.
How DNA Shows Evolutionary Relationships
Comparing nucleotide sequences
- DNA sequences of the same gene can be compared across species.
- Fewer differences → species share a recent common ancestor.
- More differences → species diverged long ago.
Identifying homologous genes (orthologues)
- Genes with the same function in different species.
- Similarity indicates shared ancestry.
Constructing phylogenetic trees
- DNA similarities are used to draw evolutionary trees.
- Branch length reflects the number of differences between sequences.
- Shows the order of divergence from common ancestors.
🔹 Examples
- Humans and chimpanzees: DNA sequences are ~98–99% identical → very close evolutionary relationship.
- Comparison of mitochondrial DNA can show maternal lineage and divergence times.
- Ribosomal RNA genes used to compare distantly related species (e.g., bacteria vs eukaryotes).
📌 Key Points
- DNA sequence comparisons provide molecular evidence of evolution.
- Can reveal recent and ancient relationships.
- More accurate than comparing only physical traits, which may be affected by convergent evolution.
Speciation: Formation of New Species
🔹 What is Speciation?
- Speciation is the process by which new species form from pre-existing species.
- Occurs when populations become genetically isolated and cannot interbreed.
🌍 1. Allopatric Speciation (Geographical Separation)
- Definition: Speciation caused by physical barriers that separate populations.
- Process:
- Population is divided by a geographical barrier (mountains, rivers, oceans).
- Separated populations cannot interbreed → gene flow stops.
- Mutations, natural selection, and genetic drift cause populations to diverge genetically.
- Over time, populations become distinct species.
- Example: Darwin’s finches on different Galápagos islands → isolated populations evolved distinct beak shapes.
🌿 2. Sympatric Speciation (Ecological and Behavioural Separation)
- Definition: Speciation occurs without physical separation, due to differences in ecology or behaviour.
- Process:
- Populations share the same habitat but exploit different niches or resources.
- Behavioural differences (e.g., mating rituals) reduce interbreeding.
- Genetic divergence occurs due to selection pressures in different niches.
- Eventually, populations become separate species.
- Example: Apple maggot flies: some lay eggs on apples, others on hawthorns → reduced interbreeding → genetic divergence.
📌 Key Points
| Type | Cause | Mechanism | Example |
|---|---|---|---|
| Allopatric | Geographical barrier | Isolation → genetic divergence | Galapagos finches |
| Sympatric | Ecological/behavioural differences | Niche or mating differences → genetic divergence | Apple maggot flies |