Evolutionary trees (AQA GCSE Biology Combined Science): Revision Notes
Evolutionary trees
What are evolutionary trees?
An evolutionary tree is a diagram that shows how different species are related to each other through evolution. Think of it like a family tree, but for all living things!
These trees help scientists understand:
- Which species evolved from common ancestors
- When different species separated during evolution
- How closely related different organisms are
Evolutionary trees are also called phylogenetic trees or family trees of life. They're one of the most important tools in modern biology for understanding how all living things are connected through evolutionary history.
How do evolutionary trees work?
Evolutionary trees use branches to show relationships between different species and their evolutionary history:
- Branch points show where species separated from a common ancestor
- The further back in time you go, the more distant the common ancestor
- Descendants are species that evolved from the same ancestor
Reading an evolutionary tree
Understanding how to read these trees is essential for interpreting evolutionary relationships:
- Species at the tips of branches are often living today
- Species at branch points are usually extinct common ancestors
- The closer two species are on the tree, the more recently they shared a common ancestor
A common mistake is thinking that species listed next to each other on a tree are more closely related. What actually matters is how far back you need to go to find their common ancestor - not their position on the diagram!
Evidence used to build evolutionary trees
Scientists use two main types of evidence to construct accurate evolutionary trees:
Fossil data:
- Shows relationships between extinct species
- Helps work out when species lived
- Provides evidence of evolutionary changes over time
Modern classification data:
- DNA analysis compares genetic similarities between living species
- Species with similar DNA are more closely related
- Chemical analysis of proteins and other molecules
Modern DNA sequencing technology has revolutionised how we build evolutionary trees. Scientists can now compare thousands of genes between species, giving us much more accurate pictures of evolutionary relationships than ever before.
Background: classification systems
Three-domain system
Carl Woese created the three-domain system in 1977. It groups all life into three main categories:
- Eukaryota - organisms with a nucleus (plants, animals, fungi, protists)
- Bacteria - true bacteria and cyanobacteria
- Archaea - primitive bacteria that live in extreme conditions (very hot or salty places)
How classification has developed
Classification systems have evolved significantly as our understanding of life has improved:
- The old Linnaean system had just two kingdoms (plants and animals)
- This expanded to six kingdoms or more as we learned more about life
- Simple organisms are harder to classify than complex ones
- New technology like microscopes and chemical techniques help us understand organisms better
The development of classification systems shows how science is constantly improving. As we discover new evidence and develop better tools, our understanding of how life is organised continues to evolve.
Worked example: reading an evolutionary tree
Worked Example: Interpreting Relationships
Looking at a tree showing cow, whale, pig, hedgehog, human, and chimpanzee:
Step 1: Identify the overall structure All six animals share a common ancestor from long ago
Step 2: Find the most recent relationships Humans and chimpanzees share the most recent common ancestor (they're most closely related)
Step 3: Identify other groupings Cows, whales and pigs also share a more recent common ancestor than with other animals
Step 4: Understand what branch points represent The common ancestors shown at branch points are now extinct
Key Points to Remember:
- Evolutionary trees show relationships between species through common ancestors
- Branch points represent extinct common ancestors
- Closer branches mean more recent common ancestors and closer relationships
- Trees are built using fossil evidence and DNA analysis
- The three-domain system groups all life into Eukaryota, Bacteria, and Archaea
- Modern technology helps scientists understand relationships between organisms better