Evolutionary trees (AQA GCSE Biology): 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. These trees help scientists understand which organisms share common ancestors and when they separated during evolutionary history.
Think of it like a family tree, but for all living things. The branches show how species split apart over millions of years.
The family tree analogy is particularly helpful because it shows relationships and connections between different groups, just like how your family tree shows how you're related to your cousins, grandparents, and other relatives.
The three-domain system
Carl Woese's classification
In 1977, scientist Carl Woese created a new way to group all living things. He used chemical analysis to study organisms and proposed dividing life into three main groups called domains.
Woese's three-domain system revolutionised how we classify life by using molecular evidence rather than just physical characteristics. This was a major breakthrough in understanding the relationships between all living organisms.
The three domains
The Three Domains of Life:
1. Eukaryota
- Includes plants, animals, fungi and protists
- These organisms have cells with a nucleus
2. Bacteria
- Contains true bacteria and cyanobacteria
- Cyanobacteria can do photosynthesis
3. Archaea
- Primitive bacteria that live in extreme places
- Found in very hot or very salty environments
How classification systems develop
Classification systems have changed over time as scientists learn more about living things.
From old to new systems
- The Linnaean system started with just two kingdoms (plants and animals)
- This grew to six kingdoms or more as scientists discovered new organisms
- Simple organisms are harder to classify than complex ones
Why new systems were needed
Scientists developed better tools that helped them understand organisms better, leading to more accurate classification systems.
Scientists developed better tools that helped them understand organisms better:
- Microscopes - let scientists see the internal structures of tiny organisms
- Chemical techniques - helped scientists understand biochemical processes inside cells
Reading evolutionary trees
Understanding the branches
- Each branching point shows a common ancestor
- The common ancestor is the last species that existed before two groups split apart
- Species that branch off later are descendants of earlier species
Time relationships
- The bottom or left side usually shows the past
- The top or right side shows the present
- The further back you go, the older the common ancestor
Example interpretation
Worked Example: Human and Chimpanzee Relationship
If you see a tree with humans and chimpanzees:
- They share a recent common ancestor
- This ancestor is now extinct
- Both humans and chimpanzees descended from this ancestor
This tells us that humans and chimpanzees are closely related because they separated relatively recently in evolutionary time.
How evolutionary trees are made
Scientists use two main types of evidence to construct evolutionary trees:
Modern evolutionary trees combine multiple types of evidence to create the most accurate picture possible of how species are related.
Fossil data
- Shows relationships between extinct species
- Helps work out when species lived
Modern classification data
- DNA analysis of living species
- Shows how closely related current organisms are
- More accurate than just looking at physical features
DNA analysis has become increasingly important because it reveals relationships that might not be obvious from physical appearance alone. Sometimes organisms that look very different are actually closely related!
Remember!
Key Points to Remember:
- Evolutionary trees show how species are related through common ancestors
- The three-domain system groups all life into Eukaryota, Bacteria, and Archaea
- Branching points on trees represent common ancestors that are now extinct
- Scientists use both fossil evidence and DNA analysis to build these trees
- Classification systems keep improving as technology gets better