🧬 Genes
Darwin did not know how characteristics were passed on. It has since been shown that characteristics are passed on to offspring in genes. So useful characteristics can be inherited by the next generation, just as Darwin's theory needs.
Understanding Genetics and Evolution » Evidence for Evolution and Fossils
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.3.4, 4.6.3.5
- The evidence that supports the theory of evolution by natural selection
- What fossils are and the three ways they form
- Why the fossil record is incomplete
- How to read evolutionary trees, charts and tables
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Unlock This CourseThe theory is now widely accepted
Charles Darwin put forward the theory of evolution by natural selection in the 19th century. Today it is widely accepted by scientists. That is because there is now a lot of evidence that supports it, including evidence that was not available to Darwin himself.
There are three main pieces of evidence you need to know:
- Genes: characteristics are passed on to offspring in genes. This gives natural selection a way to work.
- The fossil record: fossils show how living things have changed over a very long time.
- Antibiotic resistance in bacteria: we can see evolution happening in front of us.
Key terms:
- Fossil: the remains of an organism from millions of years ago, found in rocks.
- Fossil record: all the fossils that have been found, put together as evidence.
Evidence from genes and from bacteria
🦠 Antibiotic resistance
Some bacteria have become resistant to antibiotics. Scientists now know how this resistance evolves. It is natural selection that we can watch happening, and it is more evidence for the theory.
You will find out exactly how bacteria become resistant in the lesson called Extinction and Resistant Bacteria. For now, remember that it counts as evidence for evolution.
What are fossils?
Fossils are the 'remains' of organisms from millions of years ago, which are found in rocks. A fossil might be a shell, a bone, a leaf or a footprint. Together they make up the fossil record, which helps us see what lived on Earth long ago.
Fossils can form in three different ways.
🦴 Parts that have not decayed
Parts of an organism do not decay if one or more of the conditions needed for decay are absent. The parts are then preserved.
💎 Replaced by minerals
Parts of the organism are replaced by minerals as they decay. The minerals take the shape of the original part.
👣 Preserved traces
The organism itself may not be kept, but traces of it are. These include footprints, burrows and rootlet traces.
Worked example
A scientist finds the shape of a shell in a rock, but the shell itself has gone and the shape is made of mineral. This fossil formed when parts of the organism were replaced by minerals as they decayed. A footprint in hardened mud is different: it is a preserved trace.
Why is the fossil record incomplete?
The fossil record has gaps. Many early forms of life were soft-bodied, which means they have left few traces behind. The traces that there were have mainly been destroyed by geological activity.
This is why scientists cannot be certain about how life began on Earth. There is not enough evidence left in the rocks.
Key terms:
- Soft-bodied: having no hard parts such as bones or shells, so leaving few traces.
- Geological activity: changes and movements in the Earth's rocks that destroyed many traces of early life.
What can we learn from fossils?
We can learn from fossils how much or how little different organisms have changed as life developed on Earth. Putting fossils in order of age shows the changes over time. Some organisms have changed a lot. Others have changed very little.
Reading evolutionary trees and data
You need to be able to take information from charts, graphs and tables, including evolutionary trees. An evolutionary tree is a branching diagram that shows how organisms are related to each other.
- Each branch point shows where one group split into two.
- Organisms whose branches split more recently are more closely related.
- Organisms that split a long time ago are more distantly related.
Worked example
A tree shows species A and B splitting from each other recently. Species C split off much earlier. Which species is B most closely related to? Species A, because their branches split most recently.
You may also have to read a table, such as how many fossils of one organism are found in rocks of different ages. Read the headings and units first, then look for the pattern.
Common mistakes
Saying that fossils form from every dead organism. They do not, so the record is incomplete. Another mistake is saying that fossils show exactly how life began. Scientists cannot be certain about this.
Exam-style question
(a) Describe one way in which a fossil may be formed. [2 marks]
(b) Explain why scientists cannot be certain about how life began on Earth. [2 marks]
(c) Give two pieces of evidence for the theory of evolution by natural selection. [2 marks]
Model answer
(a) Parts of the organism are replaced by minerals as they decay (1), so a fossil forms in the shape of the part (1). Other answers: parts did not decay because a condition for decay was absent (1), or preserved traces such as footprints (1).
(b) Early forms of life were soft-bodied, so left few traces (1). Many of the traces that did form have been destroyed by geological activity (1).
(c) The fossil record (1). Antibiotic resistance in bacteria (1). Or: characteristics are passed on to offspring in genes (1).
Exam tip
When asked why the fossil record is incomplete, give two linked ideas: soft bodies leave few traces, and geological activity destroyed many of those traces.