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Evolution is the gradual change in the inherited characteristics of a population over many generations. This may lead to the formation of new species. In this lesson we examine the evidence that supports the theory of evolution.
Fossils are the preserved remains or traces of organisms that lived in the past. They provide a window into the history of life on Earth, spanning billions of years.
Fossils form through several processes:
| Method | Description | Example |
|---|---|---|
| Mineralisation (petrification) | Hard parts (bones, shells, teeth) are gradually replaced by minerals over millions of years as water seeps through sediment | Dinosaur bones, ammonite shells |
| Casts and moulds | An organism is buried in sediment; the body decays, leaving a hollow mould. Minerals may fill the mould to create a cast | Trilobite casts |
| Preservation in amber | Organisms (often insects) become trapped in tree resin, which hardens into amber, preserving the whole organism | Insects in Baltic amber |
| Preservation in ice | Organisms are frozen rapidly, preventing decay; soft tissues may be preserved | Woolly mammoth remains in Siberian permafrost |
| Preservation in peat | Acidic, anaerobic (oxygen-free) conditions in peat bogs slow decomposition dramatically | Bog bodies, preserved plant material |
| Trace fossils | Preserved evidence of an organism's activity rather than its body | Footprints, burrows, coprolites (fossilised faeces) |
Fossils found in deeper, older rock layers tend to be simpler organisms. Fossils in younger, shallower layers are generally more complex. This pattern supports the idea that life has evolved from simple to more complex forms over time.
Exam tip: You do not need to memorise every method of fossilisation, but you must be able to describe at least two methods and explain why certain conditions are needed.
The fossil record is incomplete. There are significant gaps, and for many organisms we have no fossils at all. The reasons include:
Despite these gaps, the fossil record remains one of the strongest lines of evidence for evolution. Each new discovery adds to our understanding.
A pentadactyl limb is a limb with five fingers or toes. It is found in many different vertebrates — mammals, birds, reptiles and amphibians — even though these animals use their limbs for very different purposes.
| Organism | Limb Use | Adaptation |
|---|---|---|
| Human | Manipulation, grip | Long, flexible fingers |
| Whale/dolphin | Swimming | Short, wide flipper with finger bones inside |
| Bat | Flying | Elongated finger bones support wing membrane |
| Horse | Running | Single large digit (toe) for speed |
| Mole | Digging | Short, broad hand with strong claws |
All these limbs share the same basic bone structure: one upper bone (humerus), two lower bones (radius and ulna), a group of small wrist/ankle bones (carpals/tarsals), and five digits (phalanges).
The simplest explanation for this shared structure is that all these animals descended from a common ancestor that had this five-digit plan. Over millions of years, natural selection modified the limb for different functions in different environments. This is called homologous structures — structures that are similar because they were inherited from a common ancestor, not because they serve the same function.
Exam tip: If asked "How does the pentadactyl limb provide evidence for evolution?", always state: (1) the limbs have the same basic bone structure, (2) they are used for different functions, (3) this suggests they evolved from a common ancestor, (4) natural selection adapted the limb for different environments.
One of the most compelling pieces of evidence for evolution is that we can observe it happening today in populations of bacteria.
MRSA (methicillin-resistant Staphylococcus aureus) is a strain of bacteria that is resistant to several antibiotics. It is a serious concern in hospitals because infections are very difficult to treat.
Antibiotic resistance shows natural selection in action:
This is exactly the mechanism Darwin described — applied to a timescale we can observe directly.
Exam tip: Bacteria do NOT "become resistant" by being exposed to antibiotics. The mutation already exists before the antibiotic is applied. The antibiotic acts as a selection pressure.
Modern technology allows scientists to compare the DNA sequences of different species. The more similar the DNA, the more closely related the species are and the more recently they shared a common ancestor.
| Comparison | DNA Similarity |
|---|---|
| Human vs Chimpanzee | ~98.7% |
| Human vs Mouse | ~85% |
| Human vs Banana plant | ~60% |
Even the fact that all living organisms use DNA as their genetic material suggests a universal common ancestor.
Scientists can also compare the amino acid sequences of specific proteins (such as cytochrome c or haemoglobin) across species. The fewer differences, the more closely related the organisms.
Exam tip: When asked about evidence for evolution, try to mention at least three types: the fossil record, comparative anatomy (pentadactyl limb), and molecular/DNA evidence. This shows breadth of knowledge.
| Evidence | What It Shows |
|---|---|
| Fossil record | Organisms have changed over time; simpler organisms in older rocks |
| Pentadactyl limb | Shared bone structure suggests a common ancestor |
| Antibiotic resistance | Natural selection can be observed in real time |
| DNA/protein comparisons | Similar sequences indicate common ancestry; quantitative measure of relatedness |
| Term | Definition |
|---|---|
| Evolution | Gradual change in inherited characteristics of a population over generations |
| Fossil | Preserved remains or traces of organisms from the past |
| Pentadactyl limb | Five-digit limb structure shared by many vertebrates |
| Homologous structures | Structures with shared evolutionary origin but different functions |
| MRSA | Methicillin-resistant Staphylococcus aureus — an antibiotic-resistant bacterium |
Exam tip: Practice writing a six-mark answer on "Describe the evidence for evolution." Structure your answer with one paragraph per type of evidence, and include specific examples.