Evidence for Evolution

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.

The Fossil Record

What Are Fossils?

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.

How Fossils Form

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)

The Sequence of Fossils

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.

Gaps in the Fossil Record

The fossil record is incomplete. There are significant gaps, and for many organisms we have no fossils at all. The reasons include:

Soft tissue decay — Many organisms have soft bodies (e.g., jellyfish, worms) that decay quickly before fossilisation can occur. Only hard structures like bones, shells and teeth fossilise easily.
Conditions not suitable — Fossilisation requires specific conditions (rapid burial, appropriate minerals, low oxygen). Most dead organisms simply decompose.
Geological activity — Tectonic plate movements, volcanic eruptions and erosion can destroy fossils that did form.
Undiscovered fossils — Many fossils remain buried deep underground or in locations that have not been explored.

Despite these gaps, the fossil record remains one of the strongest lines of evidence for evolution. Each new discovery adds to our understanding.

Comparative Anatomy — The Pentadactyl Limb

What Is a Pentadactyl Limb?

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.

Examples of the Pentadactyl Limb

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

Why Is This Evidence for Evolution?

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.

Antibiotic-Resistant Bacteria — Evolution in Action

One of the most compelling pieces of evidence for evolution is that we can observe it happening today in populations of bacteria.

How Antibiotic Resistance Develops

Within a population of bacteria, there is genetic variation — some bacteria have random mutations that make them resistant to an antibiotic.
When an antibiotic is used, most bacteria are killed, but the resistant individuals survive.
The resistant bacteria reproduce rapidly (bacteria can divide every 20 minutes), passing on the resistance gene.
Over time, the whole population becomes resistant.

MRSA — A Real-World Example

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.

Why This Is Evidence for Evolution

Antibiotic resistance shows natural selection in action:

There is variation (some bacteria carry the resistance gene).
There is a selection pressure (the antibiotic kills non-resistant bacteria).
Better-adapted individuals survive and reproduce.
The frequency of the advantageous allele increases in the population over generations.

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.

Molecular Evidence — DNA and Protein Comparisons

DNA Analysis

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.

Protein Comparisons

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.

Why Molecular Evidence Is Powerful

It provides a quantitative (numerical) measure of relatedness.
It can be used for organisms that leave no fossils (e.g., bacteria, soft-bodied organisms).
It can confirm or challenge classifications made from physical features alone.
It has led to reclassification of some organisms when DNA evidence contradicted the original grouping.

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.

Summary

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

Key Vocabulary

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.