Evidence for evolution: fossils, antibiotic resistance and speciation

What you'll learn

This revision guide covers three key pieces of evidence that support the theory of evolution: fossil records showing how organisms have changed over millions of years, antibiotic resistance demonstrating natural selection in action, and speciation explaining how new species arise. You'll understand how scientists use these observations to support Darwin's theory and what they mean for modern medicine and conservation.

Key terms and definitions

Evolution — the gradual change in the inherited characteristics of a population over time through natural selection

Fossil — the preserved remains or traces of organisms that lived millions of years ago, typically found in rocks

Antibiotic resistance — the ability of bacteria to survive exposure to antibiotics that would normally kill them or stop their growth

Natural selection — the process by which organisms better adapted to their environment survive, reproduce and pass on their characteristics

Speciation — the formation of new species when populations of the same species become so different that they can no longer interbreed to produce fertile offspring

Extinction — the permanent loss of all members of a species

Mutation — a random change in an organism's DNA that can be inherited

Resistant strain — a population of bacteria that has evolved the ability to survive treatment with specific antibiotics

Core concepts

How fossils provide evidence for evolution

Fossils form when dead organisms are buried quickly in sediment before they decay completely. Over millions of years, minerals replace the organic material or leave an impression in rock. The fossil record provides several types of evidence for evolution:

Dating fossils and rock layers

Scientists determine the age of fossils using:

• The rock layer (stratum) in which they're found — deeper layers are older
• Radioactive dating techniques that measure the decay of radioactive isotopes in surrounding rocks
• Structural features that show how organisms have changed gradually over time

What fossils reveal

The fossil record shows:

• Simple organisms appear in older rocks; more complex organisms in younger rocks
• Extinct species that no longer exist today, such as dinosaurs and trilobites
• Transitional forms showing intermediate features between groups (e.g., Archaeopteryx has both reptile and bird characteristics)
• How species have changed gradually over millions of years

Limitations of fossil evidence

The fossil record is incomplete because:

• Most organisms decomposed completely before fossilisation could occur
• Soft-bodied organisms rarely form fossils (no hard parts to preserve)
• Many fossils remain undiscovered deep underground or have been destroyed by geological activity
• Early life forms were soft-bodied, leaving few traces

Antibiotic resistance as evidence for evolution

Antibiotic resistance demonstrates natural selection occurring over a short timescale, providing observable evidence for evolutionary processes.

How antibiotic resistance develops

1. Random mutation occurs in bacterial DNA during reproduction
2. Some mutations make bacteria resistant to antibiotics by chance
3. When antibiotics are used, non-resistant bacteria die
4. Resistant bacteria survive and reproduce rapidly (bacteria can divide every 20 minutes)
5. The resistant gene is passed to offspring
6. The population becomes predominantly resistant — a resistant strain has evolved

This process demonstrates natural selection because:

• There is variation in the population (some bacteria are resistant, most are not)
• There is competition for survival when antibiotics are present
• Resistant bacteria are better adapted to survive
• Resistant bacteria reproduce and pass on their genes
• The population changes over time (evolves)

MRSA as a case study

MRSA (methicillin-resistant Staphylococcus aureus) is a bacterial strain resistant to many commonly used antibiotics. It evolved through natural selection and causes serious infections, particularly in hospitals. MRSA demonstrates evolution because:

• It developed from non-resistant bacteria through mutation and natural selection
• It shows how populations change in response to environmental pressures (antibiotics)
• New strains continue to evolve resistance to different antibiotics

Preventing antibiotic resistance

To slow the development of resistant strains:

• Complete the full course of antibiotics (kills all bacteria, preventing resistant survivors)
• Only use antibiotics for bacterial infections (not viral infections like colds)
• Restrict use of antibiotics in agriculture
• Develop new antibiotics (though bacteria evolve resistance to these too)
• Improve hygiene in hospitals to prevent spread

Speciation and the formation of new species

Speciation occurs when populations of the same species become so different they can no longer interbreed to produce fertile offspring.

The process of speciation

1. Isolation: Two populations of a species become separated by a physical barrier (geographical isolation)

   • Examples: mountain ranges, rivers, oceans, or distance
   • Populations can no longer interbreed

2. Different environments: Each population experiences different selection pressures

   • Different climates, food sources, predators or diseases
   • Different characteristics become advantageous in each environment

3. Natural selection: In each population, organisms with advantageous characteristics survive and breed

   • Different characteristics are selected in each population
   • Populations gradually become more different

4. Genetic divergence: Over many generations, the populations become so genetically different they can no longer interbreed

   • Even if the populations meet again, they cannot produce fertile offspring
   • Two separate species have formed

Darwin's finches — a classic example

Charles Darwin studied finches on the Galápagos Islands, which demonstrate speciation:

• An ancestral finch species from mainland South America colonised the islands
• Populations on different islands became geographically isolated
• Different food sources on each island created different selection pressures
• Beak shapes evolved to suit available food (large beaks for cracking seeds, thin beaks for catching insects, pointed beaks for boring into cacti)
• Eventually, different species evolved with different beak shapes and feeding behaviours

The timeframe of evolution

Evolution by natural selection requires many generations to produce significant changes:

• In bacteria: changes can be observed in days or weeks (very rapid reproduction)
• In insects: resistance to pesticides can develop in a few years
• In mammals: changes typically take thousands to millions of years
• Speciation usually takes hundreds of thousands to millions of years

The fossil record shows evolution occurring over geological timescales, while antibiotic resistance shows the same process accelerated in organisms with rapid reproduction rates.

Extinction and evolution

Extinction occurs when all members of a species die out. Causes include:

• Environmental changes (climate change, habitat destruction)
• New diseases
• New predators or competitors
• Catastrophic events (volcanic eruptions, asteroid impacts)

Extinction is important to understanding evolution because:

• It explains gaps in the fossil record
• It shows that species unable to adapt die out
• It creates opportunities for other species to evolve and fill ecological niches
• Over 99% of species that have ever lived are now extinct

Why evolution is a theory, not "just a theory"

In science, a theory is an explanation supported by extensive evidence. The theory of evolution is supported by:

• Fossil evidence showing gradual change
• Observable natural selection (antibiotic resistance, pesticide resistance)
• Genetic evidence (DNA similarities between related species)
• Geographical distribution of species
• Anatomical similarities between related organisms

Scientists continue to refine understanding of how evolution works, but the basic principle that species change over time through natural selection is supported by overwhelming evidence.

Worked examples

Example 1: Interpreting fossil evidence (4 marks)

Question: The diagram shows fossils of horse ancestors found in different rock layers. Fossil A is found in the oldest rocks, fossil D in the youngest.

Explain what this fossil evidence suggests about how horses have evolved. (4 marks)

Model answer:

• Horses have gradually increased in leg length over time ✓
• This suggests they became better adapted for running / escaping predators ✓
• The number of toes decreased from 4 to 1 ✓
• This suggests adaptation for running on harder ground / grasslands / increased speed ✓

Mark scheme notes: Accept any reasonable explanation linking the changes to adaptation or natural selection. Award marks for identifying trends and explaining their adaptive significance.

Example 2: Antibiotic resistance (6 marks)

Question: A patient has a bacterial infection and is prescribed antibiotics. Explain how populations of antibiotic-resistant bacteria can develop if the patient does not complete the full course of antibiotics. (6 marks)

Model answer:

• Random mutations in bacterial DNA occur ✓
• Some mutations make bacteria resistant to the antibiotic by chance ✓
• When antibiotics are taken, most bacteria are killed ✓
• But if the course is not completed, some bacteria (including resistant ones) survive ✓
• These resistant bacteria reproduce rapidly and pass on the resistance gene ✓
• The population becomes mostly resistant bacteria / a resistant strain develops ✓

Mark scheme notes: Award marks for a logical sequence showing mutation → variation → selection pressure → survival of resistant bacteria → reproduction → population change. Accept equivalent phrasing.

Example 3: Speciation (4 marks)

Question: Scientists studied two populations of squirrels living on opposite sides of the Grand Canyon. They found that the squirrels looked different and could no longer breed together. Explain how these two populations became separate species. (4 marks)

Model answer:

• The two populations were geographically isolated by the canyon / could not interbreed ✓
• Different environmental conditions / selection pressures on each side ✓
• Natural selection favoured different characteristics in each population ✓
• Over many generations the populations became so different they could no longer breed to produce fertile offspring ✓

Mark scheme notes: Must mention isolation, different selection pressures, and inability to interbreed. Accept "climate" or "food availability" as examples of different conditions.

Common mistakes and how to avoid them

• Confusing evolution with individual change: Organisms do not evolve during their lifetime; populations evolve over many generations. Individual bacteria don't become resistant — resistant bacteria already exist due to mutation and are selected for.

• Saying organisms "need" to evolve or adaptations happen "because" they're needed: Evolution is not goal-directed. Random mutations create variation; natural selection acts on this variation. Bacteria don't become resistant because they need to survive — resistant bacteria happen to survive by chance.

• Writing "survival of the fittest" without explanation: This phrase alone gains no marks. Explain that organisms with advantageous characteristics are more likely to survive, reproduce and pass on their genes.

• Thinking fossils are only dinosaurs: Fossils include any preserved remains — bacteria, plants, shells, footprints and impressions all count as fossils.

• Misunderstanding the timescale: Speciation takes many generations (usually thousands of years for most organisms). Don't suggest complex organisms like mammals evolve new species in a few years.

• Not explaining the complete process: In 4-6 mark questions about antibiotic resistance or speciation, include all steps: variation (mutation), selection pressure, survival/reproduction, inheritance, and population change over time.

Exam technique for "Evidence for evolution: fossils, antibiotic resistance and speciation"

• "Explain" questions require mechanisms: Don't just describe what happens — explain why at each stage. For antibiotic resistance, state why resistant bacteria survive (antibiotics kill non-resistant bacteria) and why the population changes (resistant bacteria reproduce).

• Use the mark allocation as a guide: A 4-mark question needs 4 distinct points. For antibiotic resistance, a good structure is: mutation → selection pressure → survival of resistant → reproduction/inheritance → population change (5 points for a 4-6 mark question).

• Command words matter: "Describe" = state what happens; "Explain" = give reasons why; "Suggest" = apply your knowledge to an unfamiliar context. Antibiotic resistance questions often use "explain" — you must include causes, not just observations.

• Link evidence to theory: When discussing fossils or antibiotic resistance, explicitly state how they support evolution or natural selection. Don't assume the examiner will make the connection for you.

Quick revision summary

Evolution is the gradual change in inherited characteristics over time. Fossils provide evidence by showing how organisms have changed over millions of years, with simpler forms in older rocks. Antibiotic resistance demonstrates natural selection in action: random mutations create resistant bacteria that survive antibiotics, reproduce and pass on resistance genes, changing the population. Speciation occurs when populations become isolated, experience different selection pressures, and evolve into separate species unable to interbreed. These three types of evidence — from the distant past, present-day observations, and ongoing processes — provide strong support for evolution by natural selection.