Evidence for evolution: fossils and data interpretation

Inheritance, variation and evolution • The development of understanding of genetics and evolution

Flashcards

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What information does the fossil record provide?

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The fossil record provides a time-ordered sequence of past organisms, evidence of gradual change, examples of transitional forms and patterns of extinction and diversification.

Key concepts

What you'll likely be quizzed about

Fossils and the fossil record

Fossils are physical remains or traces of organisms preserved in rocks or sediments. Fossil formation occurs when soft tissue does not decay (for example in oxygen-poor acidic peat bogs), when hard parts are replaced by minerals during burial under sediments, or when traces such as footprints or burrows are preserved. Fossils found in layered rocks produce a fossil record that places organisms in a time sequence because lower layers are older than upper layers. The fossil record shows gradual changes in some lineages and stability in others, but it contains gaps because not all organisms fossilise and some fossils are destroyed by geological processes.

Examples from the fossil record

Specific fossils provide direct evidence of evolutionary transitions and environmental change. Archaeopteryx shows a mix of reptile and bird features, supporting the origin of birds from reptiles. The coelacanth demonstrates long-term morphological stability in some lineages, while fossil sequences for horses document gradual changes in foot structure linked to drying habitats. Individual fossil examples support broader patterns seen across the fossil record.

Antibiotic resistance as modern evidence

Antibiotic resistance demonstrates evolution by natural selection acting on genetic variation across short timescales. Random mutations produce resistant variants; antibiotic treatment kills susceptible bacteria, so resistant bacteria survive and reproduce. Over successive generations, resistant strains can become dominant in a population. Methicillin-resistant Staphylococcus aureus (MRSA) provides a clinically important example of this process, with resistance increasing after widespread antibiotic use. Overuse and misuse of antibiotics accelerate the spread of resistance.

Reading fossil data, graphs and evolutionary trees

Graphs, scatter plots, bar charts and evolutionary trees summarise fossil counts, ages and relationships. The vertical sequence of rock layers gives relative ages; numerical axes on graphs give absolute or relative times. Evolutionary trees display patterns of relatedness and common ancestry: branching points represent common ancestors, and branch lengths may indicate time or degree of change depending on the diagram. Careful reading of axes, keys and labels allows extraction of trends, rates and timing of divergences from tables and graphs.

Key notes

Important points to keep in mind

Fossils form by preservation, mineral replacement, or trace preservation; identify which process produced a specimen.

Lower rock layers are older; use stratigraphy to place fossils in sequence.

Mutation creates variation; selection by antibiotics increases resistant bacteria frequency.

Read graph axes, units and keys before extracting trends or rates.

Branching order on evolutionary trees shows relatedness; check whether branch lengths represent time.

Use multiple lines of evidence (fossils, morphology, DNA, observed evolution) to strengthen evolutionary conclusions.

Recognise fossil record gaps and state limiting factors when drawing conclusions from fossil data.