What does the fossil record tell us about evolution and mass extinctions?
Evolution and the fossil record: evidence for evolution from the fossil record (morphological change through time, transitional forms, adaptive radiation); the models of evolutionary change (gradualism versus punctuated equilibrium); mass extinctions and their causes and effects (for example the end-Permian and end-Cretaceous events); the incompleteness and biases of the fossil record.
A focused answer to the OCR H414 dot point on evolution. Covers the fossil evidence for evolution (morphological change, transitional forms, adaptive radiation), the gradualism and punctuated equilibrium models, mass extinctions (the end-Permian and end-Cretaceous events) and their causes and effects, and the incompleteness and biases of the fossil record.
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What this dot point is asking
OCR wants you to describe the fossil evidence for evolution (morphological change, transitional forms, adaptive radiation), to distinguish the gradualism and punctuated equilibrium models, to describe mass extinctions and their causes and effects with named examples, and to recognise the incompleteness and biases of the fossil record.
The answer
Evidence for evolution from fossils
The fossil record provides direct evidence that life has changed through time:
- Morphological change through time. Successive strata show the same lineage changing form gradually (for example the evolution of the horse, or of ammonite suture patterns).
- Transitional forms. Fossils with features of two groups (for example Archaeopteryx, with reptilian and bird features) show intermediate stages between major groups.
- Adaptive radiation. After new opportunities arise (a new habitat, or vacated niches after an extinction), a group rapidly diversifies into many forms adapted to different ways of life.
Models of evolutionary change
Two models describe the rate of change:
- Gradualism. Slow, continuous, small changes over long periods; predicts a smooth series of transitional forms in the fossil record.
- Punctuated equilibrium. Long periods of little change (stasis) interrupted by short bursts of rapid change (often at speciation or environmental upheaval); predicts long intervals of stability and then an abrupt appearance of new forms, with few intermediates preserved.
The real fossil record shows examples of both, and the apparent "jumps" of punctuated equilibrium are partly explained by the record's incompleteness.
Mass extinctions
Named examples and likely causes:
- End-Permian (about years ago), the largest extinction, removing most marine species, linked to massive volcanism (the Siberian Traps), climate change and ocean anoxia.
- End-Cretaceous (about years ago), which ended the non-avian dinosaurs and ammonites, widely linked to a large meteorite impact (and possibly volcanism), causing climate change and food-chain collapse.
Effects. Mass extinctions empty ecological niches and remove dominant groups, reducing competition. Survivors then undergo adaptive radiation, rapidly diversifying to fill the vacated niches (for example mammals radiating after the dinosaurs). So mass extinctions are turning points that reset the course of evolution, and they conveniently mark the boundaries between eras.
Incompleteness and bias of the record
The fossil record is incomplete and biased, because preservation is rare and uneven:
- Preservation bias. Hard-bodied, marine and widespread organisms are over-represented; soft-bodied and terrestrial organisms are under-represented.
- Sampling bias. Only exposed, accessible rocks can be studied, and erosion destroys part of the record.
So the record under-samples real diversity, which is why some evolutionary transitions appear sudden.
Examples in context
Example 1. Mammals after the dinosaurs. The end-Cretaceous extinction cleared the large-animal niches, and mammals radiated rapidly to fill them, illustrating how extinctions drive subsequent diversification.
Example 2. Ammonite zones recording evolution. The rapid morphological change of ammonites through the Mesozoic both demonstrates evolution and provides the short-ranging zone fossils used to subdivide and correlate the strata.
Try this
Q1. State one type of fossil evidence for evolution. [1 mark]
- Cue. For example, morphological change of a lineage through successive strata, transitional forms (Archaeopteryx), or adaptive radiation.
Q2. Describe how punctuated equilibrium would appear in the fossil record. [2 marks]
- Cue. Long intervals of little change (stasis) interrupted by the abrupt appearance of new forms, with few transitional fossils preserved.
Q3. Explain one effect of a mass extinction on later evolution. [2 marks]
- Cue. It empties ecological niches and reduces competition, allowing surviving groups to undergo adaptive radiation and diversify rapidly (for example mammals after the dinosaurs).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20204 marksExplain the difference between the gradualism and punctuated equilibrium models of evolution, and describe how each would appear in the fossil record.Show worked answer →
Define each model, then its fossil signature.
- Gradualism
- Evolution proceeds by slow, continuous, small changes over long periods. In the fossil record this would appear as a gradual series of forms showing steady, incremental morphological change from ancestor to descendant, with many transitional forms.
- Punctuated equilibrium
- Species remain largely unchanged for long periods (stasis), interrupted by short bursts of rapid change, often associated with speciation or environmental change. In the fossil record this would appear as long intervals of little change, then an abrupt appearance of a new form, with few preserved intermediates.
- The key contrast
- Gradualism predicts a smooth, continuous transition; punctuated equilibrium predicts long stasis punctuated by sudden jumps.
Markers reward the defining feature of each model and its predicted appearance in the fossil record (continuous change versus stasis plus sudden jumps).
OCR H414/02 20186 marksDescribe what is meant by a mass extinction, using a named example, and discuss the effects of mass extinctions on the subsequent evolution of life.Show worked answer →
A level-of-response answer; define, exemplify, then discuss the aftermath.
- Definition
- A mass extinction is a relatively sudden event in which a large proportion of species across many groups becomes extinct worldwide in a short geological time.
- Named example
- The end-Cretaceous event (about years ago), in which the non-avian dinosaurs and ammonites died out, widely linked to a large meteorite impact (and possibly massive volcanism), which caused climate change and collapse of food chains. (The end-Permian event, the largest, removed an even greater proportion of marine species.)
- Effects on later evolution
- Mass extinctions empty ecological niches, removing dominant groups and reducing competition. This allows surviving groups to undergo adaptive radiation, rapidly diversifying to fill the vacated niches. For example, after the end-Cretaceous extinction, mammals radiated rapidly and came to dominate. So mass extinctions act as major turning points, resetting the course of evolution.
Top-band answers define a mass extinction, give a correct named example with a plausible cause, and discuss the clearing of niches and subsequent adaptive radiation of survivors.
Related dot points
- Fossils: the conditions that favour preservation (rapid burial, anoxia, hard parts, fine sediment); the modes of preservation (moulds and casts, permineralisation, carbonisation, and preservation in amber or ice); the properties of a good index (zone) fossil (abundant, widespread, easily recognised, short stratigraphic range); the distinction between body and trace fossils.
A focused answer to the OCR H414 dot point on fossils. Covers the conditions favouring preservation, the modes of preservation (moulds and casts, permineralisation, carbonisation, amber and ice), the properties of a good index or zone fossil, and the distinction between body and trace fossils.
- Relative dating: the principles used to order geological events (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession); the recognition of way-up evidence; the application of these principles to construct the geological history of a cross-section, including faults, intrusions and unconformities.
A focused answer to the OCR H414 dot point on relative dating. Covers superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession, way-up evidence, and how to apply these principles to reconstruct the geological history of a cross-section with faults, intrusions and unconformities.
- The geological record: the hierarchy of the geological time scale (eon, era, period, epoch) and the major divisions (Precambrian and the Phanerozoic eras); correlation of strata by lithostratigraphy (matching rock units) and biostratigraphy (matching fossils and biozones); the use of marker horizons such as volcanic ash bands; the distinction between rock units (systems) and time units (periods).
A focused answer to the OCR H414 dot point on the geological time scale and correlation. Covers the eon, era, period and epoch hierarchy and the major divisions, correlation by lithostratigraphy and biostratigraphy, the use of marker horizons such as ash bands, and the distinction between rock units and time units.
- Radiometric dating: radioactive decay of unstable parent isotopes to stable daughter isotopes; the concept of half-life as a constant; the use of parent-to-daughter ratios to calculate absolute ages; the main isotopic systems (uranium-lead, potassium-argon and carbon-14) and their suitable age ranges; the assumptions and limitations of radiometric dating; the combination of absolute and relative dating.
A focused answer to the OCR H414 dot point on radiometric dating. Covers radioactive decay of parent to daughter isotopes, half-life as a constant, calculating absolute ages from parent-to-daughter ratios, the uranium-lead, potassium-argon and carbon-14 systems and their ranges, the assumptions and limitations, and combining absolute with relative dating.
- Sedimentary environments: the concept of facies as a body of rock reflecting a particular depositional environment; sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks and desiccation cracks) and their interpretation; the characteristics of the main environments (fluvial, deltaic, shallow marine, deep marine and desert); the construction and interpretation of sedimentary logs to reconstruct environmental change.
A focused answer to the OCR H414 dot point on sedimentary environments. Covers facies, sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks, desiccation cracks) and their interpretation, the fluvial, deltaic, shallow-marine, deep-marine and desert environments, and how sedimentary logs reconstruct environmental change.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)