AP Syllabus focus:
‘Geological formations and species’ geographic distributions reveal patterns of change consistent with evolution over time.’
Evolution leaves signatures not only in DNA and anatomy, but also on maps and in rocks. By linking Earth’s changing geology with where organisms live, biologists infer shared ancestry, divergence, and historical movement of lineages.
Core idea: Earth history shapes biological history
Geological change alters habitats, connectivity, and barriers to gene flow, producing predictable geographical patterns in relatedness.
Biogeography connects place to evolutionary relationships
Biogeography: The study of the geographic distribution of organisms across space and time and how those distributions reflect evolutionary and environmental history.
Biogeographic patterns support evolution when:
Nearby regions tend to contain more closely related species than far-apart regions with similar climates.
Unique (endemic) species cluster in isolated places (islands, isolated lakes, mountaintops), consistent with descent and divergence after isolation.
Species distributions align with known geological events, indicating that populations changed as Earth changed.
Geological formations that generate evolutionary patterns
Plate tectonics and continental drift
As plates move, continents split, collide, and rearrange, changing where lineages can live and disperse.
A global tectonic-plate map showing plate names, boundary types (divergent, convergent/subduction, transform), and arrows indicating plate motion. This helps connect specific geological mechanisms (rift formation, mountain building, seaway opening/closing) to predictable changes in isolation and gene flow among populations. Source
Vicariance: a widespread population becomes separated when geology creates a barrier (continent breakup, seaway formation), and descendants diverge on each side.
Long isolation can yield distinct regional biotas (for example, lineages concentrated on particular southern landmasses after ancient separations).
Mountain building, river formation, and seaways
Physical structures can divide populations for long periods.
Orogeny (mountain uplift) can separate lowland populations into east–west lineages and create new high-elevation niches.
Rivers and canyons can act as semi-permeable barriers; limited crossing can produce related but distinct populations on opposite banks.
Seaways fragment coastlines and isolate terrestrial populations; later regression can reconnect regions, leaving mixed distributions.
Glaciation and climate-driven range shifts
Ice ages and interglacial warming repeatedly compress and expand habitats.
Refugia (ice-free pockets) can preserve isolated populations; later expansion can produce contact zones where closely related lineages meet.
Latitudinal and elevational shifts can explain why related species occur in separated patches of suitable habitat.
Land bridges and corridors
Temporary connections can explain “unexpected” distributions.
When sea level falls, land bridges permit dispersal; when sea level rises, those migrants become isolated, often diverging.
Map outlining the Beringia region, illustrating how lowered sea levels during glacial periods exposed land connecting Asia and North America. It visualizes how temporary corridors can enable dispersal and later isolation, producing the “unexpected” but explainable geographic distributions highlighted in biogeography. Source
Narrow habitat corridors can allow movement for some organisms but not others, producing taxon-specific distribution patterns.
Geographical patterns that indicate evolutionary change
Island patterns: isolation, colonisation, and divergence
Islands are natural experiments in geography-driven evolution.
Conceptual graph of the Equilibrium Theory of Island Biogeography showing immigration rates decreasing with increasing species richness and extinction rates increasing, with their intersection predicting an equilibrium number of species. It links island distance (immigration) and island size (extinction) to expected biodiversity patterns, providing a quantitative framework for interpreting endemism and divergence on islands. Source
Island species often resemble those of the nearest mainland, consistent with colonisation followed by divergence.
Different islands can contain closely related but distinct forms, reflecting limited gene flow and local adaptation.
High endemism supports descent with modification after a founding event and subsequent isolation.
“Similar environments, different ancestors”
If two regions share climate and vegetation but have different dominant lineages, that pattern supports evolution from local ancestors rather than separate creation for each habitat.
Example pattern: deserts on different continents often have ecologically similar organisms, yet their closest relatives live on the same continent, implying independent evolutionary histories shaped by geography.
Using geology to test evolutionary hypotheses
Geological and geographical data strengthen evolutionary explanations when they are consistent with:
Timing: the barrier/connection existed long enough to influence divergence.
Directionality: likely dispersal routes match ocean currents, prevailing winds, or stepping-stone habitats.
Nested relatedness: distributions align with branching descent (regional clusters of related taxa rather than random scattering).
FAQ
They compare likely routes and barriers (currents, wind belts, distances) and check whether multiple unrelated taxa show the same split, which is more consistent with vicariance.
Dispersal ability and life history matter.
Flight, seeds, or larval stages can cross water
Habitat specialists may be blocked by small unsuitable gaps
An area of endemism is a region containing multiple endemic taxa. Overlap among endemics can signal long-term isolation or stable habitats that promoted independent evolutionary histories.
River capture and course shifts can swap drainage basins, moving aquatic lineages without ocean dispersal. This can create close relatedness between species in currently separate river systems.
They create new habitats in predictable locations, enabling repeated colonisation and isolation. Over time this can generate clusters of closely related endemics associated with the hotspot’s track.
Practice Questions
Explain how the formation of a mountain range can lead to patterns of species distribution consistent with evolution over time. (2 marks)
Mountain range acts as a geographic barrier reducing gene flow between populations (1).
Isolated populations diverge over generations, producing related but distinct species/lineages on each side (1).
A chain of volcanic islands increases in age with distance from the mainland. Closely related bird species occur on multiple islands, and each island has at least one endemic species. Explain how geological and geographical information supports evolution in this scenario. (5 marks)
Volcanic origin and age gradient implies islands formed sequentially, providing a timeline for colonisation (1).
Initial colonisation from mainland (or older island) explains close relatedness to nearby source populations (1).
Isolation limits gene flow among islands, enabling divergence (1).
Endemism is expected if populations evolve independently after colonisation (1).
Stepping-stone dispersal among islands explains a pattern of related species across the chain rather than random distributions (1).
