AP Syllabus focus:
‘Divergent evolution and adaptive radiation occur when populations adapt to new habitats, increasing phenotypic diversification and speciation rates.’
Divergent evolution explains how related populations become increasingly different as they adapt to different environments. Adaptive radiation is a prominent form of divergence that rapidly generates many species from one ancestor when new habitats and opportunities arise.
Core idea: divergence produces biodiversity
Divergent evolution
Divergent evolution occurs when populations sharing a common ancestor experience different environmental conditions and, over generations, accumulate differences in traits linked to survival and reproduction.
Divergent evolution: evolutionary change in which related populations become less similar over time because different environments favor different heritable traits.
Divergence is driven by natural selection acting on heritable variation, so trait differences increase when selection pressures differ among habitats.
Frequency histograms of Darwin’s finch beak size (PC1) from different locations and years on Santa Cruz Island. The plots illustrate how the distribution of a fitness-relevant trait can differ among habitats, including cases with clear bimodality (two peaks) consistent with disruptive selection and emerging ecological divergence. Source
Adaptive radiation
Adaptive radiation is a specific pattern of divergence marked by unusually fast lineage splitting and ecological diversification, often visible as many closely related species with different adaptations.
Photo-based overview pages on the Hawaiian silversword alliance, a classic plant example of adaptive radiation on islands. The images emphasize how closely related species can evolve strikingly different growth forms and leaf/flower traits when they adapt to distinct habitats across an archipelago. Source
Adaptive radiation: rapid diversification of a single ancestral lineage into multiple species, each adapted to a distinct habitat or ecological role.
Adaptive radiations raise phenotypic diversification (observable trait variety) and increase speciation rates because multiple populations adapt along different ecological pathways at the same time.
How new habitats increase diversification and speciation rates
Ecological opportunity and selection
When populations encounter new habitats (new islands, newly formed lakes, newly opened niches after extinctions, or novel resources), selection can favor different trait combinations in different places or microhabitats.
Key outcomes of adapting to new habitats:
Different resources (food type, feeding location, activity time) favor different morphologies and behaviors.
Different abiotic conditions (temperature, salinity, moisture, light) favor different physiological tolerances.
Different biotic interactions (predators, competitors, pathogens) favor different defenses and life-history strategies.
As habitat use diverges, populations often experience reduced gene flow because individuals are less likely to encounter and mate with members using different habitats. Reduced gene flow allows differences to accumulate faster, increasing the likelihood of speciation (formation of distinct species).
Trait divergence linked to fitness
Divergence is most pronounced in traits that strongly affect fitness in a particular habitat, such as:
Feeding structures (beak/jaw size and shape) matched to resource type
Locomotion traits (limb length, body shape) matched to substrate or water flow
Physiological traits (water conservation, detoxification ability) matched to local conditions
Signals and preferences (coloration, calls) that can shift with habitat and contribute to mating divergence
Even small differences in habitat can produce consistent selection for different optima, making populations increasingly distinct in phenotype and, eventually, in reproductive compatibility.
Recognising patterns consistent with adaptive radiation
Common signatures
Patterns that commonly indicate adaptive radiation include:
Many species that are closely related (share a recent common ancestor)
Species occupying different habitats or ecological roles
Trait differences that appear functional for those roles (e.g., feeding or habitat-use adaptations)
Rapid appearance of multiple lineages over relatively short evolutionary time (inferred from genetic and/or fossil evidence)
Why speciation can accelerate
Speciation rates can increase during adaptive radiation because:
Multiple habitats create multiple, simultaneous selection regimes
Populations become spatially or ecologically separated, lowering gene flow
Strong selection on habitat-linked traits can indirectly promote divergence in mating or breeding patterns
FAQ
Triggers often include habitat formation (e.g., islands, lakes), resource novelty, or competitor removal. These increase unoccupied ecological roles that selection can favour in different ways.
A key innovation is a trait that enables access to new resources or habitats. By expanding ecological opportunity, it can increase the number of viable niches for descendant populations.
Yes. Divergence can occur across microhabitats (depth, host species, soil type) if individuals preferentially use different habitats and gene flow drops as a result.
They test whether trait differences correlate with habitat/resource use and whether diversification rates exceed background expectations, using phylogenies plus ecological and functional data.
They combine isolation (limiting gene flow from outside) with ecological release (fewer competitors/predators), making it easier for populations to diversify into distinct roles.
Practice Questions
Explain how divergent evolution can lead to increased phenotypic diversification when populations adapt to different habitats. (2 marks)
Different habitats impose different selection pressures, so different heritable traits are favoured. (1)
Over generations, populations accumulate differences in phenotype, becoming less similar. (1)
Describe adaptive radiation and outline two reasons why it can increase speciation rates when a lineage encounters new habitats. (5 marks)
Defines adaptive radiation as rapid diversification from a common ancestor into multiple species adapted to different habitats/ecological roles. (2)
New habitats provide ecological opportunity (new resources/conditions) leading to divergent natural selection among populations. (1)
Reduced gene flow due to spatial or ecological separation allows divergence to build more quickly. (1)
Multiple distinct selection regimes operate simultaneously, accelerating lineage splitting/speciation rate. (1)
