Character Displacement
Character displacement is an evolutionary pattern in which closely related species exhibit greater morphological, ecological, or behavioral divergence in geographic regions where their ranges overlap (sympatry) than where each species occurs alone (allopatry).
Character displacement is an evolutionary and community-ecological phenomenon in which two or more closely related species exhibit significantly greater phenotypic divergence in geographic regions where they coexist in sympatry than in regions where each species occurs in isolation in allopatry. Formally conceptualized and named by the systematists William L. Brown Jr. and Edward O. Wilson in 1956, character displacement arises when natural selection acts to minimize costly interspecific interactions within zones of geographic overlap. Depending on the selective mechanism driving divergence, biologists distinguish two primary forms: ecological character displacement (ECD), in which interspecific competition for shared limiting resources favors divergence in trophic morphology (such as beak depth, jaw mechanics, or body size) and microhabitat partitioning; and reproductive character displacement (RCD), in which selection against maladaptive hybridization or heterospecific courtship waste (including reinforcement) drives divergence in mating signals, courtship acoustics, coloration, or floral structure.
Theoretical Foundations and Quantitative Lotka-Volterra Mechanics
In quantitative community ecology, ecological character displacement links microevolutionary quantitative genetics directly to LotkaโVolterra resource competition theory. Consider two consumer species utilizing a continuous one-dimensional resource axis x (for example, seed hardness or prey body length) with individual utilization curves of width ฯ and mean trait values z1 and z2. Under Robert MacArthur and Richard Levins's (1967) Gaussian utilization formulation, the interspecific competition coefficient ฮฑ12 decreases exponentially with the squared phenotypic distance between the two species' mean trophic traits:
- Competition Coefficient Decay: ฮฑ(z1, z2) = exp[โ(z1 โ z2)2 / (4ฯ2)].
- Frequency-Dependent Disruptive Selection in Sympatry: When two allopatric populations with similar mean phenotypes (z1 โ z2) expand their geographic ranges into secondary contact, individuals in the overlapping tails of their phenotypic distributions experience intense interspecific resource depletion and reduced reproductive fitness. By contrast, extreme phenotypes in the opposite outer tails of each species' distribution exploit underutilized resources free from interspecific competitors.
- Quantitative Genetic Trajectory: Provided that the trophic trait possesses additive genetic variance (h2 > 0), directional selection drives the mean phenotype of Species 1 downward (ฮz1 < 0) and Species 2 upward (ฮz2 > 0) until the reduction in interspecific competition balances the stabilizing selection imposed by the ambient resource spectrum.
Schluter and McPhail's Six Rigorous Criteria for Ecological Character Displacement
Because geographical differences in morphology between sympatric and allopatric populations can arise from abiotic clines, random genetic drift, or non-random species sorting during colonization rather than in situ evolutionary divergence, Dolph Schluter and J. Donald McPhail (1992) established six operational criteria required to demonstrate ecological character displacement:
- Rejection of Null Chance Models: The observed phenotypic difference between sympatric populations must exceed expectations under null permutation or random-walk models of allopatric population variation.
- Evolutionary Shifts vs. Ecological Sorting: The sympatric-allopatric divergence must reflect an evolutionary genetic change inside the lineage rather than the differential extinction of pre-existing similar populations during community assembly.
- Heritable Genetic Basis: The displaced morphological or physiological differences must have a verified additive genetic basis rather than representing purely environmental phenotypic plasticity.
- Direct Link to Resource Use: The morphological trait shift (e.g., gill-raker spacing or beak depth) must demonstrably alter the dietary or microhabitat resource spectrum utilized by the organism.
- Independent Environmental Controls: Sites of sympatry and allopatry must not differ systematically in abiotic climate or baseline food availability in a way that could independently explain the trait shift.
- Independent Replicate Evolution: The strongest proof comes from replicated natural or experimental systems in which the same divergence pattern evolves independently across multiple isolated sympatric arenas (such as post-glacial lakes or oceanic islands).
Classic Empirical Systems in Evolutionary Ecology
Several long-term field and genomic studies satisfy all six SchluterโMcPhail criteria and illustrate the genomic architecture of character displacement:
| Taxonomic System & Geographic Arena | Displaced Phenotypic Trait | Allopatric State (Solitary Populations) | Sympatric State (Coexisting Populations) | Underlying Genetic / Selective Mechanism |
|---|---|---|---|---|
| Galรกpagos Ground Finches (Geospiza fortis & Geospiza fuliginosa) | Beak depth (mm) and bite force (N) for seed cracking | Intermediate beak depth (~10.0โ10.5 mm) on islands where either species occurs alone (Daphne Major pre-1982, Los Hermanos) | G. fuliginosa evolves shallow beaks (~8.2 mm, small soft seeds) while G. fortis evolves deep beaks (~12.4 mm) on co-occurring islands | Bone morphogenetic protein BMP4 and HMGA2 haplotype frequency shifts observed in real time during the 2004โ2005 Daphne Major drought after Geospiza magnirostris colonized |
| Threespine Sticklebacks (Gasterosteus aculeatus species pairs, British Columbia) | Body size, jaw gape, and gill-raker number/length | Solitary post-glacial lakes contain a single intermediate trophic generalist eating both littoral invertebrates and zooplankton | Sympatric lakes contain a small, slender limnetic planktivore (long, dense gill rakers) and a large, deep-bodied benthic macroinvertebrate feeder | Divergent selection on craniofacial quantitative trait loci (Eda, Pitx1 regulatory enhancers) combined with size-assortative mating |
| North American Spadefoot Toads (Spea multiplicata & Spea bombifrons) | Tadpole trophic polyphenism (omnivore vs. carnivore morph frequency) and female mating call preference | S. multiplicata produces ~25โ40% large-jawed carnivore tadpoles in allopatric desert ponds | In sympatry with the superior carnivore S. bombifrons, S. multiplicata shifts almost exclusively (< 5% carnivore) to small detritus-feeding omnivore tadpoles | Maternal investment and thyroid-hormone developmental plasticity paired with reproductive character displacement in male pulse-rate acoustic calls |
| Eurasian Rock Nuthatches (Sitta tephronota & Sitta neumayer) | Bill length (mm) and facial black eye-stripe width | Both species have virtually identical bill lengths (25.5โ26.0 mm) and narrow eye stripes in allopatric Balkans vs. Central Asia | Where ranges overlap across the Zagros Mountains of Iran, S. tephronota evolves a 29 mm bill and broad facial stripe while S. neumayer evolves a 23 mm bill and minimal stripe | Combined ecological divergence in arthropod prey size and visual plumage recognition blocking interspecific territorial aggression and hybridization |
Reproductive Character Displacement and Agonistic Character Displacement
When two incipient species with incomplete postzygotic isolation come into secondary contact, interspecific matings produce sterile or inviable hybrids (as governed by Haldane's rule and BatesonโDobzhanskyโMuller incompatibilities). Natural selection then acts directly on prezygotic mate-recognition traits via reinforcement, generating reproductive character displacement:
- Acoustic and Pheromonal Divergence: In Australian chorus frogs (Pseudophryne) and Drosophila pseudoobscura / Drosophila persimilis, sympatric females exhibit heightened choosiness and males evolve displaced acoustic pulse rates or cuticular hydrocarbon profiles compared to allopatric populations.
- Floral Color Displacement in Plants: In the Texas wildflower Phlox drummondii, allopatric populations produce light-blue corollas indistinguishable from the closely related Phlox cuspidata. Within the sympatric overlap zone across central Texas, P. drummondii has evolved dark-red petals via regulatory mutations in two anthocyanin biosynthetic genes (F3'5'H and R2R3-MYB), causing shared lepidopteran pollinators to maintain floral constancy and reducing costly interspecific pollen transfer by 68%.
- Agonistic Character Displacement: Even when two species do not hybridize, misdirected interspecific territorial aggression wastes energetic reserves and elevates predation risk, selecting for divergent territorial song frequencies or badge plumage (as documented in European Ficedula pied and collared flycatchers).
References
- Brown, W. L., & Wilson, E. O. (1956). Character displacement. Systematic Zoology, 5(2), 49โ64. https://doi.org/10.2307/2411924
- Schluter, D., & McPhail, J. D. (1992). Ecological character displacement and speciation in sticklebacks. The American Naturalist, 140(1), 85โ108. https://doi.org/10.1086/285404
- Grant, P. R., & Grant, B. R. (2006). Evolution of character displacement in Darwin's finches. Science, 313(5784), 224โ226. https://doi.org/10.1126/science.1128374
- Pfennig, K. S., & Pfennig, D. W. (2009). Character displacement: ecological and reproductive responses to a common evolutionary problem. The Quarterly Review of Biology, 84(3), 253โ276. https://doi.org/10.1086/605079