Ecological character displacement Flashcards
Define ecological character displacement (ECD)
Where species (that share a common ancestor) evolve different traits in sympatry to reduce competition for similar resources
The Ghost of Competition Past (Connell 1980)
- Used to explain why co-occurring species differ in niche.
- Proposes that competition causes character displacement, where divergent selection acts to reduce niche overlap.
- Outcome: Species coexist with minimal competition.
What are the 2 steps involved in ECD? and the outcomes.
- Interspecific competition for limited resources = natural selection favours individuals better at partitioning resources
- This drives pops. to diverge by changing trait means or reducing trait variance (example of divergent selection)
Outcomes
- Exaggerated divergence in sympatry
- Trait overdispersion
What is the relationship between interspecific competition and ECD? and what are the outcomes of ECD?
Interspecific competition is the proximate cause of ECD, creating resource use partitioning, which is the ultimate cause of phylogenetic divergence
What is Trait Overdispersion?
- Phenomenon where the variation in traits (e.g., size, shape, behavior) among co-occurring species is greater than expected, often due to competition
- Occurs in sympatric sp, a result of ECD
Describe how threespine sticklebacks are an example of ECD
Benthic and limnetic morphs
- When in allopatry size and body shape are the same
- But when in sympatry they become more exaggerated
Describe how tadpoles of the Mexican spadefoot toad are an example of ECD
- Omnivore morphs more frequently in sympatry
- Carnivore morphs (larger etc.) more frequent in allopatry
Describe how the North American Weasel are an example of ECD
- Sexual dimorphism in skull size
- Example of community wide character displacement, trait overdispersion
List Schulters 6 criteria to diagnose ECD
- Genetic Basis– Trait differences in sympatry vs. allopatry must be evolved, not due to phenotypic plasticity.
- Not Due to Chance – The pattern of divergence must be statistically unlikely to have occurred randomly.
- Evolution, Not Species Sorting – Differences must be caused by adaptive evolution, not just species with pre-existing traits sorting into different locations.
- Trait Matches Resource Use – Divergent traits must directly correlate with ecological function (e.g., beak size → food type).
- Similar Environments – Sympatric and allopatric sites should not differ significantly in environment, ensuring competition (not habitat differences) drives the pattern.
- Evidence of Competition – Species with similar traits must compete for limited resources, supporting competition as the selective force behind divergence.
How do Darwin’s finches demonstrate Ecological Character Displacement (ECD)?
Species Involved:
G. fortis (medium ground finch) – has two beak morphs (small & large).
G. magnirostris (large ground finch) – specialist on large seeds.
Key Event (Daphne Major, 2004 Drought):
- Drought caused depletion of seed resources, increasing competition between the two species.
- G. fortis with large beaks competed directly with G. magnirostris for large seeds.
- Selection favored smaller-beaked G. fortis, reducing niche overlap and minimizing competition.
Outcome:
- Trait divergence in G. fortis: Beak size shifted smaller to reduce competition.
- Direct evidence of ECD: Competition drove adaptive evolution in real-time.
Which of Schluter’s six tests for Ecological Character Displacement (ECD) does the Darwin’s finches example satisfy?
✅ 1. Genetic Basis – Beak size evolution in G. fortis was heritable and not due to plasticity.
✅ 2. Not Due to Chance – The shift in beak size directly followed a major drought and competition event, making a random explanation unlikely.
✅ 3. Evolution, Not Species Sorting – The population of G. fortis remained, but the trait distribution changed due to selection, showing adaptive evolution rather than species sorting.
✅ 4. Trait Matches Resource Use– Smaller beak size in G. fortis matched a shift in resource use (smaller seeds).
✅ 5. Similar Environments – All individuals experienced the same drought conditions, meaning environmental differences did not drive the change.
✅ 6. Evidence of Competition – Direct competition was observed between G. fortis and G. magnirostris for large seeds, driving selection for smaller beaks.
Conclusion: The Darwin’s finches example satisfies all six of Schluter’s tests for ECD
How strong is the evidence for Ecological Character Displacement (ECD)?
- Data from 144 studies:
- Only 1–4 cases meet at least 4 of Schluter’s criteria.
- Weakest support for criterion 6 (direct evidence of competition).
- Cases meeting all 6 criteria are extremely rare.
❓ Does this mean ECD is rare?
➡ Probably not. The issue is more likely that ECD is difficult to study, rather than it being rare.
➡ Directly proving competition-driven evolution is hard, requiring long-term data and genetic evidence.
Stroud et al. 2024
What is the background of the ECD study on Anolis species in the wild?
Species Studied:
- Brown Anole (Cuba, Bahamas)
- Crested Anole (Puerto Rico)
- Divergence: Diverged ~50MYA and do not hybridize.
- Ecological Similarity: Highly convergent in ecology, morphology, and behavior, classified as the same ‘trunk ground’ ecomorph.
- Range Expansion: Both species moved from native ranges to the mainland US.
- Brown Anole: Widespread across Florida.
- Crested Anole: More limited to Miami.
- In Native Ranges: Both are habitat generalists occupying similarly structured environments.
- In South Florida: Occupy a gradient of habitats, from closed canopy forests to urban areas.
Stroud et al. 2024
How did researchers measure character displacement in Anolis species?
Study Focus: Resource use (habitat, diet), movement behavior, and morphology.
- Sites: 9 sites across Miami.
- 3 Allopatric Sites (for each species).
3 Sympatric Sites (both species present).
Resource Use:
- Recorded habitat use (e.g., perch height).
- Analyzed diet (extracted from stomach contents).
- Movement: recorded movement frequency between trees and changes in perching location.
- Morphology: Measured limb length and body size.
Stroud et al. 2024
What were the key findings regarding resource use, movement behavior, and morphology in the Anolis species?
Resource Use:
- Allopatry: Both species had similar habitat use, typically around the base of trees.
- Sympatry:
Crested Anole: Perched higher in the tree.
Brown Anole: Stayed closer to the ground.
Movement Behavior:
- Allopatry: Movement patterns were similar for both species.
- Sympatric : Brown Anole moved across the ground 4.5x more frequently than when in allopatry.
Morphology:
- Body Size: No significant difference between sympatric and allopatric populations.
- Hind Limb Length:
Brown Anole: Sympatric lizards had longer hind limbs.
Crested Anole: No morphological differences between sympatric and allopatric populations.
Stroud et al. 2024
How was directional selection demonstrated in the Brown Anole?
- Pre-Invasion: Survival not associated with limb length.
- Post-Invasion: Survival depended on the relative hind and forelimb length.
- Conclusion: Evidence for directional selection in sympatric populations.
Stroud et al. 2024
What was the overall conclusion from the Anolis character displacement study?
- Compelling case for ECD in Brown Anole.
- Strong evidence for directional selection in sympatric conditions.
- No evolutionary change in the next 4-5 generations observed.
- Further study is needed to rule out phenotypic plasticity as the cause of trait differences.
Compare ECD to Brownian motion
- Brownian is an example of a stochastic event, a random process where species are independent
- ECD is deterministic and depends on the extent of niche overlap between competing species
Describe the expected repulsion effect when species are well diverged in allopatry
If sp well diverged in allopatry, when they come into sympatry the repulsion effect will be less
Describe the expected repulsion effect if species have recently diverged in allopatry
Species that have recently speciated in allopatry (high niche overlap) = strong expected repulsion effect in sympatry and will evolve further apart
→ over time, repulsion effect becomes weaker as species become more different
→ Overlap will eventually = 0 (no competition) = Brownian motion
What is theProtracted Speciation Model, and how does it describe speciation?
- Speciation is a gradual process rather than a single event
- Incipient Speciation: Species are in the early stages of diverging, but they are not yet fully distinct.
- Good Species: When speciation is complete, the populations have fully diverged and are considered distinct species.
How does competition influence the Protracted Speciation Model?
- Inititaion rate not influenced by competition
- Completion rate depends on how much divergence accumulates between pops (more diffs = more likley to complete speciation)
- Good and incipient pops driven to extinction if they fail to become different from competing lineages (according to the principles of limiting similarity and competitive exclusion)
= More different you become the less likely you are to go extinct
Why is the the Protracted Speciation Model useful
- From process model → make predictions of observable outcomes
- Simplifies evolutionary processes
- Emergent patterns can be measured in real data (but this is hard)
- Provide new insight into the origins and dynamics generating the diversity of life on Earth
How does the Protracted Speciation Model relate to species richness?
- Competition influences both speciation and extinction.
- Competition speeds up speciation completion by increasing trait divergence.
- Divergent species are more likely to coexist, increasing species richness.
- However, competition can also suppress richness through competitive exclusion, where similar species are driven to extinction.
How does competition affect speciation in the Protracted Speciation Model?
- Competition increases the overall speciation rate by driving trait divergence.
- Speciation rate declines over time as ecological opportunity decreases.
- Each speciation event takes longer as available niches fill up, making divergence harder.
How does competition affect net diversification in the Protracted Speciation Model?
When competition influences both speciation & extinction:
- Net diversification rates are suppressed due to competitive exclusion.
- Speciation duration is shorter, as species must diverge quickly or risk extinction.
How does competition affect trait evolution in the Protracted Speciation Model?
Competition allowed to influence speciation but not extinction
- Results in greater diversity of trait value (higher disparity)
- Stronger competition → Faster rates of trait evolution, as species must diverge more rapidly to reduce competition.
What are the outcomes of a bounded trait space in the Protracted Speciation Model?
-Trait space width determines the limits on how much traits can evolve.
- Narrow bounds → Lower species richness, slower diversification, and reduced trait disparity.
- Stronger constraints limit how much species can diverge, restricting evolution.
What Determines Whether ECD Causes Divergent or Directional Selection in Sympatry?
Resource Overlap & Competition:
- Strong, equal competition → divergent
- Asymmetric competition → directional.
Initial Trait Differences:
- Pre-existing differences favor divergence
- Similarity may lead to directional shifts.
Resource Distribution:
- Bimodal resources (large or small seeds) → divergent
- Unimodal (mostly large seeds) → directional
Competitive Ability:
- Equal competitors diverge
- Weaker species shifts.
Frequency-Dependent Selection :
Rare trait advantage → divergent
Dominant trait advantage → directional
Define phenotypic plasticity
The ability of a single genotype to produce different phenotypes in response to environmental cues, not due to evolutionary changes
E.g Chemical cues from predators cause Daphnia (water fleas) to produce longer spikes on their bodies, making them harder to consume
