How does niche occupation limit adaptive radiations?
- As the number of occupied niches increased, the number of evolved morphotypes decreased.
- Lack of ecological opportunity (unoccupied niches) constrains divergence.
- Consistent with the theory of diversity dependence, where limited ecological opportunity reduces potential for diversification.
How does clade growth in modern birds show evidence for ecological limits?
→ far fewer species exist than we would expect mathematically
→ suggesting there are ecological limits to diversity
What is asymptotic diversity dependence?
- Where the rate of diversification slows down as species richness (diversity) approaches a maximum limit (number of niches)
- Diversification slows over time and eventually levels off, with no significant new species emerging as ecological niches are filled.
Example: In a saturated ecosystem, the introduction of new species becomes less likely as more niches are occupied, leading to a plateau in species richness.
What is Darwinian diversity dependence?
- The concept that as species diversity increases, the rate of diversification slows due to natural selection and ecological constraints.
- Selection Pressure: Increased competition for limited niches and resources.
- Limited Niches: Fewer unfilled niches make it harder for new species to emerge.
- Slowdown in Diversification: As diversity grows, new species evolve more slowly due to stronger selection pressures and tighter ecological limits.
What are the main assertions of the Ecological Limits Hypothesis (ELH)?
- Dynamic Equilibrium: Species richness at biogeographic scales exists in a state of dynamic equilibrium.
- Diversity Dependence: Speciation and extinction rates depend on biodiversity levels.
- Resource Constraints: Evolutionary rates are limited by total resource availability, constraining overall diversity.
What is diversity dependence?
-Idea that the rate of evolutionary change (speciation, competition etc.) is influenced by the no. of sp already present in a community or clade
- Positive: low diversity = many niches = increase speciation
- Negative: higher diversity = increase competition = niche saturation = decrease speciation rate
What does the equilibrial carrying capacity model predict?
That species diversity approaches an upper limit due to ecological constraints.
- Finite Carrying Capacity: Ecosystems can only support a limited number of species.
- Slowdown in Diversification: As species richness increases, competition, resource limits, and niche saturation reduce the rate of new species formation.
- Equilibrium: Over time, speciation and extinction rates balance, leading to a relatively stable number of species.
What are waiting times?
- The time between successive speciation events is modeled as the waiting time.
- Model assumes that diversification is a continuous process, and the number of lineages increases exponentially as long as speciation events continue at the same rate
How does diversity dependence manifest in phylogenies?
Species richness shows three distinct phases:
- Exponential Rise: Rapid diversification initially.
- Deceleration: Slower diversification as niches fill.
- Saturation: Species richness plateaus at equilibrium.
Example: North American wood warblers follow this pattern
What do Anolis lizards reveal about diversity dependence?
- Observation: Decline in diversification rates correlates with island size (ecological opportunity).
- Larger islands support more species but show slower declines in speciation rates.
- Supports the idea of ecological limits on species richness.
What are the arguments for ecological limits in macroevolution?
For Ecological Limits:
- Rabosky (2013): Studied ray-finned fishes, showing strong influence of ecological limits on macroevolutionary patterns.
- Rabosky & Hurlbert (2015): Focused on global patterns of avian diversity, concluding species richness is constrained by ecological factors.
Describe the Diversity-Dependence Model (DDM)
- Rate of evolutionary change (such as speciation) is expected to slow as diversity increases
- Because the community becomes more saturated with species, leading to stronger competition.
How is Darwinian diversity dependence different from asymptotic diversity dependence?
Darwinian Diversity Dependence: Slowdown in diversification due to increased competition and natural selection pressures as more niches are filled.
Asymptotic Diversity Dependence: Diversification slows and eventually levels off as species richness approaches a maximum limit (e.g niche, resources) , regardless of evolutionary constraints.
Asymptotic diversity dependence can correspond to several limiting factors:
- Number of ecological niches.
- Resource availability (food, space, etc.).
- Habitat space.
- Predator-prey interactions.
- Environmental stability.
- Genetic constraints.
- Competition.
- Climate/environmental changes.
What are the arguments against ecological limits in macroevolution?
Diversity is Dynamic & Unbounded
- Wiens (2011): Studied amphibians, found no strict cap on diversity.
- Harmon & Harrison (2015): Examined lizards and mammals, argued diversity is not ecologically limited.
Key Reasons:
- Niche Expansion & Innovation: Evolution creates new niches
- Changing habitats allow species persistence.
- Competition can drive speciation instead of limiting diversity.
- Speciation Outpaces Extinction: Continents allow dispersal and diversification.
- Scale-Dependent Limits: Local constraints may exist, but at larger scales, diversity continues to grow.
Describe how competition can limit speciation
- Competitive Exclusion: The weaker species may go extinct before divergence occurs.
- Resource Limitation: Scarce resources prevent populations from growing and specializing.
- Stabilizing Selection: Strong competition reinforces existing traits, preventing divergence.
E.g Invasive species outcompeting natives, reducing diversification opportunities.
Describe how competition can drive speciation
- ECD: Species evolve differences to reduce niche overlap
- Adaptive Radiation: High competition in new environments leads to rapid diversification
- Disruptive Selection: Extreme traits are favored, splitting populations into distinct groups.
Example: Beak size divergence in Darwin’s finches to specialize on different seed types.
