Evolutionary Ecology and Eco-Evolutionary Dynamics

Core Concepts of Evolutionary Ecology

Definition of Evolutionary Ecology: Evolutionary ecology focuses on the interplay between ecology and evolution. It examines how ecological processes and interactions (e.g., competition, predation, environment) influence evolution, and conversely, how evolutionary processes and patterns affect ecology.
Integration Goals: The field seeks to gain new insights into biology by integrating these two disciplines rather than treating them as separate entities.
Learning Objectives: - Understand how ecology and evolution interact across different timescales. - Grasp how genetic composition within a species impacts ecological dynamics. - Identify the mechanisms through which genetic diversity impacts communities. - Examine the impact of rapid evolution on ecology. - Familiarize with experimental methods used to test the ecological impacts of rapid evolution.

Evolutionary and Ecological Interactions Across Timescales

Long Timescales (Macroevolution): - Evolutionary Responses (Ecology ➙ Evolution): Ecological conditions determine the direction and rate of evolution over long periods. This involves processes such as: - Speciation. - Molecular, phenotypic, and species diversification. - Macroevolutionary trends. - Ecological Responses (Evolution ➙ Ecology): Evolutionary history shapes the ecology of species and their current interactions. This includes: - Speciation and Niche evolution. - The fundamental principle (Wagner et al. 2013) that all species, traits, and species interactions currently studied by ecologists were ultimately created by evolution.
Short Timescales (Microevolution): - Evolutionary Responses (Ecology ➙ Evolution): Ecological contexts drive local adaptation and rapid change in trait frequencies within a population. - Specific areas of focus include: Local adaptation, coevolution, resistance evolution, sexual selection, specialization, and life-history evolution. - Ecological Responses (Evolution ➙ Ecology): Known as Community Genetics and Eco-Evolutionary Dynamics. Genetic composition and rapid evolution actively shape populations and communities in real-time.

Traditional Ecological Research vs. Modern Reality

Gause’s Paramecium Experiments: Gause studied P. aurelia and P. caudatum separately and in mixed populations to understand competition and niche space.
Traditional Competition Formulas: - Separate population growth: $\frac{dN_1}{dt} = r_1 N_1 (1 - \frac{N_1}{K_1})$ - Mixed population growth (accounting for interspecific competition): $\frac{dN_1}{dt} = r_1 N_1 (1 - \frac{N_1 + \alpha_{12} N_2}{K_1})$ $\frac{dN_2}{dt} = r_2 N_2 (1 - \frac{N_2 + \alpha_{21} N_1}{K_2})$
Foundational Assumptions in Traditional Ecology: 1. Individuals are identical: Taxa are represented by a single parameter value (e.g., all individuals have the same competition coefficient). 2. Variation is unimportant: Intraspecific variation among individuals does not significantly affect population dynamics. 3. Slow Evolution: Species trait values evolve too slowly to matter over ecological timescales.
Refuting the Assumptions: - Intraspecific Variation: Research shows that populations are NOT made of identical individuals. On average, intraspecific variation accounts for $25\%$ of total trait variation within communities and $32\%$ of total trait variation among communities. - Phenotypic Differences: Modern theory (e.g., Hart, Schreiber, Levine in Ecology Letters) suggests individual phenotypic differences can fundamentally change species coexistence and ecological dynamics.

Ways Evolution Alters Populations

Evolution is not just a background process; it actively changes populations by: 1. Altering genetic composition (genotype frequency). 2. Altering the extent of genetic variation. 3. Causing traits to evolve physically and behaviorally.
Central Question: Does genetic composition and rapid evolution within populations impact short-term ecological dynamics and interactions?

Genotype Identity and Community Genetics

Genotype Identity: Refers to an individual possessing a particular genetic makeup.
The Approach: Compare the ecological impact of individuals (genotypes) that differ genetically.
Case Study: Mexican Bean Beetle and Soybeans (Underwood & Rausher 2000, 2002): - Researchers examined insect population dynamics on four different soybean genotypes. - They used Common Garden Experiments: - Replicates of each genotype are used. - Genotypes are randomized in a shared environment. - This design eliminates environmental effects to isolate genetic impacts.
Populus (Cottonwoods) Research (Thomas Whitham): - Used common garden trials of replicated clones and experimental crosses to map ecologically important traits and quantify heritability. - Percent Variation Explained by Plant Genetic Identity: - Nutrient Cycles: $34-65\%$ - Plant Growth Rate Constant: $45\%$ - Belowground Carbon Storage & Root Production: $77\%$ - Water Cycles: $35-40\%$ - Biodiversity (associated species): $43-78\%$ - Community Stability: $32\%$

Genetic Diversity and Its Ecological Impacts

Definition: The magnitude of genetic variation within a population.
Measurement Methods: - Number of genotypes. - Quantitative genetic variance or heritability of a trait. - Molecular genetic diversity.
Theoretical Connection to Species Biodiversity: Species biodiversity is generally associated with increased stability, resistance to invasion/disturbance, productivity, and diversity across trophic levels. Scientists hypothesize that genetic diversity (diversity below the species level) has similar effects.
Empirical Evidence: - Species Diversity in Microcosms (Booth & Grime 2003): In communities of 11 species, those with higher genetic diversity ( $1, 4, \text{ or } 16$ genotypes per species) saw impacts on overall species diversity. - Seagrass (Zostera marina) (Randall Hughes 2004): Increased genotypic diversity leads to increased population stability, resilience to perturbations, and faster recovery. - Abiotic Stress (Heat Waves) (Reusch et al. 2005): Populations with six genotypes performed significantly better/had more stems during and after heat stress compared to those with one or three genotypes. - Multi-Trophic Effects: Increasing the number of plant genotypes (e.g., in Solidago or Oenothera) correlates with higher species richness of associated insects ( $r^2 = 0.16, P = 0.008$ ). - Ecosystem Properties (Crutsinger et al. 2006): Increased plant genotypic diversity correlates with higher Aboveground Net Primary Productivity (ANPP) (P < 0.01, r^2 = 0.19).

Mechanisms of Genetic Diversity Impact

Inbreeding Avoidance: High diversity prevents inbreeding depression, which would otherwise reduce population persistence.
Ecological Interactions among Genotypes: - Niche Complementarity: Occurs when a mixture of genotypes utilizes a greater quantity of resources (due to reduced niche overlap), thereby increasing the total productivity of the population compared to single-genotype groups. - Facilitation: One genotype may improve the environment for another.
Sampling Effects: Larger pools of genetic diversity are statistically more likely to include a "super-productive" genotype that has a disproportionately large impact on the total ecological effect.

Comparing Genetic and Species Diversity

Qualitative Similarities: Both types of diversity have similar effects on stability and productivity.
Quantitative Similarities: Effects can be identical in magnitude.
Caveat: In most cases with unbiased sampling, species diversity will have larger impacts simply due to the greater range of phenotypic variation present across different species compared to within a single species.

Significance in the Real World

Stability: Greater diversity leads to more stable natural ecosystems.
Conservation: Protecting genetic diversity is necessary to maintain complex community interactions.
Sustainability: Leveraging genetic diversity may increase agricultural sustainability and reduce the need for pesticides.

The Rate of Evolution and Eco-Evolutionary Dynamics

Historical View: - Darwin (1859): Witnessed nothing of "slow changes in progress" until the passage of ages. - Slobodkin: Postulated that "ecological time" (~10 generations) is distinct from "evolutionary time" (~half a million years).
Contemporary Rates vs. Fossil Rates: - Fossil rates are measured in "Darwins" ( $d$ ). Fossil mean is approximately $0.08d$ . - Guppy Introduction in Trinidad (Reznick et al. 1997): Guppies moved from high-predation to low-predation environments evolved at rates up to $395,000\,Darwins$ . High-predation guppies show faster growth and smaller size; low-predation guppies show slower growth and larger size.
Evolution on "Ecological Timescales": - Darwin's Finches: < \text{few years} - Aphids: $1\,summer$ - Stickleback: $12\,years$ - Mustard plant: $7\,years$ - Soapberry Bugs: < 41\,years
Eco-Evolutionary Dynamics: Defined as reciprocal interactions between ecology and evolution that play out on contemporary timescales (typically less than a dozen years to a few centuries, or $1-50$ generations).

Conceptual Framework (Hendry 2016)

Eco-to-Evo: Ecological changes (Population dynamics, community structure, ecosystem function) lead to Phenotypic and Genomic changes.
Evo-to-Eco: Phenotypic and genomic changes feed back to influence population dynamics, community structure, and ecosystem function.

Experimental Methods to Test Rapid Evolution

Post-Divergence Design: - Step 1: Evolve: Allow populations to evolve under different treatments (e.g., Managed vs. Non-Managed). - Step 2: Remove Phenotypic Plasticity: Standardize the environment (Common Garden) for a few generations to ensure observed differences are genetic. - Step 3: Test Ecological Impact: Re-introduce the evolved individuals into a test scenario (e.g., an invasion test). - Example: Dr. Taylor Zallek's duckweed study (Lemna minor) showed that managed populations evolved to be more invasive than non-managed ones.
Concurrent Evolution Design: - Compare ecological impacts of evolving treatments vs. non-evolving treatments as the evolution occurs. - Difficulty: How to manipulate evolution to test immediate impact? - Manipulating Selection: Risky because the selective agent itself (like a predator) affects the ecology, confounding the evolutionary effect. - No-Evolution Control (Replacement Approach): Every generation, individuals in one treatment are replaced with individuals from a non-evolving source population. This allows the ecological variable to change while keeping the genetics static. - No-Evolution Control (Removing Genetic Variation): Compare a population with genetic variation (can evolve) against a population consisting of a single clone (cannot evolve).

Case Study: The Green Peach Aphid

Organism Traits: Sap-sucking, invasive crop pest, cyclical parthenogen (reproduces clonally), generation time < 1\,week.
Experimental Proof of Eco-Evo Dynamics: - Compare an evolving population (mixture of Genotype A and B) against non-evolving populations (pure A or pure B). - Findings: 1. The evolving population changed in genotypic frequency over time ( $P = 0.016$ ). 2. Evolving populations grew much faster ( $+35\%\text{ exponential growth rate}$ ) than non-evolving populations. 3. Confirms that rapid evolution within populations impacts short-term ecological dynamics.