Evolutionary Constraints and Plasticity

Evolutionary Constraints

• Evolutionary genetic responses have constraints.
  • Evolution of any phenotype is limited by constraints.

Biomechanical Constraints

• Camels cannot evolve wings due to their weight and biomechanical limitations.
  • Large animals + No physiological adaption \rightarrow No flight

Genetic Variation Constraints

• Evolution requires genetic variation.
  • If trait variation is phenotypic and not heritable (all environmental), there can be no selection response.
• Artificial selection experiment in Drosophila species:
  • Goal: Identify genes underlying desiccation tolerance.
  • Species studied:
    • Drosophila melanogaster
    • Drosophila simulans
    • Drosophila serrata
      • Found in widespread variable habitats.
    • Drosophila birchiae
      • Found only in wet tropics (limited to tropical rainforests).
  • Results:
    • Drosophila birchiae did not respond to selection for desiccation tolerance.
      • Trait could not evolve, even after 30-50 generations of selection.
• Further investigation revealed:
  • Zero heritability for desiccation tolerance in Drosophila birchiae, regardless of population.
  • Variation existed, but no genetic component.
  • Tropical species generally had low heritability for desiccation tolerance compared to widespread species.
    • Species might be stuck in their environment because they can't evolve to move out.
    • The only options will be movement or plasticity.
    • The same pattern was observed for cold tolerance.
• This was not due to inbreeding or low genetic variation overall:
  • DNA variation and heritability for other traits (e.g., wing size) were normal.
  • Constraint was specific to traits limiting distribution.

Phylogenetic Constraint

• Evolutionary pathways can be constrained by phylogeny.
  • Mammals are unlikely to evolve wings like birds.

Trait and Gene Constraints

• Fitness depends on multiple traits, each underpinned by multiple genes.

Selective and Genetic Constraints

• A single gene affecting two traits might have opposing effects on each trait.

Fitness Landscapes

• Fitness landscapes illustrate how different gene combinations or alleles affect fitness.
• Example: Two genes with different allele combinations:
  • Mid to high values of gene 1 and low to mid of gene 2 result in highest fitnesss.
• Interactions between genes affect fitness.
  • Different gene combinations yield different fitness levels.
• Fitness landscapes can also be considered in terms of traits:
  • High value of one trait can lower the fitness of individual if paired with combination of trait.
  • Fitness landscapes predict population movement towards higher fitness combinations.

Selection Constraints

• Example: Plant traits (number of leaves vs. flowering time):
  • More leaves and earlier flowering are individually expected to increase fitness.
  • Combined selection might not push in the same direction.
  • Early flowering might limit resource allocation for leaf production.
  • Observed trait distribution can differ from the expected direction of selection due to constraints.

Evolution of Plasticity

• Plasticity itself can evolve when we think about responses to environmental change.
• Reaction norm: Phenotype expression across different environments.
  • Slope = estimate of plasticity.
• Evolution requires variation due to genes.

Genotype-by-Environment Interaction:

• Different genotypes exhibit different plasticities.
  • This indicates genetic variation for plasticity.
  • Allows for measurement of genetic variation for plasticity.
• In changing environments, certain plasticity levels can have higher fitness.
  • Consider overall fitness in temporally varying environments.
  • If there is no environmental variation, no plasticity is favored.
  • If there is temporal variation, some intermediate level of plasticity may be selected.
• Selection can act against other reaction norms if there are costs to having large differences in phenotype.
  • Sometimes the presence of plasticity is not necessarily adaptive due to potential costs.

Plasticity and Evolutionary Response

• Environmental change and sustained environmental change can influence the evolutionary response.

Plasticity as a Shield:

• Plasticity might reduce evolutionary response.
  • If organisms can track the environment through plasticity, selection for specific genotypes might be reduced.

Genetic Assimilation/Plasticity First:

• Plastic response followed by selection against plasticity.
  • Sustained environmental change can lead to selection for a fixed, plastic response.
• Study on C. elegans and heat tolerance:
  • Selection for heat tolerance led to decreased plasticity over time.
  • Reaction norm changed: Ancestral lines had plasticity, but selected lines lost plasticity.
• This shows stable environments can lead to reduction in plasticity.

Ecological Consequences of Environmental Heterogeneity

Responses to Changing Environments:

• Plasticity: Matching environment within a lifetime (reversible or irreversible).
• No change: Fixed phenotype with high fitness in one environment and low in another.
• Generalist: Good in all environments, but not great in any.

Fitness Trade-offs:

• Different genotypes (blue and white) have high fitness in corresponding environments.
• Plasticity allows for maintaining high fitness in multiple environments.
• Generalist strategy: Average fitness in all environments, avoiding extremes. (Jack of all Trades, master of none).

Specialist vs. Generalist Phenotypes:

• Specialist: High fitness in specific environments.
• Generalist: Moderate fitness across environments.

Climatic Variability Hypothesis:

• Organisms from variable climates have higher plasticity or broader thermal tolerance.
• Broader thermal tolerance: Wider performance breadth.
  • Specialists do better in the right enviroment.
• Predictions:
  • Higher plasticity in variable environments.
  • Wider thermal breadth in temperate habitats compared to tropical.
• Meta-analysis of plasticity across ectotherms:
  • No evidence for climatic variability hypothesis for plasticity across latitude.
  • Small relationship found between cold tolerance and seasonality.
• Evidence for broader thermal performance curves:
  • Taphole species: Tropical species have higher Ctmax and Ctmin, but temperate have a wide range.
  • Monkey flower: There was not a significant thermal breadth, but the most variable population had the narrowest in breadth.

Invasive Species and Plasticity:

• Invasive species might have higher plasticity or greater thermal breath than noninvasive species.
• Study comparing plasticity in invasive vs. non-invasive plants:
  • Invasive species tend to be more plastic.
  • But further studies have shown that that is only sometimes the case. It is not predictable.

Eco-Evolutionary Dynamics

• Evolutionary responses can alter ecological interactions.
• Guppy example: Predation pressure leads to evolutionary changes in guppies:
  • High predation: Early maturation, frequent reproduction, small offspring.
  • Low predation: Slower development, fewer and larger offspring.
• Evolutionary changes in guppies affect ecosystem structure and predator fitness.
  • This causes feedback systems.
• Evolutionary change in one species can influence the fitness/ecology of other species.

Integration of Concepts

• Environmental grain (temporal and spatial variation) influences evolutionary and plastic responses.
  • Temporal and Spatial variation can be fine or coarse, changing in space and time.
• Coarse grain (temporal and spatial): Local adaptation and potential speciation.
• Less Coarse (Predictable): Evolution of developmental or non-reversible plasticity.
• When there is more prediction that is evolutionary in nature you get non-reversible plastic responses.
• Fine grain (temporal): Reversible plasticity and behavioral responses.
• Fine Spatial, course environment:
  • Can select habitat.
• Coarse Spatial, fine temporal:
  • Must relay on adaptive trait and responses.
• Relationship between the the choice to move and the environment.
  • No choice: have to show adaptive responses.
• Specialization vs. Plasticity:
  • Specialization: Temporally constant environments.
  • Plasticity: Temporally varying environments.