Coevolution in Biology

Coevolution

Introduction to Coevolution

  • Definition: Coevolution refers to reciprocal evolutionary change between interacting species that is driven by natural selection.

  • Key Concept: Under certain conditions, species interactions can lead to natural selection, sometimes resulting in co-evolution.

Case Study: Garter Snakes and Rough-skinned Newts

Overview of the Case Study

  • 1970 Incident: Local legend recounts that three hunters were found dead at their campsite, which led to investigations into the local fauna, particularly the rough-skinned newt (Taricha sp.) and its toxicity.

  • Study Reference: Williams, Becky (2007) "Biological Warfare and the Coevolutionary Arms Race".

  • Initial Findings: Unusual items found at the campsite included a boiled rough-skinned newt.

Characteristics of the Rough-skinned Newt

  • Appearance: Rough-skinned newts are characterized by their brown top and brightly colored underbelly, a common trait among poisonous animals.

  • Toxicity Validation: To determine if the newts were poisonous, researchers:

    • Injected potential predators (birds, reptiles) with newt skin extract.

    • Fed the newts to these predators.

    • Observed symptoms including muscle weakness, vomiting, drops in blood pressure, and paralysis.

Hypothesis on Toxicity Adaptation

  • Hypothesis: The hypothesis posits that poisonous newts (Taricha sp.) are favored by natural selection, as their poison serves as a defense mechanism against predation.

Evidence for Natural Selection on Newt Toxicity

  • Criteria for demonstrating that natural selection is acting on newt toxicity includes:

    1. Trait variation in the population.

    2. Heritability of traits (TTX production).

    3. Differential reproductive output due to traits (survival rates of toxic vs. non-toxic newts).

Predictions about TTX in Newts

  • Expected Adaptation: It is predicted that the newt population should, on average, maintain just enough TTX to effectively kill their predators. This is likely a balancing act influenced by evolutionary trade-offs.

Evolutionary Trade-offs in Newt Toxicity

  • Consequences of TTX Production:

    • Newts that produce too much TTX may have fewer offspring due to increased energy costs or greater natural predation risks.

    • Conversely, producing too little TTX increases the likelihood of being eaten by predators.

TTX Toxicity Levels

  • Critical Data: On average, the TTX content in one rough-skinned newt can kill up to 100 humans.

  • Question Raised: Why would newts evolve such high toxicity?

Coevolution: Snakes and Newts

  • Adaptive Responses: In response to the newts’ toxicity, garter snakes evolved increased resistance to TTX over time.

  • Research Findings: Studies show that garter snakes commonly prey on newts, indicating a coevolutionary arms race where each species develops defenses or counter-defenses.

Coevolutionary Arms Race

  • Dynamic Interaction: Each coevolutionary pair evolves increasingly effective weapons and defenses that cycle to match and eventually overcome their opponent’s adaptations.

Variation in TTX Production and Resistance

Variation Among Newts

  • Observations: Levels of TTX production varied among individual newts as well as across different populations.

Variation Among Snakes

  • Research Method: Garter snakes were tested by injecting them with standardized doses of TTX and observing their speed on a racetrack. Variations in speed indicated differing levels of resistance to the toxin.

Genetic Basis of TTX Production and Resistance

Heritability of TTX Production

  • Assessment Method: Ideal strategy would have involved capturing mother newts and raising their offspring to test TTX levels. Due to challenges in raising them in captivity, indirect inferences were drawn from similar tests

  • Inference: In related species (poison dart frogs), toxin acquisition through diet was established. Newts fed with TTX-infused crickets showed no acquired toxicity, suggesting TTX is likely a heritable trait rather than dietary.

Heritability of Resistance in Snakes

  • Findings: Experiments with lab-reared snakes demonstrated that resistance to TTX is heritable.

Impacts on Reproductive Success

TTX Production in Newts

  • Inference on Success: Newts exhibiting very high levels of TTX generally avoid predation, thus benefiting their reproductive success (survival leading to offspring production).

Resistance in Garter Snakes

  • Consequences of Resistance Levels: Snakes with low resistance exhibit intoxication, lower mobility, and higher predation risk. Under common predation by newts, those with higher resistance possess greater fitness advantages.

Evidence Required to Demonstrate Natural Selection

  • Required Evidence: To confirm natural selection acting on newt toxicity and snake resistance, it is essential to show:

    • Variation in traits.

    • Heritability of traits.

    • Differential reproductive success linked to these traits.

  • Much of the evidence established was inferential rather than directly observed.

Evolutionary Trade-offs for Adaptation

Specific Trade-offs

  • Newt Trade-offs:

    • Too little TTX: Increased predation risk; However, produce more offspring.

    • Too much TTX: Lower predation risk; However, produce fewer offspring.

  • Snake Trade-offs:

    • Too little TTX resistance: Unable to consume toxic newts; Higher speed.

    • Too much resistance: Able to consume toxic newts; Lower speed.

Implications of Trade-offs

  • Due to these trade-offs for both predator (snakes) and prey (newts), it is hypothesized that newts should produce just enough TTX to avoid being eaten, while snakes should develop just enough resistance to enable consumption of newts.

Biological Patterns Across Populations

  • Diversity Among Populations: Species are typically a collection of populations across variable environments; different populations may exhibit traits that 'match up' in terms of toxicity and resistance.

Geographic Distribution of TTX and Resistance

Specific Populations Examined

  • Texada Island, British Columbia: Exhibits no TTX production or resistance.

  • Olympic Peninsula, Washington: Different levels of TTX and resistance.

  • Examples of TTX Relationships: Biologists correlate the amount of TTX produced in local newts with the resistance level in local snake populations.

Evidence Supporting Coevolution Hypothesis

  • Conclusion on Findings: The evidence supports the hypothesis of co-evolution between newts and garter snakes. However, further convincing data could strengthen the argument.

Questions for Consideration

  1. Why might some newt/snake populations not engage in an arms race?

  2. What might happen if a population of newts becomes so toxic that snakes cannot adapt?

  3. Concept of Resistance is Futile: Implications of extreme toxicity could lead to new ecological dynamics.

Species Interactions and Coevolution Types

Types of Interactions and Effects

  • Consumption (+/-): Interaction example is garter snakes having a negative impact on rough-skinned newts due to predation.

  • Competition (-/-): Both species detrimentally affect each other’s fitness.

  • Mutualism (+/+): Both species benefit from the interaction, further examples include fig trees and fig wasps.

Resources for Further Research

  • Provided links to articles for detailed insights into biological warfare and coevolution.