Intimate Partnerships: How Species Adapt to Each Other

THE WEB OF LIFE

  • Food Chains vs. Food Webs (Section 15.1):

    • Food Chain: Only considers trophic levels.
      • Trophic Levels: The feeding relationships among various species.
      • Bottom Trophic Level (Producers): Consists of plants and other organisms that perform photosynthesis. These organisms convert and store solar energy as chemical energy in the form of organic molecules\text{organic molecules}.
      • Energy Transfer: A food chain reflects the upward transfer of energy through trophic levels.
    • Food Web: Reflects interactions between species regardless of their specific trophic level. Every species exists within a complex web of ecological interactions (Zimmer & Emlen, 2016; Fig. 15.2).

  • Species Relationships (Table 15.1):

    • Mutualism (+/+): A relationship that raises the fitness of both species involved.
      • Pollination: Insects/animals visit flowers for nectar; plants benefit from pollen dispersal. Coevolved adaptation: Bright colors attract pollinators; hummingbirds have slender bills for flower tubes.
      • Seed Dispersal: Birds/mammals eat fleshy fruits; seeds pass through digestive tracts. Plant benefits from wide dispersal.
      • Nutrient Exchange (Mycorrhizae): Soil fungi deliver minerals/nutrients to plant roots; plants provide organic carbon to fungi.
      • Farming: Certain ant species rear "mushroom gardens" in nests. The farmed fungi only grow inside the nests; ants carry antibiotic-producing bacteria to kill pathogenic fungi.
      • Animals and Microbiota: Humans and animals depend on microbes for food digestion and vitamin synthesis.
      • Cleaners: Fish species eat ectoparasites off the skin of other fish.
    • Commensalism (+/Neutral): One species benefits while the other suffers no fitness loss.
      • Remoras: Remora fish attach to larger fish (like sharks) using specialized attachment structures and detach to feed on prey killed by the larger fish.
    • Antagonism (-/+): One species benefits at the expense of another.
      • Predators/Prey: Animals ingesting other animals. Adaptations: Prey produce toxins; predators evolve defenses (e.g., Garter snake and rough-skinned newt).
      • Herbivores/Plants: Animals feeding on plants. Adaptations: Plants produce sticky latex; insects evolve behaviors to avoid triggering latex.
      • Deceptive Pollination: A flower tricks an insect into visiting without providing nectar. Adaptations: Flowers produce pheromones and structures looking like female insects; males eventually learn to avoid them.
      • Host and Parasite: Organisms (viruses, worms) living on or in a host, causing disease or death. Parasites may castrate hosts or alter behavior; host immune systems attack pathogens.

VARIATION & POPULATIONS: THE BUILDING BLOCKS OF COEVOLUTION

  • Coevolutionary Partnerships (Section 15.2):

    • Coevolution Definition: The process of reciprocal evolutionary change between ecologically intimate species, driven by natural selection.
    • Primary Requirement: Traits relevant to the ecological interactions of the species must demonstrate heritable variation.
    • The Henter Experiment (1995):
      • Setup: Focused on parasitoid wasps that lay eggs in aphids. Wasp larvae feed on and eventually kill the aphid host.
      • Aphid Defense: Production of immune cells that form a suffocating wall around wasp eggs/larvae.
      • Method: A group of genetically identical aphids (clones) were introduced to parasitoid wasps.
      • Results: Parasitism rates varied from 7−100 %7-100 \, \%. Related wasps showed equal success levels.
      • Conclusion: Wasp performance (skill) was found to be heritable. A follow-up experiment with similar wasps and non-related aphids showed variability in aphid defenses, concluding aphid variability is also present.

  • Reciprocal Selection:

    • As one species adapts to its partner, the partner evolves in response.
    • Strength and direction vary by population due to small population sizes or selection on other traits with pleiotropic effects (a single gene having multiple effects on an individual).
    • European Fruit Flies and Parasitoid Wasps Example:
      • Flies can encapsulate (enclose) wasp eggs; wasps can resist encapsulation.
      • Trade-off: Better encapsulation ability in fly larvae makes them less competitive with other larvae for food.
      • Result: In environments with limited food resources, selection for wasp resistance is weaker.

  • Antagonists:

    • Negative Frequency-Dependent Selection: Relative fitness is high when a phenotype is rare and low when it is common. Parasites often adapt to the most common host phenotype.
    • Coevolutionary Alternation: Occurs when one species in an antagonistic relationship interacts with several other species. A predator may focus on the least-defended prey; as that prey's numbers drop or its defenses improve, the predator switches to a different prey species with weaker defenses. This creates alternating bouts of pairwise coevolution.

COEVOLUTIONARY ARMS RACES

  • Arms Race (Coevolutionary Escalation):

    • An extreme expression of antagonistic coevolution that imposes significant costs on both species.
    • Rough-skinned Newt and Common Garter Snake:
      • Newts (a type of salamander/amphibian) produce tetrodotoxin (TTX), a neurotoxin that locks onto nerve cell receptors, causing fatal paralysis.
      • Garter snakes on the US West Coast have evolved resistance to TTX.
      • Costs: High TTX production diverts energy from other life processes for newts. Resistant snakes move slower, making it harder to catch prey or avoid their own predators.
      • Geographic Variation: Traits are well-matched in some areas (high toxicity/high resistance) and mismatched in others. Mismatches are likely due to different speeds of evolution: snake resistance requires only 11 to a few mutations, while increasing TTX strength requires a series of mutations.
      • The arms race essentially ends when the snake population becomes so resistant that no small increase in toxin strength confers a predatory advantage.

  • Attenuated Coevolution:

    • Intense antagonistic interactions gradually decrease (attenuate) until each species exerts only weak selection on the other, slowing the rate of coevolution.
    • Australian Rabbits and Myxoma Virus:
      • Rabbits were introduced in 18691869. The rabbit myxoma virus (biological control from South America) was released in the 1950s1950s via mosquitoes.
      • Initial Phase: In dense rabbit populations, the most virulent strains reproduced fastest and spread easily despite killing hosts.
      • Middle Phase: As rabbit density thinned, virulent strains killed hosts before they could be transmitted. Selection then favored less virulent strains, allowing rabbits to survive, recover, and reproduce.
      • Current State: There is intermediate viral virulence and some resistance in rabbits. Populations have recovered but remain at lower levels than before the virus.

MUTUALISM AND MIMICRY

  • Mutualist Dynamics:

    • Positive Frequency-Dependent Selection: Favored alleles are those that provide benefit rather than harm. The most common genotype of one species is most likely to interact with the most common genotype of the partner species.
    • Pollination Examples: Long-tongued flies and Zaluzianskya plants show regional variation in tongue/floral tube lengths. The tube-lipped bat (Anoura fistulata) was discovered in 20052005 in Ecuadorian cloud forests.

  • Mimicry Types:

    • Müllerian Mimicry: A form of convergent coevolution where two or more toxic/unpalatable species converge to look alike. Predators learn to avoid one and thus avoid all mimics. Example: 77 species of toxic millipedes in the Appalachians (each possessing enough cyanide to kill 1818 pigeon-sized birds) and Heliconius butterflies in the Amazon.
    • Batesian Mimicry: A non-toxic "mimic" species evolves to look like a toxic "model" species. This is considered "cheating."
      • Dynamics: If the mimic is rarer than the model, predators learn to avoid the pattern, and species remain unchanged. If the mimic is too common, predators do not learn to avoid them, causing the model to evolve away and the mimic to follow (a coevolutionary chase involving directional selection).

  • Cheaters in Mutualism:

    • Cheating (e.g., Batesian mimics or Disa draconis orchids that mimic nectar-producing flowers) can cause mutualisms to collapse if it becomes too common.
    • Punishment: Some species maintain stability by punishing cheaters. Fig trees will abort flowers or fruit (killing the wasp larvae inside) if the fig wasps do not provide enough pollination. This relationship has remained stable for ≥80 million years\ge 80 \text{ million years}.

COEVOLUTION AS AN ENGINE OF BIODIVERSITY

  • Geographic Mosaic:

    • Diversifying Coevolution: Occurs when there are ≥2\ge 2 competing selection pressures on a characteristic, leading to wide ecological outcomes.
    • Red Crossbills and Pine Cones:
      • Birds with bigger bills inhabit areas with bigger/thicker cones. However, pine defenses against crossbills (thicker scales) are ineffective against Red Squirrels, which tear scales off.
      • Where Red Squirrels are present, pine cones have thinner scales but more seeds, and crossbills have shorter bills. This creates a geographic mosaic where other species (Clark's nutcracker, pine borer moth) also influence selection.

  • Speciation:

    • Coevolution drives speciation. As mutualist pairs evolve together, they can become isolated from other populations.
    • Milkweeds: Plants evolve new toxins/hairs; insects evolve specialized enzymes/mouthparts. This leads to rhythmic radiation and speciation in both groups.
    • Elaiosomes: Fleshy wads of lipids and proteins on seeds produced by ∼11,000\sim 11,000 plant species. Ants take seeds to nests, eat the elaiosomes, and discard seeds in safe chambers. Elaiosomes evolved independently ≥101×\ge 101 \times. Plant lineages with ant-dispersal have >2×> 2 \times as many species as closely related lineages.

  • Extinction Risk:

    • Losing one partner in a mutual association can lead to the extinction of the other or a shift to a new partner.
    • K-T Mass Extinction (66 mya66 \text{ mya}): Mutualistic corals (which rely on photosynthetic dinoflagellates for ∼50 %\sim 50 \, \% of energy needs) suffered 4×4 \times more extinctions than non-mutualistic corals.
    • New Zealand Study (2011): Mammal introductions led to a 49 %49 \, \% extinction rate in bird species. This resulted in fewer fruits on the mainland due to the loss of bird pollinators/dispersers.

ENDOSYMBIOSIS

  • Process: One coevolutionary species (mutualist) lives inside the other. Over millions of years, endosymbionts lose genes because the partner produces the necessary proteins. Phylogenetic trees of endosymbionts often show the same branching pattern (topology) as their hosts.
  • Example: Aster leafhoppers harbor two bacterial species acquired ∼260−280 mya\sim 260-280 \text{ mya}. These bacteria cannot survive freely but retain genes to produce amino acids for the leafhopper.
  • Origin of Mitochondria: Oxygen-consuming bacteria began living inside cells ∼2 billion years ago\sim 2 \text{ billion years ago}, likely to utilize host ATP. Evidence includes a single circular chromosome, bacteria-like genes, a double membrane, and reproduction similar to bacteria.
  • Serial Endosymbiosis: The theory that mitochondria evolved first, followed by chloroplasts in the origin of photosynthetic eukaryotic cells.

QUESTIONS & DISCUSSION

  • Question: What do "producers" actually produce?
    • Response: They produce organic molecules by converting solar energy into chemical energy.
  • Question: What does the Henter experiment suggest about the heritability of wasp skills?
    • Response: Since related wasps were equally successful, it suggests performance is a heritable trait.
  • Review Question: What are pleiotropic effects?
    • Response: These refer to a single gene having multiple effects on an individual (from Chapter 6).
  • Review Question: What is negative frequency-dependent selection?
    • Response: It is when the relative fitness of a phenotype is high when it is rare and low when it is common (from Chapter 6).
  • Thought Question: Does mutualism result from positive or negative frequency-dependent selection?
    • Response: Positive frequency-dependent selection.
  • Mimicry Questions: Why not use "model" and "mimic" for Müllerian mimicry?
    • Response: Because both species are toxic/distasteful; they both act as models.
  • Mimicry Questions: Which type of mimicry is basically "cheating"?
    • Response: Batesian mimicry.
  • Selection Scenarios: Is the coevolutionary chase in Batesian mimicry (model moving away, mimic following) diversifying, directional, or stabilizing selection?
    • Response: Directional selection.
  • Review Question: How long ago was the K-T extinction?
    • Response: Approximately 66 million years ago66 \text{ million years ago} (from Chapter 14).