Overview

“Survival of the fittest” makes nature sound like a constant war. But nature is also a giant cooperative. Symbiosis (“living together”) is when two species hook up to survive. Sometimes they help each other, sometimes one mooches off the other, and sometimes one eats the other from the inside out.

Core Idea

The core Idea is Interdependence. No organism is an island. Life is a web of relationships. Evolution often favors cooperation over competition because it is more efficient to share resources than to fight for them.

Formal Definition

A close and long-term biological interaction between two different biological organisms.

  • Mutualism (+/+): Both benefit (Clownfish and Anemone).
  • Commensalism (+/0): One benefits, the other doesn’t care (Remora fish on a Shark).
  • Parasitism (+/-): One benefits, the other is harmed (Tick on a Dog).

Intuition

  • Roommates (Mutualism): You pay rent, I buy food. We both live cheaper.
  • Couch Surfer (Commensalism): I sleep on your couch. You don’t mind, but you don’t get anything out of it.
  • Thief (Parasitism): I break into your house and eat your food.

Examples

  • Mitochondria: The powerhouse of the cell. Billions of years ago, a big cell swallowed a little bacteria. Instead of digesting it, they made a deal. The little bacteria provided energy, and the big cell provided protection. This is the origin of all complex life (Endosymbiotic Theory). We are all walking colonies of bacteria.
  • Coral Reefs: Coral is an animal (polyp) that has a plant (algae) living inside its tissue. The algae makes food from sunlight (photosynthesis) and feeds the coral. If the water gets too hot, the coral spits the algae out (bleaching) and starves to death.
  • Gut Bacteria: You have 2kg of bacteria in your colon. They digest your food and make vitamins for you. You give them a warm home. You literally cannot live without them.

Common Misconceptions

  • Symbiosis means “good”: In common speech, we use it to mean “helping each other.” In biology, it includes parasitism. A tapeworm is a symbiont.
  • It’s a choice: It’s usually obligate. The clownfish needs the anemone to hide from predators. It’s not a friendship; it’s a survival contract.
  • Coevolution: When two species evolve together. Flowers evolved bright colors to attract bees; bees evolved hair to catch pollen. They shaped each other.
  • Holobiont: The idea that an organism (like a human) plus its microbiome constitutes a single ecological unit.

Applications

  • Agriculture: Nitrogen-fixing bacteria live in the roots of bean plants. Farmers use this to fertilize soil naturally.
  • Probiotics: Eating yogurt to restore the “good” symbionts in your gut after antibiotics kill them.

Criticism / Limitations

  • Cheating: Symbiotic relationships are fragile. If one partner cheats (e.g., a flower that smells like nectar but has none), the relationship can collapse.

Further Reading

  • Margulis, Lynn. Symbiotic Planet. 1998. (The scientist who proved the Endosymbiotic Theory).
  • Yong, Ed. I Contain Multitudes. 2016.