Co-Evolution—Survival of the Friendliest?

By now, if you’ve been reading carefully and thinking deeply, you’ve realized that the rainforest is an extraordinarily complex biome. It should be clear that the organisms there are not adapted just to the environment, but to the other organisms in the rainforest.


It’s common to wonder “what came first” or to speculate on how such a complex system could have emerged together. There are the straightforward relationships—like the algae and fungi that make up a lichen, or the hoatzin bird and the water lettuce you read about in Week 1. Then there are more subtle relationships, like the philodendron and the tree on which it climbs. At first, the plant is an epiphyte using another tree as a perch. Eventually, it sends long roots to the forest floor and brings water up like a natural aqueduct system for the other plants that perch in the canopy.


The relationships are intricate, but the clues are all around. When you see flowers in the canopy that are brightly colored and cup shaped, it’s likely they are pollinated by birds. White nocturnal blooms are often bat-pollinated and they often have a lot of nectar. If a plant has a rotten or mildew-like smell, it’s probably trying to attract flies for pollination. To attract butterflies, the odor may be mild but the color is often red or orange—colors butterflies can see.


For the answer to the “what came first?” conundrum, it’s often important to keep energy in mind. Most of the things that animals do for plants could be done without the animal help, but with great difficulty. Each small step (mutation) that makes variation (cross-pollination) a little more likely is a step in the direction of greater reproductive success for that plant. It’s a way of saving energy—and becoming “fitter” in the evolutionary sense.






Sex and the Single Rainforest


In the language of natural selection, being the “fittest” means having the most offspring, and the most varied offspring. The closeness and the tight segregation of rainforest strata prevent some of the common strategies that plants would normally use to cross-pollinate. 


There are many reproductive strategies that are unique—or at least best suited for—rainforests. The seeds of the plant below are covered with a bright red "paprika" spice that prevents them from being eaten. (Children use the powdery spice as a joke--like we'd use a water bolloon.)


Wind pollination (which is the most common method of mixing genes in prairies and deserts)  is very rare in the rainforest. But since hybridization is the best genetic route to greater variation, and more effective evolution, there have to ways to achieve genetic variation even in the darkest and most humid corners of the rainforest. Many trees use “outcrossing” mechanisms—they are dioecious, meaning that a tree is either male or female. That encourages a mixing of genetic material in an area that doesn’t lend itself to a lot of distant seed dispersal.  Animal pollination is a very common method of achieving greater variation; bats, hummingbirds, bees and even a few mammals help the plants cross-pollinate.


In the language of survival, “fittest” often also means having the means to encourage your offspring to quickly move far away from home, to make sure they don’t compete for resources “at home.” Animals help with this too. Bats, birds and primates may carry fruit and the seeds they contain to new areas. Some 70% of rainforest canopy plants rely on animals for some part of their lifecycle. These are examples of co-evolution described above.