Coevolution and Symbiosis Coevolution
Biological coevolution is the evolutionary change of one living thing triggered by change in another, interacting living thing. Such evolutionary change can be interspecific (between two different species), or intraspecific (within a single species). For now, we will continue to focus on interspecific coevolution.

Example: Flowers compete for pollinators. The plants that produce flowers (with colors, shapes and scents specific to particular pollinators) that attract the most effective pollinators will set the most seed and have the most offspring.

The various species in an ecosystem have evolved over the millennia in response to pressures from both the environment and from other species with which they live and interact.

One hypothesis suggests that the explosion in diversity of insects and flowering plants, which occured at the same time in evolutionary history, may have been driven--at least in part--by coevolution of pollinators and their host plants.

  • honeybee-pollinated flowers tend to be
  • bumblebee-pollinated flowers tend to be

  • butterfly-pollinated flowers tend to be

  • moth-pollinated flowers tend to be

  • beetle-pollinated flowers tend to be

  • bird-pollinated flowers tend to be

    What do you suppose a bat-pollinated flower should look like? (Remember the movie!)

    Many flowers have even evolved color patterns to guide their pollinators to the correct spot. These are called nectar guides.

    Because many insects and birds can see ultraviolet radiation that's invisible to us, many flowers pollinated by them have ultraviolet nectar guides invisible to us, but clear as a landing pad to the pollinators.


    Some terminology we might use for various interacting species:


    A Strange Partnership The caterpillars of Lycaenid Butterflies have a mutualistic relationship with ants:


    Caution: When You ASSuME... Just as correlation does not imply cause and effect (Let's think of violent behavior vs. video games for a moment...), coadaptation does not necessarily imply coevolution.

    While it is tempting to assume that such closely co-adapted species evolved in response to each other's activities, the good scientist must be very careful.

    It has been pointed out (by evolutionary ecologist Dan Janzen) that the ants might already have had anti-wasp behaviors, and the caterpillars might already have been producing honeydew (for another reason) before the species ever came into contact.

    Their association would thus have been "waiting to happen" when they finally did arrive in the same ecosystem.

    Unfortunately, such things are mind-bogglingly difficult (or impossible) to subject to experimental manipulation...especially since the evolution has already happened. There may sometimes be no way to discover whether the coadapted traits evolved together or independently.


    The Evolutionary Arms Race When a population (A) benefits at the expense of another population (B), there is usually an evolutionary response from the latter. The result can be a sort of "evolutionary arms race", in which "B" individuals best suited to avoid being harmed by "A" individuals leave more offspring. Eventually, the genetic shift in the population will cause a shift in population "A", with those better able to capture/infect/exploit "B" individuals leaving the most offspring.

    As with a weapons arms race, evolutionary arms races can result in "escalation" of characters used by each population to exploit or escape its symbiotic partner.

    As ecological competitors--parasites vs. hosts or predators vs. prey--evolve against each other, both must keep up. If one population fails to do so, it may go extinct.

    Three Flavors of Coevolutionary Competition In parasite/host, predator/prey evolution, there are three possible scenarios:

    The Red Queen Model has been invoked as one of the forces behind the evolution of sex. And that's where we're going next.