When Two Species Evolve Together

Coevolution (when two organisms evolve together) is one of the most beautiful features of nature.  A previous article of mine delved into one example of mutualism – the bobtail squid and V. fischeri bioluminescence/light organ relationship.  However, many taxa (a closely-related group of species) have evolved to entirely rely on another specific taxa.  Some of these you may be familiar with: lice and mammals, bees and flowering plants, and clownfish and sea anemones.  Perhaps you are unfamiliar with the fig wasp (family Agaonidae) and fig tree (genus Ficus) relationship, which has been under the pressure of selection for approximately 87 million years.  Currently, 18 genera of fig wasps are responsible for pollinating 20 types of figs, yet there is a basic life cycle that characterizes most instances of this association (Cook and Rasplus 2003).

One species of fig wasp. Left: female; right: male.

During the two-day lifespan of an adult female fig wasp, each individual must find a young fig, climb inside, and deposit its eggs.  However, this isn’t as simple as it sounds.  In some areas, fig trees are sparse, and less than 1% of wasps are able to encounter the volatile cues of a receptive fig.  In addition, entry into the fig sometimes results in the loss of both wings and antennae of the females since the tunnel is so narrow.  Once inside, the female pollinates the flowers with pollen that she collected and stored in special sacs, and then uses her ovipositor (a long projection from the abdomen) to place her eggs into the developing flowers of the fig.  Afterwards, she dies (see www.figweb.org for more information).

The inside of a ripe fig.

As the fig develops, the larvae feed on plant material and reach maturity within three to twenty weeks (depending on the species).  Once they all chew their way out of the fig wall, males mate with females and then dig a tunnel for the females to leave the fruit.  Males die shortly after crafting this pathway.  Females gather pollen and then disperse to a new host fig.  Although there are exceptions, most figs have a specific species of pollinating fig wasp.  Generally, these wasps are unable to recognize cues from any other species of fig tree than their own (Cook and Rasplus 2003).

A flowering yucca. | Photo credit: Stan Shebs

A similar mutualism exists between the yucca moth and the yucca.  The moth is responsible for pollinating the yucca’s flowers, and in turn the yucca allows the moth to develop within some of its flowers.  This occurs in a stepwise fashion: a yucca moth will lay some eggs in a flower, then pollinate that flower, lay more eggs in a different flower, then pollinate that flower, etc.  Each partner is prohibited from cheating this system with life-threatening sanctions.  The yucca moth cannot lay too many eggs, or else the yucca will selectively abort those flowers, ending its supply of food.  Additionally, it can be hypothesized that the yucca moth will no longer pollinate flowers if the yucca continually aborts all of its larvae (Pellmyr 2003).

How can such relationships exist?  Essentially, the fig tree pays the wasp for its duties as a pollinator by feeding its larvae. The yucca plant also reinforces the pollinating activities of the yucca moth by providing its larvae with food and a place to develop.  Their relationships are so exact that both can only barely survive with the other.  The body of literature documenting and analyzing relationships like these is vast- the intense complexity gives rise to many questions regarding how these relationships evolved, how they can be so exceptionally synchronized, and how they can continue to exist for millions of years. Now think about how that lice got into your hair.



Cook, J.M. and J. Rasplus. 2003. Mutualists with an attitude: coevolving fig wasps and figs. Trends in Ecology and Evolution 18(5): 241-248.

Pellmyr, O. 2003. Yuccas, yucca moths, and coevolution: A review. Annals of the Missouri Botanical Garden 90(1): 35-55.

5 Comments on “When Two Species Evolve Together”

  1. I’m not entirely sure from this how these two species *evolve* together, which is what your title states. the article seems merely to describe mutualism – a way of living together beneficially – but how is this relatioship affecting evolution, not just reproduction/survival? Similarly, I’m not sure how humans rely on lice in a similar manner to bees and flowers…

  2. Two species have to evolve a mutualism, it doesn’t just happen suddenly. Though the article doesn’t mention how the fig wasp and fig tree may have come to rely on one another, the mutualism had to *evolve* at some point. Once the mutualism is established, if one species evolves, the other must also evolve.

    1. Exactly- within a mutualism or a parasitism, both species force selective pressures on the other. When it is mutualistic, both species evolve to synchronize their activities. If it is parasitic, then one species is driven to evolve defenses while the other continually is driven to evolve new ways to exploit its host. It is successful reproduction and survival that drives evolution itself; those that are most fit will be able to reproduce and pass on their genes.

  3. In many cases, what we see as mutualism is actually a case of commensalism or low-cost parasitism where individuals (host) best co-opted with their parasite have higher fitness. If an organism is able to cheat on its “cooperating partner” it would do so if there is a benefit associated with it. In fact this is often the case, a tug of war between benefits and costs. Costly cheating may have prevented directional selection towards commensalism or parasitism, so instead we get stabilizing selection towards a mutualistic strategy (which may manifest itself in the complex “synchronizing” adaptations). It is only after this specialization caused by the many generations of co-option that leads to interspecies dependence and later, co-evolution and speciation. Perhaps it should be called beneficial parasitism instead.

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