Few species go through life without interacting with an other, but some interactions are more intimate than others. Pollination is an example of an interaction that, in some species, has become very intimate indeed! Most dedicated pollinators show adaptations to this, such as pollen baskets in bees, but these are often generic adaptations that enable the individual to visit many different species of plant. Equally, plants have adaptations to attract a variety of different insects. Different pollinators (bees, birds, moths) have different visual systems, and thus different flower colouration can be used to attract pollinators of different species. The timing of flower and pollinator emergence is also carefully timed in order to ensure maximum cross-over between the two. Pollinators generally gain food from the relationship, whilst plants achieve dispersal of their genes without having to physically move themselves.

Some plant-pollinator interactions are more intimate, more specific. This can lead to more extreme adaptations, as the two species become increasingly specialised for interacting with one another. Possibly the most extreme plant-pollinator relationship exists between the fig and the fig wasp. Young fig wasps emerge as larvae inside a tiny fig. The larvae feed on the fruit of the fig until they are ready to mature into adults, which again occurs within their fig prison. As adults, the wasps mate, collecting pollen from their birth fig before they leave. The male fig wasps then dig their way out of the fruit, creating a path for the females to emerge from. The male fig wasps are not well suited to life outside the fig, however, and often die shortly after making their escape. The females fly off and find a new fig plant where they can lay their eggs. Squeezing through the tiny entrance hole, known as the ostiole, the female enters a new fig and deposits her eggs inside the fruit, simultaneously depositing pollen on the fig’s reproductive parts.

Of Figs and Fig Wasps

There are over 700 species of fig which harbour a wasp symbiont, and in all cases, both parties are entirely dependent upon the other for survival. The wasp can no more survive without the fig than the fig can live without its wasp. However, this level of dependence leaves both parties open to exploitation, and there are some wasps who try to cheat the system, laying eggs without providing pollen to the fig. The wasps that hatch from these cheater eggs have the benefit of feeding on the nutritious fig, but their parents did not have to suffer the cost of pollen collection and transport. The fig plant is not totally helpless in this interaction, however. Individual fig fruits can be cut off from their sap supply if they contain too many parasites. When this occurs, every wasp inside that fruit dies, and thus cheating is subject to frequency dependent selection – the more cheaters there are in a population, the less benefit there is to cheating. This kind of natural selection prevents the system from collapsing.

Further conflicts must be resolved between the fig and the wasp in terms of which figs should harbour wasps at all. From the perspective of an adult female, every fig could provide a home to her offspring, however the plant must use some figs to produce seeds, in order to spread it’s genes to the next generation. The proportion of seed-bearing figs varies from species to species. The fig plant can afford to produce relatively few seed-bearing figs, as long as pollination is ensured in the other figs – so some species use an active method of pollen transfer to the wasps, which allows them to concede a greater number of figs to their wasp mutualist. Other species have separated the roles out entirely, with female plants producing only seeds and male plants producing only wasp-housing. It is these species from which we get edible figs (from the female plant only) – which is why you don’t get a mouthful of dead wasps when you eat a fig.

Darwin’s Orchid

Extremes of coadaptation can be found in physical attributes, as well, and Darwin’s orchid, Angraecum sesquipedal, is a good example of this. It was studied by (surprise, surprise!) Darwin, and he discussed its unusual characteristics in his 1862 book “On the various contrivances by which British and foreign orchids are fertilized by insects, and on the good effects of intercrossing”. The orchid has an unusually long spur, an elongated spike containing nectar (and pollen), which extends below the flower. Many plants have spurs, and these plants can only be pollinated by insects, birds or bats with a tongue long enough to reach the base of the spur. Based on comparison with these species, Darwin postulated that a moth must exist with the 35cm long proboscis (like a tongue) necessary to pollinate the orchid. For this, he was ridiculed. However, in 1903, years after his death, the exact moth he predicted 40 years earlier was found in Madagascar; Morgan’s Sphinx (Xanthopan morgani) – a large hawk moth with an absurdly long proboscis.

The previous example illustrates not just how complex and intimate interactions can be between species, but also how evolution can be a predictive as well as an explanatory science. Both the fig wasps and the hawk moths are examples of coevolution; when two species evolve in adaptation to interacting with one-another. Coevolution is an elegant example of evolution at work; over many thousands of generations, each member of the mutualistic partnership gained a benefit from the interaction, and thus selection favoured those individuals better suited to it. For each evolutionary change in one partner, a complementary change in the other was favoured, and over time, under the right circumstances, evolution can favour such intimate partnerships. This is just one side of the coin, however, as in other circumstances evolution can favour exploitation and parasitism as well as cooperation.

In a world where extinction exists, which I think we must all agree is the case, what intelligent designer would create a species so totally dependent upon another that the extinction of one would surely mean the annihilation of the other?

Articles in this Series:

1. Intro: Reasons Why Evolution is True
2. Part One: The Panda’s Thumb
3. Part Two: Parasitoid Wasps
4. Part Three: Ring Species
5. Part Four: Galapagos Finches
6. Part Five: The Quirky Human Eye
7. Part Six: Homology
8. Part Seven: Co-evolution
9. Part Eight: PreCambrian Rabbits
10. Part Nine: DIY Evolution
11. Part Ten: Convergent Evolution

Want to Know More?

• Figs and Fig Wasps
• Kjelberg et al (2001) Pollination mode in fig wasps: the predictive power of correlated traits. Proceedings of the Royal Society London B, 268; 1113 – 1121
• Darwin (1862) On the various contrivances by which British and foreign orchids are fertilized by insects, and on the good effects of intercrossing. John Murray, Albemarle Street, London.