Symbiosis
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[edit] Definition
[edit] Example: bees and flowering plants
One example of symbiosis which is obligatory for both parties is the relationship between bees and angiosperms (flowering plants). Bees are obligate feeders from flowers; and many species of flowering plants would go extinct without bees to transport their pollen, because they don't produce enough pollen for wind-borne distribution to be effective.
It is interesting to ponder how such a situation evolved; fortunately, there is sufficient evidence in the fossil record to answer this question confidently.
To begin with, it is clear from the fossil record that the earliest seed-bearing plants were gymnosperms --- non-flowering plants which distribute their pollen on the wind. Conifers are examples of living gymnosperms. When winged insects arose, they started feeding off this pollen, which is a rich source of nutrients.
This was, of course, bad for the plants, since eating a plant's pollen reduces its chances of pollination. However, there was a slight compensation --- sometimes pollen would stick to a winged insect, and be borne to another plant with greater accuracy than could be achieved by wind-borne pollen. You will note that this is not yet a symbiotic relationship, since the insects are doing net harm to the plants whose pollen they eat.
So, at this point the plants are harmed by having their pollen eaten; they are benefited by insects inadvertently carrying pollen from plant to plant; and the activities of insects are a net disadvantage to the plants. At this point, any mutation which makes the pollen more likely to stick to insects is beneficial to the plants, and will be favored by natural selection. This reduces the harm done by the insects. As the pollen becomes more and more adapted to sticking to insects, it reduces the harm done to to zero; and finally the pollen is so sticky that insects are a more efficient way of distributing pollen than just scattering it on the wind, and the plants start to benefit from insect activity.
![The pollen of a sunflower (ESM image courtesy of Dartmouth College[1]). Note the cocklebur form of the pollen grains, suitable for tangling in the fur of bees.](/index.php/Image:Pollen.jpg)
Now we have symbiosis, though it is not yet obligate on either side. So at this point natural selection favors a new set of adaptations to the plants, because it is now beneficial for plants with sticky pollen to attract insects, by means of visual markers (flowers) olfactory signals (scent) and nutritional rewards (nectar). Also, the plants can reduce the amount of pollen they need to produce, since the large quantities of pollen produced by gymnosperms is only necessary because of the inefficiency of wind-borne pollination. It is this factor that makes the symbiotic relationship obligate for the plants, since they are now producing so little pollen that if they had to rely on the wind to distribute it, this would be ineffective and they'd go extinct.
Meanwhile, the supply of nectar as well as pollen means that the plants are producing a balanced diet suitable for insects, and there is now a niche in nature suitable for specialized flower-feeders, such as bees, which have become so specialized in their habits and feeding parts that they couldn't live off anything else. The relationship is now obligatory on both sides.
There is one final refinement. If some group of insects is better at having pollen stick to it than others, than it is to the advantage of plants to attract and reward that group of insects relative to other groups. This means that, other things being equal, an improvement in the insects' ability to carry pollen will be adaptive from the point of view of the insects, since it will elicit adaptations on the part of the flowers which will benefit the insects. This effect will be strongest when it comes to the insects which are most specialized as feeders from flowers. So we find that not only is pollen adapted to stick to bees, but bees are adapted to have pollen stick to them.
This narrative is borne out by the fossil record, in which non-flowering plants come before winged insects, which come before flowering plants, which come before specialized flower-feeders such as bees.
[edit] Misconceptions
[edit] Creationists and symbiosis
An obligate symbiotic relationship between two organisms is perhaps the simplest example in nature of an irreducibly complex system, so of course creationists are completely unable to understand how it evolved. In the sweet, endearing way they have, they blink like fawns in the headlights of oncoming knowledge, and plaintively demand to be told which of the organisms "evolved first".
As you can see from the example above, the obligate symbiotic relationship is caused by the co-adaptation of two organisms which start off in a relationship which is neither obligate nor symbiotic.
[edit] Crocodile Birds
The relationship between crocodiles and crocodile birds is often given as an example of symbiosis. Sadly (for the concept is a charming one) is seems that the alleged symbiotic relationship does not exist: for more information, see the main article on crocodile birds.
