Selection and Coevolution - A story of the Bahama Senna and a Pierid butterfly.

Price defines coevolution as "reciprocally induced evolutionary change between two or more species or populations".

In 1964, Ehrlich and Raven proposed the following idea of how this reciprocally induced change might occur: Suppose that a plant is subject to attack by an insect herbivore. (An animal which attacks a plant host is called a herbivore.) Genetic mechanisms which we have described, like recombination, allow the plant to produce a chemical which is toxic to the herbivore - a chemical defense!!!! You have already met a genus of plants which produce such defenses - many species of Erythrina produce alkaloids and other toxic chemicals. The plant now can radiate into new adaptive zones because it is protected against insect attack. But, the herbivores can also respond by developing chemical mechanisms to detoxify and render harmless the defensive chemicals produced by the plants. If they do, then the insect herbivores can radiate into new adaptive zones. So, the idea of coevolution is that one type of organism might develop a defense and then radiate into new adaptive zones. Then the other organism might develop a defense against the plant defense and have a burst of radiation. This cycle would continue with the result that the two species would be closely associated with each other.

An example of this sort of host plant/insect coevolution are butterflies from the family Pieridae and leguminous plants. In South Florida we have the Pierid butterfly Phoebis sennae and the Bahama Senna, Cassia bahamensis:

It's likely that you have seen this butterfly or a closely related species! The Cassia has a nice yellow flower which matches the butterfly's color, though I don't have a picture of the flower to show you. Look in the Gifford Arboretum for the plant and the butterfly - Both have been seen there!

The evidence is fairly good that insects have developed defense mechanisms against the toxic chemicals in plants. But, it is less clear that plants have developed toxic chemicals as an adaptive response to the herbivory by insects. Price points out that "many chemicals in plants that play important defensive roles are also very active in the plants, apparently closely associated with primary metabolic pathways". Nicotine is one of these toxic compounds which are used to make other components. Nicotine is used in tobacco plants to produce amino acids and sugars. In order to have coevolution, there must be selection by both the plants and the insect herbivores.

Mutualisms are interactions between two species, each of which benefits from the relationship. Mutualistic relationships are found mostly in the tropics and appear to have developed by coevolution.

The relationships between populations of two interacting species, A and B, can be described by making a chart of all possible types of interactions. Interactions may be positive (+), in which a species is positively affected by the interaction or negative (-), in which a species is negatively affected by the interaction. For example, if a predator eats another animal species, the prey, the predator is affected positively by the interaction (predator rubs belly, feeling quite satisfied by the meal) and the prey is affected negatively (prey is slowly digested into component molecules while family waits in vain). All the possible interactions can be described by the following chart:

  A B  
Neutralism 0 0 Populations of A and B are independent and don't interact.
Mutalism + + Both species benefit from the relationship.
Commensalism + 0 One species benefits from the relationship but the other is neither harmed nor benefited
Competition - - Both species have a negative effect on each other since they are competiting for the same resources.
Predation + - The predator (A) benefits while the prey (B) is negatively affected.
Parasitism + - The parasite (A) benefits while the host (B) is negatively affected.

Commensalism and mutualism are often grouped together and called symbiosis.

Many interesting coevolutionary relationships are mutualisms. Mutualism seems particularly important in the tropics. Price lists four observations which support this: "1) There are no obligate ant-plant mutualisms north of 14 degrees. 2) There are no nectarivorous or frugivorous bats north of 32-33 degrees. 3) There are no orchid bees north of 24 degrees. 4) Numbers of plant species with extrafloral nectaries decline rapidly north of 24 degrees." Unlike competition, predation, or parasitism (notice that predation and parasitism are essentially the same), mutualistic relationships tend to be quite unstable. Slight perturbations in conditions or population numbers and one species or the other could become extinct. It is thought that the stability of the tropical environment permits mutalisms to be stable that would be unstable at temperate or polar latitudes.

Example 1 - Pseudomyrmex ants have a mutualistic relationship with certain species of Acacia. The ant protects the plant against herbivory and the plant provides the ants with protein, carbohydrate and a place to live.

A really interesting case of mutualism is that of the relationship between ants of the genus Pseudomyrmex and plants of the genus Acacia. Acacias are legumes, often, but not always, with finely divided leaves. (Yes, they are nicely heliotropic!). In Central America, more than 90 of species of Acacia are protected by chemicals in the leaves. The rest, which do not have these chemicals, provide a wonderful environment for Pseudomyrmex ants! These acacias have swollen thorns, which provide an environment which the ants can carve out into a nest. (Hence the name "swollen thorn acacias"). The leaf petioles have nectaries which secrete carbohydrate-rich nectar which is consumed by the ants. Finally, the tips of the leaflets produce protein-rich Beltian bodies which the ants gather.

The ants return this huge energy investment by the plant by providing some useful services: They attack any herbivores! This includes not only insects but also grazing animals and even people. And, they chew other plants which grow around the acacias, thus preventing those plants from shading the acacias and supressing growth.

Example 2 - Fig wasps pollinate flowers of the fig plant and in return the fig provides food and shelter for the larval stage of the wasps.

Another really fascinating story is that of figs (genus Ficus) and fig wasps(family Agaonidae). Figs have a very unusual flowering system. The flowers are on the inside of a structure called a syconium, which will develop into the ripe fruit we know as a fig. The female fig wasp enters the syconium through an opening called the ostiole. It carries pollen and pollinates the flowers on the inside of the hollow syconium. At the same time, it lays eggs into flowers with short styles. The eggs hatch in the ovaries of the short-styled flowers and the larvae consume the ovary tissue as they grow. These flowers develop galls, which are regions of tissue which are stimulated to undergo mitosis by chemicals released by the female wasp, thus providing the larvae with an ample food source. The long-styled flowers do not have eggs laid in them, apparently because their ovaries are too deep in the fig wall for the wasp ovipositor to reach. But, the long-styled flowers do get pollinated by the fig pollen carried by the female wasp.

The larva develop into adult wasps which are barely visible to the naked eye. The male and female wasps are strikingly different in appearance. The male is a wingless creature. The males fertilize the females, chomp a hole in the wall of the now mature fig, and promply die inside the fig! The female leaves the fig through this hole, taking pollen with her.

Notice that the fig and wasp have closely coevolved. Only particular kinds of wasps can enter the the syconium. The timing of flowering is such that the female flowers, in which wasp eggs are laid, mature earlier than the male flowers. So, the male flower is not mature until close to the time at which the larvae have produced the next generation of wasps and the females are ready to leave the fig with the pollen. The female wasps have evolved just the right shape of ovipositor to place eggs in the ovary of the short-styled flowers. These and many other adaptations of both the figs and the wasps work together to ensure reproduction and development of both species.

The mature fig remains for you or some other animal to eat. The seeds pass through your digestive system, with their seed coats weakened by the acid in your stomach, and fall on the ground, germinating into another fig tree. So, you or another animal has aided in seed dispersal.

Example 3 - Protozoa live in the gut of termites and wood roaches and obtain food from the foraging activities of the insects. In return, the protozoa use their specialized enzymes to break down cellulose, a polymer of glucose, for the insects.

Think back to the protists. Remember that there is a mutualistic relationship between termites, wood roaches, and some other insects and the protozoa in their digestive system. (There is a very similar system in cows and other ruminants.) The termite has evolved a special structure for the mutualistic microorganisms to live in, as has the cow. (The cow has a rumen, hence the name ruminant.)

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