XVIII.5 Coevolution of two species is sometimes closely interrelated and leads to specific adaptations to the conditions of their coexistence in both species
Sometimes it is not, at first glance, apparent that a certain trait emerged as a product of the coevolution of two species.At other times this fact is, to the contrary, quite obvious.It is apparent for a great many pairs of symbiotic species that neither of them could survive in nature today in the absence of suitable anatomic, physiological or behavioral traits of members of the other species.A great many species of fig trees could not reproduce if specialized species of symbiotic wasps of the Agaonidae species did not live in their enclosed flowers, isolated from the surrounding environment; the larvae of these wasps feed on the fig tree flowers and their adults simultaneously pollinate the remaining flowers in the flower head (i.e. the fig) (West & Herre 1994).Some species of ants are entirely dependent for their food on the honeydew excreted by a certain species of greenfly or coccids (Homoptera), while these species of greenfly or coccids are, on the other hand, partially or completely dependent on assistance from ants in defending themselves against enemies or for transport from one attacked (and completely exhausted) plant to another.Other species of ants defend certain species of bushes against herbivore insects where, in return, these species of bushes provide domacia (Agrawal & Karban 1997), i.e. specialized anatomical structures, for example spines with capacious internal cavities, in which the relevant species of ants build their nests.The ants of a number of species cultivate fungi in their nests, which form their only food, bring them a suitable nutrient substrate, for example the leaves of plants, and the young queens bring an inoculum of this fungus in a specialized body organ to their new home when establishing new colonies (Schultz 1999).The fungus itself apparently does not occur at all outside of these nests and is completely dependent on this species of ant for its reproduction and nutrition.In fact, tripartate symbiosis exists for ants of the Atta species; the second of the partners is the symbiotic fungus of the tribe Leucocoprini, converting the leaf substrate into food for the ants, and the third consists in symbiotic bacteria of the Streptomyces genus, producing an antibiotic that prevents destruction of the leaf substrate by parasitic fungi of the Escovolopsis genus (Currie et al. 1999). The obligatory symbiosis of the roots of plants with some fungus species, mykorhiza, or with some species of bacteria is widely known (Saikkonen et al. 1998).In a great many plants, symbiosis with fungi causes that the above-ground tissues are poisonous or at least unpleasant tasting.The fungi, frequently passed down only vertically from one generation to the next, thus protect the plants from herbivores (Faeth & Fagan 2002).