The structure of mutualistic networks provides clues to processes shaping biodiversity [1-10]. Among them, interaction intimacy, the degree of biological association between partners, leads to differences in specialization patterns [4, 11] and might affect network organization [12]. Here, we investigated potential consequences of interaction intimacy for the structure and coevolution of mutualistic networks. From observed processes of selection on mutualistic interactions, it is expected that symbiotic interactions (high-interaction intimacy) will form species-poor networks characterized by compartmentalization [12, 13], whereas nonsymbiotic interactions (low intimacy) will lead to species-rich, nested networks in which there is a core of generalists and specialists often interact with generalists [3, 5, 7, 12, 14]. We demonstrated an association between interaction intimacy and structure in 19 ant-plant mutualistic networks. Through numerical simulations, we found that network structure of different forms of mutualism affects evolutionary change in distinct ways. Change in one species affects primarily one mutualistic partner in symbiotic interactions but might affect multiple partners in nonsymbiotic interactions. We hypothesize that coevolution in symbiotic interactions is characterized by frequent reciprocal changes between few partners, but coevolution in nonsymbiotic networks might show rare bursts of changes in which many species respond to evolutionary changes in a single species.
Mutualistic networks involving plants and their pollinators or frugivores have been shown recently to exhibit a particular asymmetrical organization of interactions among species called nestedness: a core of reciprocal generalists accompanied by specialist species that interact almost exclusively with generalists. This structure contrasts with compartmentalized assemblage structures that have been verified in antagonistic food webs. Here we evaluated whether nestedness is a property of another type of mutualism-the interactions between ants and extrafloral nectary-bearing plants-and whether species richness may lead to differences in degree of nestedness among biological communities. We investigated network structure in four communities in Mexico. Nested patterns in ant-plant networks were very similar to those previously reported for pollination and frugivore systems, indicating that this form of asymmetry in specialization is a common feature of mutualisms between free-living species, but not always present in species-poor systems. Other ecological factors also appeared to contribute to the nested asymmetry in specialization, because some assemblages showed more extreme asymmetry than others even when species richness was held constant. Our results support a promising approach for the development of multispecies coevolutionary theory, leading to the idea that specialization may coevolve in different but simple ways in antagonistic and mutualistic assemblages.
The boreotropical flora concept suggests that relictual tropical disjunctions between Asia and the Americas are a result of the expansion of the circumboreal tropical flora from the middle to the close of the Eocene. Subsequently, temperate species diverged at high latitudes and migrated to other continents. To test this concept, we conducted a molecular phylogenetic analysis (using cpDNA) of the Magnoliaceae, a former boreotropical element that currently contains both tropical and temperate disjuncts. Divergence times of the clades were estimated using sequences of matK and two intergenic regions consisting of psbA-trnH and atpB-rbcL. Results indicate the tropical American section Talauma branched first, followed by the tropical Asian clade and the West Indies clade. Within the remaining taxa, two temperate disjunctions were formed. Assuming the temperate disjunction of Magnolia acuminata and Asian relatives occurred 25 mya (late Oligocene; based on seed fossil records), section Talauma diverged 42 mya (mid-Eocene), and tropical Asian and the West Indies clades 36 mya (late Eocene). These events correlate with cooling temperatures during the middle to late Eocene and probably caused the tropical disjunctions.
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