Speciation by founder effect in Hawaiian cave planthoppers? (Homoptera: Fulgoromorpha: Cixiidae)
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ABSTRACT:
After decades of serious fundamental debates about speciation we are now well provided with verbal and numerical models concerning this process. It has become evident, however, that in order to further progress in our understanding of speciation, new information is needed. A new model system, the cavernicolous planthoppers of Hawaiian lava tubes, which has recently been inaugurated as a model case for rapid speciation (HOCH & HOWARTH 1993, HOCH 1997) may provide some of this information.
The main difficulty in studying and understanding speciation lies in recognizing the relative importance of the factors driving divergence - random drift, natural selection, and/or sexual selection. We are addressing this problem in a concerted effort, by comparative studies on the morphology, behavior and genetics in allopatric taxa in different stages of evolutionary divergence. The serial colonization of new lava flows as well as new islands results in the formation of naturally occurring "replicates", through which we can observe speciation at different stages, within the same taxon, and even within the same area.
In the present study I focus on the "Oliarus polyphemus"-species complex. "Oliarus polyphemus" FENNAH was described as a widespread single species on the Big Island of Hawaii. Taxa of this complex are now known from numerous caves. Prerequisite for local occurence is the availability of an adequate food resource: roots of native trees and shrubs. Evidence from morphological and behavioural studies suggests that "Oliarus polyphemus" is a young species complex in a dynamic stage of evolutionary transformation (HOCH & HOWARTH 1993, WESSEL & HOCH 1999).
For assessing the degree of phenotypic differentiation between all cave populations studied, statistical analyses were performed using 13 morphological and 17 behavioral parameters (pertaining to the time-pattern of male and female courtship calls). The discontinuities between the populations for all measured parameters are highly significant, for two populations the ranges of variation even were without any overlap. Using a statistical species criterion (WESSEL & HOCH 1999) we interpret these two populations as taxa which have acquired species status, while for the remaining 15 populations, we assume subspecies status. Based on sequence information of the mitochondrial 16s rRNA and the COI gene a phylogenetic analysis supported the monophyly of the "Oliarus polyphemus" species complex. The model system is also ideally suited for estimating divergence time. A detailed biogeographic documentation of all "O. polyphemus" populations studied was conducted and matched with existing information on the age of the corresponding lava flows/caves. Combining these different sets of data will allow us to reconstruct possible pathways of migration and colonization of caves.
Our data suggest a sequence of founder events in the course of colonizing the subterranean habitat in new lava flows. Different degrees of variability in certain morphological characters correlating with the estimated age of the populations fit an interpretation of serial founder effects sensu CARSON (see CARSON & TEMPLETON 1984). CARSON explains possible founder effects by the breaking up of co-adapted gene complexes in a phase of rapid population growth (the so-called "founder flush") against loss of genetic variation. Many questions, however, remain to be investigated, especially the role of sexual selection which might account for the rapid establishment of newly appearing song variants within a founder population.
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STATISTICS
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