Drosophila Are Single- London: Reeve, Benham & Reeve

Hume, J. P. The history of the dodo Raphus cucullatus and the penguin based largely upon sexually dimorphic characters pos- of Mauritius. Historical Biology : –. sessed by males and thought to be used mainly in court- Hume, J. P., and A. S. Cheke. The white dodo of Réunion Island: unravelling a scientifi c and historical myth. Archives of Natural History ship and mating. These characters range from elaborately : –. pigmented wings to elongate setae, cilia, and bristles Moree, P. A concise history of Dutch Mauritius, 1598–1710. on the forelegs to unique structures on the mouthparts London: Kegan Paul International. Mourer-Chauviré, C., R. Bour., S. Ribes., and F. Moutou. The avi- and forelegs (Fig. ). The wide variety of secondary sex- fauna of Réunion Island (Mascarene Islands) at the time of the arrival of ual characters possessed by males of the various species the fi rst Europeans. Smithsonian Contributions to Paleobiology : –. groups suggests that sexual selection may have played an Owen, R. Memoir of the dodo (Didus ineptus, Linn.). London: Taylor and Francis. important role in the diversifi cation of this group. Strickland, H. E., and A. G. Melville. The dodo and its kindred. Approximately % of Hawaiian Drosophila are single- London: Reeve, Benham & Reeve. island endemics, possibly owing to their relatively poor fl ight abilities and low tolerance for desiccation. These physiological constraints, coupled with the unique geo- logical history of the Hawaiian Islands, have led to a spa- tially distributed pattern of diversifi cation referred to as DROSOPHILA the progression rule, where older species are found on older islands, and younger species are found on younger PATRICK M. O’GRADY, KARL N. MAGNACCA, islands (Fig. ). AND RICHARD T. LAPOINT One explanation for the large numbers of drosophi- lid species in the Hawaiian Islands involves adaptation University of California, Berkeley to so-called empty niches. This atmosphere of reduced competition allowed these species to experiment with The genus Drosophila provides excellent opportunities to novel life history strategies, and thus to diversify. Several study evolution on island systems. The endemic Hawaiian adaptations unique to the Hawaiian Drosophilidae seem Drosophila are a classic example of adaptive radiation and to bear this out. For example, the small group of ∼ spe- rapid speciation in nature evolving in situ over the course cies placed in the Scaptomyza subgenus Titanochaeta of the past million years. Other groups of Drosophila, have specialized to oviposit in spider egg sacs. Larvae found on true islands or in island-like systems (e.g., the develop and parasitize the spider eggs while they are Madrean Archipelago), are invaluable tools to under- being guarded by adult spiders. Sixty-seven percent of standing evolutionary biology and have served as theo- Hawaiian Drosophila for which data is available breed retical and empirical model systems for over years. on only a single host plant family, whereas % are spe- cifi c to a single host substrate, such as leaves, bark, or HAWAIIAN DROSOPHILA sap fl ux (Magnacca et al., ). The endemic Hawaiian Drosophilidae, with an esti- Hawaiian Drosophila have served as a model sys- mated species, consists of two major lineages, the tem to address a number of evolutionary phenomena, Hawaiian Drosophila and the genus Scaptomyza. The high including how founder events and mating asymmetries degree of species diversity in Hawai‘i is extraordinary, can drive species formation (Fig. ). Throughout their with about one-sixth of the world’s known Drosophili- evolutionary history, Hawaiian Drosophila have repeat- dae being endemic to this small archipelago. Phylogenetic edly undergone founder events, either as they colonize analyses indicate that the family colonized the Hawai- new islands or when populations are subdivided (e.g., ian Islands only once, roughly million years ago. The by lava fl ows or erosional processes). Hampton Carson genus Scaptomyza, which also contains a large number of utilized Hawaiian Drosophila to illustrate his founder mainland taxa, seems to have escaped from Hawai‘i and fl ush theory, a type of founder effect speciation that undergone subsequent diversifi cation on the mainland proposes a reduction in intraspecifi c competition and (see below). an increase in population size following a colonizing There are currently described species of Hawaiian event. Once the population size becomes large again, Drosophila, with an additional ∼ awaiting description. selection is reasserted, and the population may consti- These species have been divided into eight species groups tute a new species (Fig. ). (picture wing, nudidrosophila, ateledrosophila, antopocerus, Alan Templeton also used Hawaiian Drosophila to modifi ed tarsus, modifi ed mouthpart, haleakalae, rustica) explain his transilience founder effect theory. Templeton 232 DROSOPHILA Gillespie08_D.indd 232 4/9/09 1:07:42 PM Kaneshiro extended founder effect theories to include the complex mating behaviors and secondary sexual char- acteristics exhibited by Hawaiian Drosophila. In most populations, females are very choosy in selecting males with which to mate. However, small, colonizing populations are initially subject to founder effects, in which the scarcity of males forces females to be less selective when choosing a mate: If they are too choosy, they will not fi nd a mate. Additionally, males from this small population display highly variable mating behaviors because of reduced intraspecifi c competition and relaxed selection in what are normally highly selected behaviors (Fig. ). Choosy females from the larger source population will not mate with the males of the founder population, although females of this founder population will accept males from the larger source population. CARIBBEAN DROSOPHILA Patterns of in situ species formation are not as clear in the Caribbean islands because many widely distributed species are also found in mainland North and South Amer- ica. Historically, this has led to a situation in which gene fl ow from closely related mainland populations can act to homogenize any unique genetic differences that may accu- mulate in island populations. In spite of the close prox- imity of mainland ancestors, some Caribbean Drosophila have become genetically distinct from these widespread ancestral species. The repleta and cardini groups highlight two factors, ecological specialization and morphological adaptation, that may act to drive the formation of new species in the presence of gene fl ow from the mainland. FIGURE 1 Phylogenetic and morphological diversity in the Hawaiian Drosophilidae. Phylogeographic patterns in the planitibia species group of Hawaiian Drosophila showing four independent examples of the progression rule in the cyrtoloma (blue), neopicta (green), planitibia (red), and picticornis (yellow) subgroups. Secondary sexual modifi cations on the forelegs of (A) D. waddingtoni and (B) D. quasiexpansa; the arista of (C) D. tanythrix; mouthparts of (D) D. xenophaga, (E) D. hystricosa, (F) D. adventitia, and (G) D. freycinetae; and the wing of D. grimshawi. discussed how, after a bottleneck derived from a founder event, a highly outbred colonizing population would expe- rience a shuffl ing between epistatic loci that were adapted to work best in certain combinations (Fig. ). This would cause an instant shift in adaptive peaks to ones optimal for these randomly recombined loci. Although he con- cludes that this is likely a rare event, the ecological and genetic nature of the Hawaiian Drosophila lend them to FIGURE 2 Comparison of founder fl ush, genetic transillience, and this sort of speciation. asymmetrical mating hypotheses. DROSOPHILA 233 Gillespie08_D.indd 233 4/9/09 1:07:42 PM Ecological adaptation may be an important component SCAPTOMYZA AND ISLAND COLONIZATION of species formation. The Drosophila repleta group con- The cosmopolitan genus Scaptomyza (Diptera: Droso- tains about described species, the majority of which are philidae), with nearly described species placed in endemic to the New World and have diversifi ed on various subgenera constitutes a major radiation within the species of necrotic cacti. One group, the mayaguana triad, family Drosophilidae. About described species are is restricted to the Caribbean and includes the widespread endemic to Hawai‘i, and an additional ∼ from this species D. mayaguana, and the more narrowly restricted archipelago await description. Phylogenetic analyses taxa D. straubae and D. parisiena. Although few morpho- strongly support the monophyly of Scaptomyza and its logical differences distinguish these species, they do have placement as the sister group to the endemic Hawaiian unique polytene chromosome inversions and ecological Drosophila. Unlike the Hawaiian Drosophila, however, associations (different host cacti) that may have acted as a Scaptomyza males generally lack secondary sexual char- reproductive barrier to generate distinct species. acteristics and do not perform elaborate courtship dis- Caribbean island Drosophila have also been used as plays. Instead, they have complex genitalia that can be models to understand morphological variation and phe- quite different, even between closely related taxa, and notypic plasticity. The Drosophila cardini group consists may act to reinforce a “lock and key” form of species of neotropical species in two subgroups, cardini and recognition. dunni. The dunni subgroup is entirely Caribbean in dis- Biogeographic analyses suggest that the

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