Speciation and phylogeography of Hawaiian terrestrial arthropods

The Hawaiian archipelago is arguably the world's finest natural laboratory for the study of evolution and patterns of speciation. Arthropods comprise over 75% of the endemic biota of the Hawaiian Islands and a large proportion belongs to species radiations. We classify patterns of speciation within Hawaiian arthropod lineages into three categories: (i) single representatives of a lineage throughout the islands; (ii) species radiations with either (a) single endemic species on different volcanoes or islands, or (b) multiple species on each volcano or island; and (iii) single widespread species within a radiation of species that exhibits local endemism. A common pattern of phylogeography is that of repeated colonization of new island groups, such that lineages progress down the island chain, with the most ancestral groups (populations or species) on the oldest islands. While great dispersal ability and its subsequent loss are features of many of these taxa, there are a number of mechanisms that underlie diversification. These mechanisms may be genetic, including repeated founder events, hybridization, and sexual selection, or ecological, including shifts in habitat and/or host affiliation. The majority of studies reviewed suggest that natural selection is a primary force of change during the initial diversification of taxa.     

Subtle population genetic structure in the Hawaiian grouper, Epinephelus quernus (Serranidae) as revealed by mitochondrial DNA analyses

The endemic Hawaiian grouper, Epinephelus quernus , is a commercially important species experiencing intense fishing pressure in part of its distributional range. We examined population genetic structure with 398 base pairs of the mitochondrial control region across a large portion of the range of E. quernus , spanning approximately 2000 km of the Hawaiian archipelago. Examination of genetic diversity shows that Gardner Island, situated midway along the island chain, harbours the most diverse haplotypes. F -statistics and Bayesian estimates of migration also reveal the mid-archipelago as genetically differentiated, where the first significant break among adjacent pairs of populations lies between the islands of Nihoa and Necker. Most island comparisons beyond Necker and Gardner to the north-west and among the lower five islands to the south-east show little to no genetic differences. Evidence of historical population expansion across the islands was also found by Maximum Likelihood analyses. The results suggest that management should be structured to reflect the genetic differentiation and diversity in the mid-archipelago, the patterns of which may be associated with oceanic current patterns.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 449–468.     

Biogeography and the evolution of flightlessness in a radiation of Hawaiian moths (Xyloryctidae: Thyrocopa)

Aim Although the ability to fly confers benefits to most insects, some taxa have become secondarily flightless. Insect flightlessness may be more likely to evolve in environments such as islands and other windswept and alpine areas, but this prediction has rarely been tested while controlling for phylogenetic effects. Here we present a phylogeny for the endemic Hawaiian Lepidoptera genus Thyrocopa, which has two flightless species that occur in alpine areas on Maui and Hawaii islands, in order to determine whether the flightless species are sister to each other or represent separate losses of flight. We also explore divergence times and biogeographic patterns of inter‐island colonization in Thyrocopa, and present the first Hawaiian study to sample a genus from nine islands. Location The Hawaiian Islands. Methods The phylogeny is composed of 70 individuals (including 23 Thyrocopa species and 7 outgroup species) sequenced for portions of cytochrome c oxidase subunit I, elongation factor 1α and wingless genes, for a total of 1964 base pairs, and was estimated using both parsimony (paup *) and Bayesian inference (Mr Bayes ). Divergence times were estimated using the beast software package. Results Our results indicate that two independent invasions of alpine habitats with concomitant loss of flight have occurred in Thyrocopa. Based on current taxon sampling, Thyrocopa colonized the Hawaiian Islands slightly before the formation of Kauai. In terms of overall patterns of diversification, subclades generally follow a progression from older to younger islands. The genus has the greatest number of species on Kauai, with species numbers generally decreasing with decreasing island age. Main conclusions Loss of flight ability has evolved twice in a short period of geological time in Thyrocopa, perhaps as a result of low temperatures, high winds and/or a lack of predation pressure. However, several other Thyrocopa species that live on small islands with consistently high winds, such as Necker and Nihoa islands, retain the ability to fly.     

Phylogeny and age of diversification of the planitibia species group of the Hawaiian Drosophila

The Hawaiian Drosophila offer a unique opportunity to examine evolutionary questions because of the known ages of the Hawaiian Islands and the large number of species endemic to this archipelago. One of the more well studied groups of Hawaiian Drosophila is the planitibia species group, a long-standing population genetic model system. Here we present a molecular phylogenetic hypothesis of all 17 taxa in the planitibia group based on nucleotide sequences from two mitochondrial (16S and COII) and four nuclear (Adh, Gpdh, Yp1, and Yp2) loci, accounting for over 4kb of sequence per taxon. We use these data to estimate major divergence times within this group. Our results suggest that the basal diversification within this group, calculated at 6.1 +/- 0.47 MY, predates the oldest high island of Kauai. The older diversifications in this group took place on Kauai, with subsequent colonization and speciation events occurring as new islands became available to Drosophila. Understanding of the phylogenetic relationships of this important group will place the existing population genetic work in a macroevolutionary context and stimulate additional work, particularly on those taxa endemic to the Maui Nui complex of islands.     

Repeated range expansion and niche shift in a volcanic hotspot archipelago: Radiation of C4 Hawaiian Euphorbia subgenus Chamaesyce (Euphorbiaceae)

Woody perennial plants on islands have repeatedly evolved from herbaceous mainland ancestors. Although the majority of species in Euphorbia subgenus Chamaesyce section Anisophyllum (Euphorbiaceae) are small and herbaceous, a clade of 16 woody species diversified on the Hawaiian Islands. They are found in a broad range of habitats, including the only known C₄ plants adapted to wet forest understories. We investigate the history of island colonization and habitat shift in this group. We sampled 153 individuals in 15 of the 16 native species of Hawaiian Euphorbia on six major Hawaiian Islands, plus 11 New World close relatives, to elucidate the biogeographic movement of this lineage within the Hawaiian island chain. We used a concatenated chloroplast DNA data set of more than eight kilobases in aligned length and applied maximum likelihood and Bayesian inference for phylogenetic reconstruction. Age and phylogeographic patterns were co‐estimated using BEAST. In addition, we used nuclear ribosomal ITS and the low‐copy genes LEAFY and G3pdhC to investigate the reticulate relationships within this radiation. Hawaiian Euphorbia first arrived on Kaua`i or Ni`ihau ca. 5 million years ago and subsequently diverged into 16 named species with extensive reticulation. During this process Hawaiian Euphorbia dispersed from older to younger islands through open vegetation that is disturbance‐prone. Species that occur under closed vegetation evolved in situ from open vegetation of the same island and are only found on the two oldest islands of Kaua`i and O`ahu. The biogeographic history of Hawaiian Euphorbia supports a progression rule with within‐island shifts from open to closed vegetation.     

Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)

The endemic Hawaiian lobeliads are exceptionally species rich and exhibit striking diversity in habitat, growth form, pollination biology and seed dispersal, but their origins and pattern of diversification remain shrouded in mystery. Up to five independent colonizations have been proposed based on morphological differences among extant taxa. We present a molecular phylogeny showing that the Hawaiian lobeliads are the product of one immigration event; that they are the largest plant clade on any single oceanic island or archipelago; that their ancestor arrived roughly 13 Myr ago; and that this ancestor was most likely woody, wind-dispersed, bird-pollinated, and adapted to open habitats at mid-elevations. Invasion of closed tropical forests is associated with evolution of fleshy fruits. Limited dispersal of such fruits in wet-forest understoreys appears to have accelerated speciation and led to a series of parallel adaptive radiations in Cyanea, with most species restricted to single islands. Consistency of Cyanea diversity across all tall islands except Hawai ;i suggests that diversification of Cyanea saturates in less than 1.5 Myr. Lobeliad diversity appears to reflect a hierarchical adaptive radiation in habitat, then elevation and flower-tube length, and provides important insights into the pattern and tempo of diversification in a species-rich clade of tropical plants.     

Incorrect handling of calibration information in divergence time inference: an example from volcanic islands

Divergence time studies rely on calibration information from several sources. The age of volcanic islands is one of the standard references to obtain chronological data to estimate the absolute times of lineage diversifications. This strategy assumes that cladogenesis is necessarily associated with island formation, and punctual calibrations are commonly used to date the splits of endemic island species. Here, we re-examined three studies that inferred divergence times for different Hawaiian lineages assuming fixed calibration points. We show that, by permitting probabilistic calibrations, some divergences are estimated to be significantly younger or older than the age of the island formation, thus yielding distinct ecological scenarios for the speciation process. The results highlight the importance of using calibration information correctly, as well as the possibility of incorporating volcanic island studies into a formal, biogeographical hypothesis-testing framework.     

Origin of the endemic fern genus Diellia coincides with the renewal of Hawaiian terrestrial life in the Miocene

The enigmatic fern genus Diellia, endemic to the Hawaiian archipelago, consists of five extant and one recently extinct species. Diellia is morphologically highly variable, and a unique combination of characters has led to several contrasting hypotheses regarding the relationship of Diellia to other ferns. A phylogenetic analysis of four chloroplast loci places Diellia within 'black-stemmed' rock spleenworts of the species-rich genus Asplenium, as previously suggested by W. H. Wagner. Using an external calibration point, we estimate the divergence of the Diellia lineage from its nearest relatives to have occurred at ca. 24.3 Myr ago matching an independent estimate for the renewal of Hawaiian terrestrial life (ca. 23 Myr ago). We therefore suggest that the ancestor of the Diellia lineage may have been among the first successful colonists of the newly emerging islands in the archipelago. Disparity between morphological and nucleotide sequence variation within Diellia is consistent with a recent rapid radiation. Our estimated time of the Diellia radiation (ca. 2 Myr ago) is younger than the oldest island of Kaua'i (ca. 5.1 Myr ago) but older than the younger major islands of Maui (ca. 1.3 Myr ago), Lana'i (ca. 1.3 Myr ago) and Hawaii (ca. 0.43 Myr ago).     

PACT in practice: comparative historical biogeographic patterns and species–area relationships of the Greater Antillean and Hawaiian Island terrestrial biotas

Aim To compare the evolutionary and ecological patterns of two extensively studied island biotas with differing geological histories (the Hawaiian Islands and the Greater Antilles). We evaluated the results from PACT (phylogenetic analysis for comparing trees), an innovative approach that has been proposed to reveal general patterns of biotic expansion (between regions) and in situ (within a region) diversification, as well as species–area relationships (SAR) and the taxon pulse dynamic. Location The Hawaiian Islands and Greater Antilles. Methods We used the PACT algorithm to construct general area cladograms and identified biotic expansion and in situ nodes. We analysed the power‐law SAR and relative contribution of biotic expansion and in situ diversification events using power‐law and linear regression analyses. Results Both biotic expansion and in situ nodes were prevalent throughout the PACT general area cladograms (Greater Antilles, 55.9% biotic expansion, 44.1% in situ; Hawaiian Islands, 40.6% biotic expansion, 59.4% in situ). Of the biotic expansion events, both forward and backward events occurred in both regions (Greater Antilles, 85.1% forward, 14.9% backward; Hawaiian Islands, 65% forward, 35% backward). Additionally, there is a power‐law SAR for the Greater Antilles but not for the Hawaiian Islands. However, exclusion of Hawai'i (the youngest, largest Hawaiian Island) produced a power‐law SAR for the Hawaiian Islands. Main conclusions The prevalence of in situ events as well as forward and backward biotic expansion events reveals that both Hawaiian and Greater Antillean biotas have evolved through alternating episodes of biotic expansion and in situ diversification. These patterns are characteristic of the taxon pulse dynamic, for which few data have previously been recorded on islands. Additionally, our analysis revealed that historical influences on the power‐law SARs are pronounced in both assemblages: old, small islands are relatively species rich and young, large islands are relatively species poor. Thus, our PACT results are consistent with hypotheses of geological influence on the evolution of island biotas and also provide greater insight into the role of the taxon pulse dynamic in the formation of island equilibria.     

Long-distance colonization and radiation in gekkonid lizards, Tarentola (Reptilia: Gekkonidae), revealed by mitochondrial DNA sequences

Morphological systematics makes it clear that many non-volant animal groups have undergone extensive transmarine dispersal with subsequent radiation in new, often island, areas. However, details of such events are often lacking. Here we use partial DNA sequences derived from the mitochondrial cytochrome b and 12S rRNA genes (up to 684 and 320 bp, respectively) to trace migration and speciation in Tarentola geckos, a primarily North African clade which has invaded many of the warmer islands in the North Atlantic Ocean. There were four main invasions of archipelagos presumably by rafting. (i) The subgenus Neotarentola reached Cuba up to 23 million years (Myr) ago, apparently via the North Equatorial current, a journey of at least 6000 km. (ii) The subgenus Tarentola invaded the eastern Canary Islands relatively recently covering a minimum of 120 km. (iii) The subgenus Makariogecko got to Gran Canaria and the western Canary Islands 7-17.5 Myr ago, either directly from the mainland or via the Selvages or the archipelago of Madeira, an excursion of 200-1200 km. (iv) A single species of Makariogecko from Gomera or Tenerife in the western Canaries made the 1400 km journey to the Cape Verde Islands tip to 7 Myr ago by way of the south-running Canary current. Many journeys have also occurred within archipelagos, a minimum of five taking place in the Canaries and perhaps 16 in the Cape Verde Islands. Occupation of the Cape Verde archipelago first involved an island in the northern group, perhaps São Nicolau, with subsequent spread to its close neighbours. The eastern and southern islands were colonized from these northern islands, at least two invasions widely separated in time being involved. While there are just three allopatric species of Makariogecko in the Canaries, the single invader of the Cape Verde Islands radiated into five, most of the islands being inhabited by two of these which differ in size. While size difference may possibly be a product of character displacement in the northern islands, taxa of different sizes reached the southern islands independently.     

The maximum age of Hawaiian terrestrial lineages: geological constraints from Kōko Seamount

Aim To determine if Kōko Seamount submerged below sea level before Kure Island and Pearl and Hermes Reef formed, resulting in a period in which there were no extant islands. A period with no islands would eliminate prior terrestrial and shallow marine biotas that could migrate from island to island and require a restart of colonization from distant shores to populate the younger islands of the Hawaiian volcanic chain. Location Emperor Seamount Chain, north‐central Pacific Ocean. Methods We estimate subsidence rates for Kōko Seamount using ages determined from fossil large foraminifera and Sr‐isotopes, and maximum depths using palaeodepth estimates based on coralline algae. These data are combined with palaeolatitude changes as the Pacific Plate moved northwards, sea level variations, and sea surface temperature variations at the seamount through time to reconstruct the time and causes of submergence. Results Rounded carbonate clasts include three facies: zooxanthelate corals, bioclastic packstones to rudstones, and rhodolith floatstones. Two rudstones contain relatively deep‐water, coralline algal rhodoliths and large foraminifera indicative of Aquitanian (20.4–20 Ma) and Burdigalian (20–16 Ma) stages of the Early Miocene, consistent with Sr‐isotope ages of algae and one sample of large foraminifera. Corals grew on Kōko Seamount from c. 50 to 27.1 ± 0.4 Ma, the youngest Sr‐isotope age of a coral sample. These shallow, warm‐water coral reefs came under increasing stress as the volcano subsided at 0.012 ± 0.003 mm yr^?1, and migrated northwards, and as global climate cooled. The summit submerged and shallow coral reef growth ceased before 29 Ma, probably around 33 Ma. The volcano continued its slow subsidence, and deep‐water carbonates accumulated until they too were unable to keep pace, dying out at c. 16 Ma. Main conclusions The final submergence of the summit of Kōko Seamount by about 33 Ma confirms that biota on older Hawaiian–Emperor Islands could not have migrated from island to island along the entire chain to eventually colonize the present Hawaiian Islands. There was a period between at least 33 and 29 Ma in which no islands existed, and distant colonization had to repopulate the younger portion of the Hawaiian chain, which began to emerge between about 29 and 23 Ma.     

Rapid parallel evolution of aberrant traits in the diversification of the Gulf of Guinea white-eyes (Aves, Zosteropidae)

Archipelago-endemic bird radiations are familiar to evolutionary biologists as key illustrations of evolutionary patterns. However, such radiations are in fact rare events. White-eyes (Zosteropidae) are birds with an exceptionally high colonization and speciation potential; they have colonized more islands globally than any other passerine group and include the most species-rich bird genus. The multiplication of white-eye island endemics has been consistently attributed to independent colonizations from the mainland; the white-eyes of the Gulf of Guinea archipelago had been seen as a classic case, spanning as great a breadth of phenotypic diversity as the family worldwide. Contrary to this hypothesis, our molecular phylogenetic analysis places the Gulf of Guinea white-eyes in just two radiations, one grouping all five oceanic island taxa and the other grouping continental island and land-bridge taxa. Numerous 'aberrant' phenotypes (traditionally grouped in the genus Speirops) have evolved independently over a short space of time from nonaberrant (Zosterops) phenotypes; the most phenotypically divergent species have separated as recently as 0.22 Ma. These radiations rival those of Darwin's finches and the Hawaiian honeycreepers in terms of the extent of adaptive radiation per unit time, both in terms of species numbers and in terms of phenotypic diversity. Tempo and patterns of morphological divergence are strongly supportive of an adaptive radiation in the oceanic islands driven by ecological interactions between sympatric white-eyes. Here, very rapid phenotypic evolution mainly affected taxa derived from the youngest wave of colonization, in accordance with the model of asymmetric divergence owing to resource competition in sympatry.     

Genotoxicity and carcinogenicity testing of 1,2-dibromopropane and 1,1,3-tribromopropane in comparison to 1,2-dibromo-3-chloropropane

The activities of 1,2-dibromopropane (DBP) and 1,1,3-tribromopropane (TBP) were studied in seven genotoxicity assays, (i) SOS-induction inE. coli, (ii) DNA repair in primary rat hepatocyte culture, (iii) theSalmonella/microsome assay, (iv) a host-mediated assay usingSalmonella, (v) the somatic mutation and recombination assay inDrosophila melanogaster, (vi) HGPRT-mutagenesis assay in ARL 18 cells, and (vii) micronucleus formation assay in mouse polychromatophylic erythrocytes (PCE), forestomach (FS), glandular stomach (GS), duodenum (D), jejunum (J), cecum (C) and liver (L). The halopropanes were also tested for tumor formation in the fishDanio rerio. DBP was active in assays (ii), (v), (vii FS) and (vii L). TBP was positive in assays (ii) and (iii), strongly positive in (vii L) and borderline positive in (iv). However, neither DBP nor TBP induced tumors in fish, in contrast to the carcinogenic 1,2-dibromo-3-chloropropane. The genotoxicity and potential carcinogenicity of DBP and TBP in mammals is discussed.Abbreviations 2-AA 2-aminoanthracene - DBCP 1,2-dibromo-3-chloropropane - DBP 1,2-dibromopropane - HGPRT hypoxanthine-guanine phosphoribosyl transferase - i.p. intraperitoneal(ly) - NQO 4-nitro-quinoline-1-oxide - PCE polychromatic erythrocytes - TBP 1,1,3-tribromopropane - WME Williams' medium E     

The diversity and biogeography of koa-finches (Drepanidini: Rhodacanthis), with descriptions of two new species

All known populations of koa-finches, genus Rhodacanthis , became extinct in the Holocene epoch. Two new species are described here from Quaternary fossil sites in the Hawaiian Islands. One new species, from Kauai and Maui, is roughly the size of the historically known greater koa-finch ( R. palmeri ) but differs in having a more robust skull and in bill morphology. The second new species, from Oahu and Maui, is similar in size to the lesser koa-finch ( R. flaviceps ) but closer to R. palmeri in qualitative osteological traits. The two species of koa-finches known historically from the island of Hawaii are distinct in osteology from the fossil koa-finches on the older Hawaiian islands, indicating that at least two of the four known speciation events in the genus took place within approximately the past 500 kyr. However, the similarity of maxillae from Pleistocene and Holocene sites on Oahu suggests that the Oahu population maintained morphological stasis through the climate changes of the late Quaternary. The evidence that speciation occurred on the youngest island in the archipelago suggests that the process of community assembly on newly emergent Hawaiian landscapes was a stimulus to evolutionary diversification in Rhodacanthis .  © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society , 2005, 144 , 527–541.     

Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands

The presence of diverse and species-rich plant lineages on oceanic islands is most often associated with adaptive radiation. Here we discuss the possible adaptive significance of some of the most prominent traits in island plants, including woodiness, monocarpy and sexual dimorphisms. Indirect evidence that such traits have been acquired through convergent evolution on islands comes from molecular phylogenies; however, direct evidence of their selective value rarely is obtained. The importance of hybridization in the evolution of island plants is also considered as part of a more general discussion of the mechanisms governing radiations on islands. Most examples are from the Hawaiian and Canarian floras, and in particular from studies on the morphological, ecological and molecular diversification of the genus Aeonium, the largest plant radiation of the Canarian Islands.     

Molecular phylogeny and biogeography of the endemic Hawaiian Succineidae (Gastropoda: Pulmonata)

The endemic Hawaiian Succineidae represent an important component of the exceptionally diverse land snail fauna of the Hawaiian Islands, yet they remain largely unstudied. We employed 663-bp fragments of the cytochrome oxidase I (COI) mitochondrial gene to investigate the evolution and biogeography of 13 Hawaiian succineid land snail species, six succineid species from other Pacific islands and Japan, and various outgroup taxa. Results suggest that: (1) species from the island of Hawaii are paraphyletic with species from Tahiti, and this clade may have had a Japanese (or eastern Asian) origin; (2) species from five of the remaining main Hawaiian islands form a monophyletic group, and the progression rule, which states that species from older islands are basal to those from younger islands, is partially supported; no geographic origin could be inferred for this clade; (3) succineids from Samoa are basal to all other succineids sampled (maximum likelihood) or unresolved with respect to the other succineid clades (maximum parsimony); (4) the genera Succinea and Catinella are polyphyletic. These results, while preliminary, represent the first attempt to reconstruct the phylogenetic pattern for this important component of the endemic Hawaiian fauna.     

Conservation implications of hybridization in Hawaiian picture-winged Drosophila

In this review, we discuss the importance of hybridization among species for the conservation of Hawaiian picture-winged Drosophila. Hybridization can be a positive evolutionary process that creates new species and increases the adaptation of populations and species through the spread of adaptive alleles and traits. Conversely, hybridization can disrupt the genetic integrity of species or populations and this may be most detrimental among taxa that are recently hybridizing due to recent ecological changes. The loss of biodiversity in Hawaiian Drosophila through hybridization may be facilitated by habitat alteration and introduced species that reduce population sizes and alter geographic distributions of native species. We briefly review the evidence for hybridization in the genus Drosophila and then focus on hybridization in the Hawaiian picture-winged Drosophila. We examine three general approaches for identifying hybrids and for assessing the factors that appear to contribute to hybridization and the potential ecological and evolutionary outcomes of hybrids in natural populations. Overall, the potential for hybridization among species will likely increase the risk of extinction for Hawaiian picture-winged Drosophila species. Thus, it is important to consider the potential for hybridization among species when developing plans for the conservation of Hawaiian Drosophila.     

Evolutionary biogeography of the terrestrial biota of the Marquesas Islands,one of the world's remotest archipelagos

Aim

Here I review phylogenetic studies concerning the biogeography of the Marquesas Islands, an oceanic hotspot archipelago in the Pacific Ocean formed <5.5 Ma, and compare patterns (particularly pertaining to colonization and diversification) within the archipelago to those reported from the Hawaiian and Society Islands.

Location

Marquesas Islands, French Polynesia (Pacific Ocean).

Methods

I reviewed 37 phylogenetic studies incorporating Marquesas‐endemic taxa. I asked the following questions: (a) where are the sister‐groups of Marquesas lineages distributed? (b) are Marquesas‐endemic “radiations” monophyletic or polyphyletic? (c) what major between‐island phylogeographic barriers are seen in the Marquesas? (d) what evidence exists for diversification within islands? (e) how old is the Marquesas biota compared to the archipelago's age? Finally, these patterns are compared with those seen in the Society Islands and Hawaii.

Results

Most Marquesan lineages have their closest known relatives on other Pacific plate archipelagos (particularly the Society, Hawaiian, and Austral islands). Most Marquesas‐endemic radiations are found to be monophyletic, and among‐island diversification appears to be common. There is limited evidence for within‐island diversification. Some radiations may be consistent with a weak progression rule in which younger lineages are on younger islands. Crown ages of no Marquesas radiations appear to be older than the age of the archipelago (with one exception).

Main conclusions

Diversification of the Marquesas biota resembles that of the Hawaiian Islands more than that of the Society Islands. Many radiations are monophyletic and some appear to diversify in parallel with the formation of the archipelago.

     

Population variation and hybridization: Comparison of finches from two archipelagos

Summary Some populations of Darwin's Finches (Emberizinae) are exceptionally variable in body size and beak traits as a result of introgressive hybridization. A study of museum specimens of honeycreeper-finches (Carduelinae) from the Hawaiian islands was undertaken to see if the same phenomenon was manifested by a different phyletic group of finches in a different archipelago. Five hundred and twenty-four specimens of the seven species with finch-like bills were measured and their coefficients of variation were compared with those of the ground finch group (six species) of Darwin's Finches. Coefficients were smaller in the Hawaiian finches. Sympatric and, hence, potentially hybridizing species on the island of Hawaii were not consistently more variable than the allopatric species on other islands in the archipelago. The one species with both sympatric and allopatric populations did not show greater variation in the sympatric population. There is little evidence from these comparisons of hybridization occurring in the last 100 years. The difference between the two finch faunas can be explained in terms of two factors. Finches have been present for a longer time in the Hawaiian archipelago than in the Galápagos archipelago and have had more time to not only diversify but to evolve pre- and post-zygotic isolating mechanisms. In the generally less seasonal and floristically richer Hawaiian islands they have evolved greater dietary specializations. Beak traits adapted to specialist feeding may have been under stronger stabilizing selection and hybrids (if formed) may have been at a strong disadvantage in the absence of an ecological niche intermediate between the niches of the two parental species. Results of published electrophoretic studies of genetic variation suggest that the early phase of differentiation, involving occasional introgressive hybridization, may last for up to 5 million years.     

Diversity despite dispersal: colonization history and phylogeography of Hawaiian crab spiders inferred from multilocus genetic data

The Hawaiian archipelago is often cited as the premier setting to study biological diversification, yet the evolution and phylogeography of much of its biota remain poorly understood. We investigated crab spiders (Thomisidae, Mecaphesa ) that demonstrate contradictory tendencies: (i) dramatic ecological diversity within the Hawaiian Islands, and (ii) accompanying widespread distribution of many species across the archipelago. We used mitochondrial and nuclear genetic data sampled across six islands to generate phylogenetic hypotheses for Mecaphesa species and populations, and included penalized likelihood molecular clock analyses to estimate arrival times on the different islands. We found that 17 of 18 Hawaiian Mecaphesa species were monophyletic and most closely related to thomisids from the Marquesas and Society Islands. Our results indicate that the Hawaiian species evolved from either one or two colonization events to the archipelago. Estimated divergence dates suggested that thomisids may have colonized the Hawaiian Islands as early as ~10 million years ago, but biogeographic analyses implied that the initial diversification of this group was restricted to the younger island of Oahu, followed by back-colonizations to older islands. Within the Hawaiian radiation, our data revealed several well-supported genetically distinct terminal clades corresponding to species previously delimited by morphological taxonomy. Many of these species are codistributed across multiple Hawaiian Islands and some exhibit genetic structure consistent with stepwise colonization of islands following their formation. These results indicate that dispersal has been sufficiently limited to allow extensive ecological diversification, yet frequent enough that interisland migration is more common than speciation.