DNA evidence for morphological and cryptic Cenozoic speciations in the Anaspididae, ‘living fossils’ from the Triassic

The speciation history of Anaspides tasmaniae (Crustacea: Malacostraca) and its close relatives (family Anaspididae) was studied by phylogenetic and molecular clock analyses of mitochondrial DNA sequences. The phylogenetic analyses revealed that the Anaspides morphotype conceals at least three cryptic species belonging to different parts of its range. The occurrence of multiple cryptic phylogenetic species within one morphological type shows that substantial genetic evolution has occurred independently of morphological evolution. Molecular clock dating of the speciation events that generated both the cryptic and the morphological species of Anaspididae indicated continuous speciation within this group since the Palaeocene ~55 million years ago. This relatively constant rate of recent morphological and cryptic speciation within the Anaspididae suggests that the speciation rate in this group does not correlate with its low extinction rate or morphological conservatism.     

AND IF ENGLER WAS NOT COMPLETELY WRONG? EVIDENCE FOR MULTIPLE EVOLUTIONARY ORIGINS IN THE MOSS FLORA OF MACARONESIA

The Macaronesian endemic flora has traditionally been interpreted as a relict of a subtropical element that spanned across Europe in the Tertiary. This hypothesis is revisited in the moss subfamily Helicodontioideae based on molecular divergence estimates derived from two independent calibration techniques either employing fossil evidence or using an Monte Carlo Markov Chain (MCMC) to sample absolute rates of nucleotide substitution from a prior distribution encompassing a wide range of rates documented across land plants. Both analyses suggest that the monotypic Madeiran endemic genus Hedenasiastrum diverged of other Helicodontioideae about 40 million years, that is, well before Macaronesian archipelagos actually emerged, in agreement with the relict hypothesis. Hedenasiastrum is characterized by a plesiomorphic morphology, which is suggestive of a complete morphological stasis over 40 million years. Macaronesian endemic Rhynchostegiella species, whose polyphyletic origin involves multiple colonization events, evolved much more recently, and yet accumulated many more morphological novelties than H. percurrens. The Macaronesian moss flora thus appears as a complex mix of ancient relicts and more recently dispersed, fast‐evolving taxa.     

Integrative taxonomy reveals extreme morphological conservatism in sympatric Mugil species from the Tropical Southwestern Atlantic

The biodiversity crisis has had particularly harsh impacts on marine environments. However, there is still considerable uncertainty about how many species have been seriously impacted and the effectiveness of protection measures (e.g., marine protected areas or MPAs) due to high levels of cryptic species in many taxa. Here, we employ an integrative taxonomy approach to mullet species in the genus Mugil. In addition to its high economic value, this genus is notable for having diversified ~29 million years ago without marked morphological and ecological divergence. We obtained 129 specimens of Mugil from the Coral Coast MPA, the largest of its kind in the Tropical Southwestern Atlantic marine province. Although morphometric and meristic traits revealed six taxonomically recognized species, only five mitochondrial lineages were observed. All individuals morphologically identified as M. incilis belonged to the mitochondrial lineage of Mugil curema, which is consistent with misidentification of morphologically similar species and an overestimation of species diversity. Remarkably, Mugil species in our sample that diverged up to ~23 million years ago are also the most morphologically similar (M. curema and M. rubrioculus), suggesting extreme morphological conservatism, possibly driven by similarities in habitat use and life‐history traits. This study demonstrates the potential utility of integrative taxonomy (including DNA barcoding) for contributing to the conservation and sustainable use of natural resources.     

Deep genetic divergences among morphologically similar and parapatric Skistodiaptomus (Copepoda: Calanoida: Diaptomidae) challenge the hypothesis of Pleistocene speciation

We used mitochondrial [cytochrome c oxidase subunit I (CO I ), cytochrome b , and 16S] and nuclear [internal transcribed spacer (ITS) phylogenies of Skistodiaptomus copepods to test hypotheses of Pleistocene divergence and speciation within the genus. Mitochondrial (mt)DNA sequence divergences do not support hypotheses for Pleistocene speciation and instead suggest much more ancient speciation events in the genus. Skistodiaptomus oregonensis and Skistodiaptomus pygmaeus (i.e. two morphologically similar and parapatric species) exhibited uncorrected mtDNA sequence divergences exceeding 20%. Similarly, we identified three divergent clades of Skistodiaptomus pallidus that exhibited mtDNA sequence divergences exceeding 15%, suggesting that even intraspecific divergence within this morphospecies predates the Pleistocene. We found clear evidence of CO I pseudogenes in S. pygmaeus , but their presence did not lead to significant overestimates of sequence divergences for this gene. Substitution saturation and strong purifying selection have most likely led to underestimates of sequence divergences and divergence times among Skistodiaptomus . The widespread phenomenon of morphological stasis among genetically divergent copepod groups indicates that speciation often occurs with little or no morphological change. Instead, morphological evolution may occur idiosyncratically after speciation and create discordant patterns of morphological similarity, shared ancestry and divergence time. Cryptic species complexes are therefore common in copepods, and morphological species concepts underestimate their true species diversity.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 150–165.     

MOLECULAR DIVERGENCE BETWEEN ASIAN AND NORTH AMERICAN SPECIES OF LIRIODENDRON (MAGNOLIACEAE) WITH IMPLICATIONS FOR INTERPRETATION OF FOSSIL FLORAS

Botanists have long been aware of the floristic similarities between eastern Asia and eastern North America. Most who have considered this classic disjunction pattern have suggested that it arose through range disruption of a flora that was once more widely distributed in the Northern Hemisphere. There is less agreement on the timing of this process, with suggestions ranging from the Paleocene to the Neogene. In this study, molecular markers from two different plant genomes were used to assess the degree of genetic divergence between the two interfertile, morphologically similar species of the genus Liriodendron, i.e., L. tulipifera and L. chinense. Resulting molecular divergence estimates were translated into approximate dates of separation, independent of evidence from the fossil record. Allozyme data (Nei's genetic identity = 0.434) suggested a divergence time of 10–16 million years before present, whereas sequence divergence in the plastid genomes (1.24%) led to an estimate of approximately 11–14 million years before present. A review of the paleobotanical literature indicated that the fossil floras that included, or might have included Liriodendron could not have survived in Beringia after the late Miocene and the onset of southward-migrating Arctic air masses on the North American continent. This interpretation suggests a minimum time of separation of approximately 13 million years before present. Thus, both molecular data sets and the paleobotanical evidence concur in suggesting a divergence time of 10–16 million years before present. Interspecific compatibility and relative morphological stasis must have, therefore, persisted from at least the late Miocene. We emphasize the need for similar studies in other genera, especially those that have both a reasonable Tertiary fossil history and extant species in mesic temperate refugia in Asia, Europe, and western as well as eastern North America.     

Geographic isolation facilitates the evolution of reproductive isolation and morphological divergence

Geographic isolation is known to contribute to divergent evolution, resulting in unique phenotypes. Oftentimes morphologically distinct populations are found to be interfertile while reproductive isolation is found to exist within nominal morphological species revealing the existence of cryptic species. These disparities can be difficult to predict or explain especially when they do not reflect an inferred history of common ancestry which suggests that environmental factors affect the nature of ecological divergence. A series of laboratory experiments and observational studies were used to address what role biogeographic factors may play in the ecological divergence of Hyalella amphipods. It was found that geographic isolation plays a key role in the evolution of reproductive isolation and divergent morphology and that divergence cannot be explained by molecular genetic variation.     

Deep-sea squat lobster biogeography (Munidopsidae: Leiogalathea) unveils Tethyan vicariance and evolutionary patterns shared by shallow-water relatives

The ecology, abundance and diversity of galatheoid squat lobsters make them an ideal group to study deep-sea diversification processes. Here, we reconstructed the evolutionary and biogeographic history of Leiogalathea, a genus of circum-tropical deep-sea squat lobsters, in order to compare patterns and processes that have affected shallow-water and deep-sea squat lobster species. We first built a multilocus phylogeny and a calibrated species tree with a relaxed clock using StarBEAST2 to reconstruct evolutionary relationships and divergence times among Leiogalathea species. We used BioGeoBEARS and a DEC model, implemented in RevBayes, to reconstruct ancestral distribution ranges and the biogeographic history of the genus. Our results showed that Leiogalathea is monophyletic and comprises four main lineages; morphological homogeneity is common within and between clades, except in one; the reconstructed ancestral range of the genus is in the Atlantic and Indian oceans (Tethys). They also revealed the divergence of the Atlantic species around 25 million years ago (Ma), intense cladogenesis 15–25 Ma and low levels of speciation over the last 5 million years (Myr). The four Leiogalathea lineages showed similar patterns of speciation: allopatric speciation followed by range expansion and subsequent stasis. Leiogalathea started diversifying during the Oligocene, likely in the Tethyan. The Atlantic lineage then split from its Indo-Pacific sister group due to vicariance driven by closure of the Tethys Seaway. The Atlantic lineage is less speciose compared with the Indo-Pacific lineages, with the Tropical Southwestern Pacific being the current centre of diversity. Leiogalathea diversification coincided with cladogenetic peaks in shallow-water genera, indicating that historical biogeographic events similarly shaped the diversification and distribution of both deep-sea and shallow-water squat lobsters.     

SPECIATIONAL HISTORY IN A DIVERSE CLADE OF HABITAT-SPECIALIZED SPIDERS (ARANEAE: NESTICIDAE: NESTICUS): INFERENCES FROM GEOGRAPHIC-BASED SAMPLING

This paper summarizes the results of an initial effort to reconstruct the speciational history of cave spiders (Nesticus) from the southern Appalachian Mountains of eastern North America. The Appalachian Nesticus fauna includes a large series of about 30 species distributed across islandlike cave and montane habitats. Many of the species are geographically restricted; all of the species are found in allopatry. Observed patterns of morphological variation and biogeographic evidence suggest that species diversification in this lineage may have occurred recently, perhaps in response to Pleistocene climatic fluctuations. To address questions about the spatial and temporal dynamics of Nesticus speciation, while accounting for potential phylogenetic difficulties, I have gathered nuclear and mitochondrial DNA sequences for a sample of individuals from 81 populations representing 28 Nesticus species. Analyses of these data indicate that considerable genetic divergence exists within and among currently recognized morphological species. Consistent with relatively deep species divergences, most of which likely predate the Pleistocene, is a prevailing pattern of phylogenetic concordance between taxonomic species and monophyletic gene tree lineages. The few deviations from monophyly detected can be tentatively attributed to a peripatric mode of speciation. Although species limits as inferred by the molecular data are generally concordant with patterns of morphological continuity and discontinuity in genitalia, there is evidence to suggest that cryptic phylogenetic lineages exist within some morphologically continuous units. This observation, in combination with the general depth of species lineages, makes any argument about rapid evolution in Nesticus genitalic characteristics unnecessary.     

Morphological stasis in the Eurytemora affinis species complex (Copepoda: Temoridae)

Morphological stasis has long been regarded as one of the most challenging problems in evolutionary biology. This study focused on the copepod species complex, Eurytemora affinis, as a model system to determine pattern and degree of morphological stasis. This study revealed discordant rates of morphological differentiation, molecular evolution, and reproductive isolation, where speciation was accompanied by lack of morphological differentiation in secondary sex characters. Comparisons were made among phylogenies based on morphometrics, nuclear (allozyme) loci, and mitochondrial DNA (mtDNA) sequences from cytochrome oxidase I, for a total of 43 populations within the complex. These systematic relationships were also compared to patterns of reproductive isolation. In addition, genetic subdivision of nuclear molecular (allozyme) markers (G _ST) and quantitative (morphological) characters (Q _ST) were determined to infer evolutionary forces driving morphological differentiation. The morphometric phylogeny revealed that all clades, excluding the European clade, were morphologically undifferentiated and formed a polytomy (multifurcation). Morphometric distances were not correlated with mtDNA distances, or with patterns of reproductive isolation. In contrast, nuclear and mtDNA phylogenies were mostly congruent. Reproductive isolation proved to be the most sensitive indicator of speciation, given that two genetically and morphologically proximate populations showed evidence of hybrid breakdown. Quantitative genetic (morphological) subdivision (Q _ST = 0.162) was lower than nuclear genetic subdivision (G _ST = 0.617) for four laboratory-reared North American populations, indicating retarded evolution of morphological characters. This result contrasts with most other species, where Q _ST typically exceeds G _ST as a result of directional selection. Thus, in all but the European populations, evolution of the secondary sex characters was marked by morphological stasis, even between reproductively-isolated populations.     

Molecular patterns of differentiation in canyon treefrogs (Hyla arenicolor): evidence for introgressive hybridization with the Arizona treefrog (H. wrightorum) and correlations with advertisement call differences

Detection of genetic and behavioural diversity within morphologically similar species has led to the discovery of cryptic species complexes. We tested the hypothesis that US populations of the canyon treefrog (Hyla arenicolor) may consist of cryptic species by examining mate‐attraction signals among three divergent clades defined by mtDNA. Using a multi‐locus approach, we re‐analysed phylogenetic relationships among the three clades and a closely related, but morphologically and behaviourally dissimilar species, the Arizona treefrog (H. wrightorum). We found evidence for introgression of H. wrightorum’s mitochondrial genome into H. arenicolor. Additionally, the two‐clade topology based on nuclear data is more congruent with patterns of call variation than the three‐clade topology from the mitochondrial dataset. The magnitude of the call divergence is probably insufficient to promote isolation of the nuclear DNA‐defined clades should they become sympatric, but further divergence in call properties significant in species identification could promote speciation in the future.     

Genetic and reproductive evidence for two species of ornate wobbegong shark Orectolobus spp. on the Australian east coast

This study reports on evidence for reproductive isolation among Orectolobus ornatus and Orectolobus halei, two previously cryptic and recently redescribed species of wobbegong shark (Orectolobiformes: Orectolobidae) from the east coast of Australia. The evidence is based on disparity in size at sexual maturity, diagnostic nuclear and mitochondrial DNA variants, and marked phylogenetic divergence. Plots of total length (L_T) and maturity for the two species were non‐overlapping and illustrative of statistically significant size dimorphism. Genetic analyses and phylogenetic reconstruction did not provide indication of hybridization between O. ornatus and O. halei. In fact, sequence divergence between them was higher than in comparisons with another congeneric and largely co‐distributed wobbegong species (Orectolobus maculatus). The assumption of a molecular clock revealed that the two species have evolved in isolation for c. 3·9 million years. These results challenge a paradigm often mentioned in the biodiversity literature that most cryptic species are the product of recent speciation events and will contribute to the development of effective management strategies for wobbegong sharks.     

Integrative species delimitation reveals cryptic diversity in the southern Appalachian Antrodiaetus unicolor (Araneae: Antrodiaetidae) species complex

Although species delimitation can be highly contentious, the development of reliable methods to accurately ascertain species boundaries is an imperative step in cataloguing and describing Earth's quickly disappearing biodiversity. Spider species delimitation remains largely based on morphological characters; however, many mygalomorph spider populations are morphologically indistinguishable from each other yet have considerable molecular divergence. The focus of our study, the Antrodiaetus unicolor species complex containing two sympatric species, exhibits this pattern of relative morphological stasis with considerable genetic divergence across its distribution. A past study using two molecular markers, COI and 28S, revealed that A. unicolor is paraphyletic with respect to A. microunicolor. To better investigate species boundaries in the complex, we implement the cohesion species concept and use multiple lines of evidence for testing genetic exchangeability and ecological interchangeability. Our integrative approach includes extensively sampling homologous loci across the genome using a RADseq approach (3RAD), assessing population structure across their geographic range using multiple genetic clustering analyses that include structure , principal components analysis and a recently developed unsupervised machine learning approach (Variational Autoencoder). We evaluate ecological similarity by using large‐scale ecological data for niche‐based distribution modelling. Based on our analyses, we conclude that this complex has at least one additional species as well as confirm species delimitations based on previous less comprehensive approaches. Our study demonstrates the efficacy of genomic‐scale data for recognizing cryptic species, suggesting that species delimitation with one data type, whether one mitochondrial gene or morphology, may underestimate true species diversity in morphologically homogenous taxa with low vagility.     

PATTERNS OF ALLOZYME RELATIONSHIPS COMPARED WITH MORPHOLOGY,HYBRIDIZATION, AND GEOLOGIC HISTORY IN ALLOPATRIC ISLAND-DWELLING MOSQUITOES

Allozyme relationships were compared with morphology, interspecific hybridization, and geologic history in 14 closely-related species in the Aedes (Stegomyia) scutellaris subgroup. The phylogeny generated by the electrophoretic data was generally in agreement with morphological classifications except that one morphologically distinct species pair (A. alcasidi and A. malayensis) showed only populational differentiation and several nearly identical morphological pairs in Polynesia (e.g., A. pseudoscutellaris and A. polynesiensis) were very distant genetically. A molecular clock, based on the ratio between genetic distance and divergence time (one unit of Nei's distance = 11 million years) inferred from a dated geologic event, was used to compare zoogeography, morphology, and biochemical relationships. Genetic distances between species from the Solomon Islands and the Vanuatu-Polynesia area correspond to an early Pliocene separation of these areas. An invasion of Micronesia occurred approximately 5 million years ago. Most remaining speciation occurred during the mid-Pliocene as islands moved into their present-day positions. The close relationship between southeast Asian and Philippine species could have resulted from the recent onset of isolation in the Pleistocene. Interspecific hybridization potential was not significantly correlated with Nei's genetic distance. Many quite distantly related species readily hybridize, while some closely related pairs show only unidirectional compatibility. Slight interspecific morphological divergence and the independence of hybridization capabilities from phylogeny are not unexpected in a group in which species are allopatric and occupy similar habitats.     

Timing the Eastern Asian–Eastern North American Floristic Disjunction: Molecular Clock Corroborates Paleontological Estimates

Sequence data of the chloroplast gene rbcL were used to estimate the time of the well-known eastern Asian–eastern North American floristic disjunction. Sequence divergence of rbcL was examined for 22 species of 11 genera (Campsis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella, Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse array of flowering plants occurring disjunctly in eastern Asia and eastern North America. Divergence times of putative disjunct species pairs were estimated from synonymous substitutions, using rbcL molecular clocks calibrated for Cornus. Relative rate tests were performed to assess rate constancy of rbcL evolution among lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 ± 4.30 million years to recent (<0.31 million years), with most within the last 10 million years (in the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This estimate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian–eastern North American floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the Northern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribution of taxa with low divergence, such as Menispermum.     

Molecular Phylogeny of Mobilid and Sessilid Ciliates Symbiotic in Eastern Pacific Limpets (Mollusca: Patellogastropoda)

The phylogenetic relationships of the ciliate subclass Peritrichia, composed of the orders Mobilida and Sessilida, have recently come under debate as morphological and molecular analyses have struck contrasting conclusions as to the monophyly of the group. We provide additional molecular data to assess the monophyly of the Peritrichia by sequencing the small subunit ribosomal RNA genes of two symbiotic peritrichs, Urceolaria korschelti and Scyphidia ubiquita, found inhabiting the mantle cavity of limpets. Although phylogenetic analyses indicated a nonmonophyletic Peritrichia, approximately unbiased tests revealed that the monophyletic hypothesis could not be rejected. With regard to the Mobilida, our analysis showed divergence within the family Trichodinidae related to host taxa—a molluscan clade and a fish clade. For the Sessilida, the family Scyphidiidae was sister to the Astylozoidae. In our sampling of U. korschelti and S. ubiquita, both species showed significant genetic divergence among geographically isolated, yet morphologically indistinguishable populations. We hypothesize that cryptic speciation has produced these morphologically identical species and argue that more extensive genomic analyses are required to fully assess the monophyly, biogeography, and ultimately biodiversity of the peritrichs.     

HOW DOES ECOLOGICAL OPPORTUNITY INFLUENCE RATES OF SPECIATION,EXTINCTION, AND MORPHOLOGICAL DIVERSIFICATION IN NEW WORLD RATSNAKES (TRIBE LAMPROPELTINI)?

Ecological adaptive radiation theory predicts an increase in both morphological and specific diversification when organisms colonize new environments. Accordingly, bursts of morphological diversification, characterized by low within‐subclade morphological disparity, may be associated with these increases in speciation rates. Conversely, increasing species density, reduction in available habitat, or increasing extinction rates are expected to cause rates of diversification to decline. We test these hypotheses by examining the tempo and mode of speciation in the lampropeltinine snakes, a morphologically variable group that colonized the New World ～24 million years ago and radiated throughout the Miocene. We show that specific diversification increased early in the history of the group, and that most morphological variation is partitioned among, rather than within subclades. These patterns provide further evidence for the hypothesis that morphological variation tends to be strongly partitioned among lineages when clades undergo early bursts of species diversification. A reduction in speciation rates may be indicative of density dependent effects due to a saturation of available ecological opportunity, rather than increases in extinction rates at the onset of the Pleistocene/Pliocene glacial cycles. This evidence runs counter to the general Pleistocene species pump model.     

Systematics and evolution of the Jacchus group of marmosets (Platyrrhini)

Interspecific differentiation and geographic variation patterns in 39 skull traits of eastern Brazilian marmosets are analyzed. Eastern Callithrix taxa are distinct morphologically, and no evidence of intergradation among taxa is observed. Instead, there are sharp, stepped morphological boundaries among taxa, consistent with species‐level distinction. The morphological similarity cluster diagram obtained from Mahalanobis distances is different from available molecular trees, and the general picture emerging is that the eastern Callithrix taxa should be considered as good species arising recently in South American history. In particular, C. kuhlii is morphologically distinct from other marmoset taxa, including C. geoffroyi and C. penicillata, which were previously hypothesized to be the parental populations that formed C. kuhlii by hybridization. Furthermore, C. kuhlii populations from southeastern Bahia do not overlap morphologically with any C. penicillata population, including the upper São Francisco River populations that display skin colors and pelage patterns to some extent similar to true Kuhli's marmosets. There is a negative, though insignificant, correlation between the morphological distance matrix and a Mahalanobis distance matrix estimated from nine climatic variables, a pattern opposite to that expected under a parapatric speciation model. This result, together with the lack of clinal variation in skull traits, suggests that an allopatric model of speciation might best explain eastern marmoset diversification. Am J Phys Anthropol 2003. © 2002 Wiley‐Liss, Inc.     

A SHIFT IN THE LONG‐TERM MODE OF FORAMINIFERAN SIZE EVOLUTION CAUSED BY THE END‐PERMIAN MASS EXTINCTION

Size is among the most important traits of any organism, yet the factors that control its evolution remain poorly understood. In this study, we investigate controls on the evolution of organismal size using a newly compiled database of nearly 25,000 foraminiferan species and subspecies spanning the past 400 million years. We find a transition in the pattern of foraminiferan size evolution from correlation with atmospheric pO₂ during the Paleozoic (400–250 million years ago) to long‐term stasis during the post‐Paleozoic (250 million years ago to present). Thus, a dramatic shift in the evolutionary mode coincides with the most severe biotic catastrophe of the Phanerozoic (543 million years ago to present). Paleozoic tracking of pO₂ was confined to Order Fusulinida, whereas Paleozoic lagenides, miliolids, and textulariids were best described by the stasis model. Stasis continued to best describe miliolids and textulariids during post‐Paleozoic time, whereas random walk was the best supported mode for the other diverse orders. The shift in evolutionary dynamics thus appears to have resulted primarily from the selective elimination of fusulinids at the end of the Permian Period. These findings illustrate the potential for mass extinction to alter macroevolutionary dynamics for hundreds of millions of years.     

Polymorphism in a common Atlantic reef coral (Montastraea cavernosa) and its long-term evolutionary implications

Recent advances in morphometrics and genetics have led to the discovery of numerous cryptic species in coral reef ecosystems. A prime example is the Montastraea annularis scleractinian coral species complex, in which morphological, genetic, and reproductive data concur on species boundaries, allowing evaluation of long-term patterns of speciation and evolutionary innovation. Here we test for cryptic species in the Atlantic species, Montastraea cavernosa, long recognized as polymorphic. Our modern samples consist of 94 colonies collected at four locations (Belize, Panamá, Puerto Rico in the Caribbean; S?o Tomé in the Eastern Atlantic). Our fossil samples consist of 78 colonies from the Plio-Pleistocene of Costa Rica and Panamá. Landmark morphometric data were collected on thin sections of 46 modern and 78 fossil colonies. Mahalanobis distances between colonies were calculated using Bookstein coordinates, revealing two modern and four fossil morphotypes. The remaining 48 of the 94 modern colonies were assigned to morphotype using discriminant analysis of calical measurements. Cross-tabulation and multiple comparisons tests show no significant morphological differences among geographic locations or water depths. Patterns of variation within and among fossil morphotypes are similar to modern morphotypes. DNA sequence data were collected for two polymorphic nuclear loci (β-tub1 and β-tub2) on all 94 modern colonies. Haplotype networks show that both genes consist of two clades, but morphotypes are not associated with genetic clades. Genotype frequencies and two-locus genotype assignments indicate genetic exchange across clades, and ϕst values show no genetic differentiation between morphotypes at different locations. Taken together, our morphological and genetic results do not provide evidence for cryptic species in M. cavernosa, but indicate instead that this species has an unusually high degree of polymorphism over a wide geographic area and persisting for >25 million years (myr).     

Morphological differentiation within the Daphnia longispina group

Although morphological evolution is assumed to be slow within Daphnia species complexes, discontinuities in morphological space can be detected. Here, morphological data derived from females of genetically-defined clones (cf. Gießler et al., 1999) are presented, in order to estimate the genetic component of phenotypic variance under standardised laboratory conditions. Animals originated from clonal assemblages of pre-alpine lakes and ponds, and a remote lake in western Germany, covering a wide range of morphotypes known from the traditional species D. cucullata, D. galeata, D. hyalina, D. rosea, and a variety of interspecific hybrids. Phenotypic analyses were based on quantitative and qualitative morphological characters of females in the first and fifth instars. Morphological divergence between clones was analysed using discriminant analysis or multidimensional scaling and the significance of the morphological groupings was estimated using neighbour-joining trees and bootstrapping. All analyses confirmed that (a) phenotypic similarities among taxa change with instar, (b) in contrast to low genetic divergence, pronounced morphological divergence exists between animals separated on the lake/pond level favouring speciation by the habitat shift hypothesis.