The domain relativity of evolutionary contingency

A key issue in the philosophy of biology is evolutionary contingency, the degree to which evolutionary outcomes could have been different. Contingency is typically contrasted with evolutionary convergence, where different evolutionary pathways result in the same or similar outcomes. Convergences are given as evidence against the hypothesis that evolutionary outcomes are highly contingent. But the best available treatments of contingency do not, when read closely, produce the desired contrast with convergence. Rather, they produce a picture in which any degree of contingency is compatible with any degree of convergence. This is because convergence is the repeated production of a given outcome from different starting points, and contingency has been defined without reference to the size of the space of possible outcomes. In small spaces of possibilities, the production of repeated outcomes is almost assured. This paper presents a definition of contingency which includes this modal dimension in a way that does not reduce it to the binary notion of contingency found in standard modal logic. The result is a conception of contingency which properly contrasts with convergence, given some domain of possibilities and a measure defined over it. We should therefore not ask whether evolution is contingent or convergent simpliciter, but rather about the degree to which it is contingent or convergent in various domains, as measured in various ways.     

New dinosaurs link southern landmasses in the Mid-Cretaceous

Abelisauroid predators have been recorded almost exclusively from South America, India and Madagascar, a distribution thought to document persistent land connections exclusive of Africa. Here, we report fossils from three stratigraphic levels in the Cretaceous of Niger that provide definitive evidence that abelisauroid dinosaurs and their immediate antecedents were also present on Africa. The fossils include an immediate abelisauroid antecedent of Early Cretaceous age (ca. 130-110 Myr ago), early members of the two abelisauroid subgroups (Noasauridae, Abelisauridae) of Mid-Cretaceous age (ca. 110 Myr ago) and a hornless abelisaurid skull of early Late Cretaceous age (ca. 95 Myr ago). Together, these fossils fill in the early history of the abelisauroid radiation and provide key evidence for continued faunal exchange among Gondwanan landmasses until the end of the Early Cretaceous (ca. 100 Myr ago).     

Dating the Monocot–Dicot Divergence and the Origin of Core Eudicots Using Whole Chloroplast Genomes

We estimated the dates of the monocot–dicot split and the origin of core eudicots using a large chloroplast (cp) genomic dataset. Sixty-one protein-coding genes common to the 12 completely sequenced cp genomes of land plants were concatenated and analyzed. Three reliable split events were used as calibration points and for cross references. Both the method based on the assumption of a constant rate and the Li–Tanimura unequal-rate method were used to estimate divergence times. The phylogenetic analyses indicated that nonsynonymous substitution rates of cp genomes are unequal among tracheophyte lineages. For this reason, the constant-rate method gave overestimates of the monocot–dicot divergence and the age of core eudicots, especially when fast-evolving monocots were included in the analysis. In contrast, the Li–Tanimura method gave estimates consistent with the known evolutionary sequence of seed plant lineages and with known fossil records. Combining estimates calibrated by two known fossil nodes and the Li–Tanimura method, we propose that monocots branched off from dicots 140–150 Myr ago (late Jurassic–early Cretaceous), at least 50 Myr younger than previous estimates based on the molecular clock hypothesis, and that the core eudicots diverged 100–115 Myr ago (Albian–Aptian of the Cretaceous). These estimates indicate that both the monocot–dicot divergence and the core eudicots age are older than their respective fossil records.     

Bone-eating Osedax worms lived on Mesozoic marine reptile deadfalls

We report fossil traces of Osedax, a genus of siboglinid annelids that consume the skeletons of sunken vertebrates on the ocean floor, from early-Late Cretaceous (approx. 100 Myr) plesiosaur and sea turtle bones. Although plesiosaurs went extinct at the end-Cretaceous mass extinction (66 Myr), chelonioids survived the event and diversified, and thus provided sustenance for Osedax in the 20 Myr gap preceding the radiation of cetaceans, their main modern food source. This finding shows that marine reptile carcasses, before whales, played a key role in the evolution and dispersal of Osedax and confirms that its generalist ability of colonizing different vertebrate substrates, like fishes and marine birds, besides whale bones, is an ancestral trait. A Cretaceous age for unequivocal Osedax trace fossils also dates back to the Mesozoic the origin of the entire siboglinid family, which includes chemosynthetic tubeworms living at hydrothermal vents and seeps, contrary to phylogenetic estimations of a Late Mesozoic–Cenozoic origin (approx. 50–100 Myr).     

The molecular evolutionary tree of lizards,snakes, and amphisbaenians

Squamate reptiles (lizards, snakes, amphisbaenians) number approximately 8200 living species and are a major component of the world's terrestrial vertebrate diversity. Recent molecular phylogenies based on protein-coding nuclear genes have challenged the classical, morphology-based concept of squamate relationships, requiring new classifications, and drawing new evolutionary and biogeographic hypotheses. Even the key and long-held concept of a dichotomy between iguanians (～1470 sp.) and scleroglossans (all other squamates) has been refuted because molecular trees place iguanians in a highly nested position. Together with snakes and anguimorphs, iguanians form a clade – Toxicofera – characterized by the presence of toxin secreting oral glands and demonstrating a single early origin of venom in squamates. Consequently, neither the varanid lizards nor burrowing lineages such as amphisbaenians or dibamid lizards are the closest relative of snakes. The squamate timetree shows that most major groups diversified in the Jurassic and Cretaceous, 200–66 million years (Myr) ago. In contrast, five of the six families of amphisbaenians arose during the early Cenozoic, ～60–40 Myr ago, and oceanic dispersal on floating islands apparently played a significant role in their distribution on both sides of the Atlantic Ocean. Among snakes, molecular data support the basic division between the small fossorial scolecophidians (～370 sp.) and the alethinophidians (all other snakes, ～2700 sp.). They show that the alethinophidians were primitively macrostomatan and that this condition was secondarily lost by burrowing lineages. The diversification of alethinophidians resulted from a mid-Cretaceous vicariant event, the separation of South America from Africa, giving rise to Amerophidia (aniliids and tropidophiids) and Afrophidia (all other alethinophidians). Finally, molecular phylogenies have made it possible to draw a detailed evolutionary history of venom among advanced snakes (Caenophidia), a key functional innovation underlying their radiation (～2500 sp.). To cite this article: N. Vidal, S.B. Hedges, C. R. Biologies 332 (2009).     

The origin of modern crocodyliforms: new evidence from the Cretaceous of Australia

While the crocodyliform lineage extends back over 200 million years (Myr) to the Late Triassic, modern forms-members of Eusuchia-do not appear until the Cretaceous. Eusuchia includes the crown group Crocodylia, which comprises Crocodyloidea, Alligatoroidea and Gavialoidea. Fossils of non-crocodylian eusuchians are currently rare and, in most instances, fragmentary. Consequently, the transition from Neosuchia to Crocodylia has been one of the most poorly understood areas of crocodyliform evolution. Here we describe a new crocodyliform from the mid-Cretaceous (98-95 Myr ago; Albian-Cenomanian) Winton Formation of Queensland, Australia, as the most primitive member of Eusuchia. The anatomical changes associated with the emergence of this taxon indicate a pivotal shift in the feeding and locomotor behaviour of crocodyliforms-a shift that may be linked to the subsequent rapid diversification of Eusuchia 20 Myr later during the Late Cretaceous and Early Tertiary. While Laurasia (in particular North America) is the most likely ancestral area for Crocodylia, the biogeographic events associated with the origin of Eusuchia are more complex. Although the fossil evidence is limited, it now seems likely that at least part of the early history of Eusuchia transpired in Gondwana.     

Molecular and fossil evidence place the origin of cichlid fishes long after Gondwanan rifting

Cichlid fishes are a key model system in the study of adaptive radiation, speciation and evolutionary developmental biology. More than 1600 cichlid species inhabit freshwater and marginal marine environments across several southern landmasses. This distributional pattern, combined with parallels between cichlid phylogeny and sequences of Mesozoic continental rifting, has led to the widely accepted hypothesis that cichlids are an ancient group whose major biogeographic patterns arose from Gondwanan vicariance. Although the Early Cretaceous (ca 135 Ma) divergence of living cichlids demanded by the vicariance model now represents a key calibration for teleost molecular clocks, this putative split pre-dates the oldest cichlid fossils by nearly 90 Myr. Here, we provide independent palaeontological and relaxed-molecular-clock estimates for the time of cichlid origin that collectively reject the antiquity of the group required by the Gondwanan vicariance scenario. The distribution of cichlid fossil horizons, the age of stratigraphically consistent outgroup lineages to cichlids and relaxed-clock analysis of a DNA sequence dataset consisting of 10 nuclear genes all deliver overlapping estimates for crown cichlid origin centred on the Palaeocene (ca 65–57 Ma), substantially post-dating the tectonic fragmentation of Gondwana. Our results provide a revised macroevolutionary time scale for cichlids, imply a role for dispersal in generating the observed geographical distribution of this important model clade and add to a growing debate that questions the dominance of the vicariance paradigm of historical biogeography.     

Historical contingency and ecological determinism interact to prime speciation in sticklebacks, Gasterosteus

Historical contingency and determinism are often cast as opposing paradigms under which evolutionary diversification operates. It may be, however, that both factors act together to promote evolutionary divergence, although there are few examples of such interaction in nature. We tested phylogenetic predictions of an explicit historical model of divergence (double invasions of freshwater by marine ancestors) in sympatric species of three-spined sticklebacks (Gasterosteus aculeatus) where determinism has been implicated as an important factor driving evolutionary novelty. Microsatellite DNA variation at six loci revealed relatively low genetic variation in freshwater populations, supporting the hypothesis that they were derived by colonization of freshwater by more diverse marine ancestors. Phylogenetic and genetic distance analyses suggested that pairs of sympatric species have evolved multiple times, further implicating determinism as a factor in speciation. Our data also supported predictions based on the hypothesis that the evolution of sympatric species was contingent upon 'double invasions' of postglacial lakes by ancestral marine sticklebacks. Sympatric sticklebacks, therefore, provide an example of adaptive radiation by determinism contingent upon historical conditions promoting unique ecological interactions, and illustrate how contingency and determinism may interact to generate geographical variation in species diversity     

Hidden mitochondrial DNA divergence in the Lake Biwa endemic goby Gymnogobius isaza: implications for its evolutionary history

The gobiid Gymnogobius isaza is an endemic species that has adapted remarkably to the pelagic environment in Lake Biwa, Japan, a representative ancient lake in East Asia. To obtain clues that would reveal the origin and evolution of this species, we conducted phylogenetic and population genetic analyses based on partial sequences of the mitochondrial cytochrome b gene. Consistent with previous studies, our Bayesian phylogenetic analysis with a relaxed molecular clock model reconfirmed a sister relationship between G. isaza and Gymnogobius urotaenia?+?Gymnogobius petschiliensis, with a divergence time of about 2.9 million years (Myr), and provided an evolutionary rate of 3.0 %/Myr (pairwise) for their clade. Population genetic analysis revealed two distinct mtDNA groups in G. isaza, which were estimated to have diverged 0.66 million years ago (Mya) at the Lake Katata stage of Paleo-Lake Biwa, preceding the origin of the present Lake Biwa environment with its extensive deep pelagic area (0.3–0.4 Mya). A dumbbell-like haplotype network and bimodal mismatch distribution suggested that the population of G. isaza experienced secondary contact between two genetically differentiated populations. Demographic parameters from mismatch distribution analysis and Bayesian skyline plot analysis suggested that both of the mtDNA groups of G. isaza exhibited a signal of sudden population expansion at approximately the same time (80–90 thousand years ago) during the last glacial period after the development of the present Lake Biwa. These results imply the complex population history of G. isaza, including genetic isolation and secondary contact following differentiation from its relatives in the Pliocene.     

Exon-intron structure of the <Emphasis Type="Italic">Xist</Emphasis> gene in elephant,armadillo, and the ancestor of placental mammals

The Xist gene belongs to the class of long noncoding regulatory RNA genes which play a key role in the process of inactivation of one of the X chromosomes in females of placental mammals. Based on inter-specific comparative sequence analysis performed using a set of bioinformatic programs and approaches, the exon-intron gene structure was first described in two species, elephant and armadillo, belonging to the most primitive placental mammal groups, Afrotheria and Xenarthra. Using multiple sequence alignment of the species representing all main groups of placental mammals (12 species), consensus sequence of the ancestral gene was reconstructed. In the gene structure four evolutionary conserved regions with the identity level of 90% and the sizes of more than 100 bp were identified. Substantial contribution of transposable elements to the gene origin, as well as mosaic evolution of certain elements of the Xist locus was demonstrated. It is likely that the ancestral gene consisted of ten exons and was formed before the radiation of placental mammals, in the period from 140 to 105 Myr ago.     

New Guinea highland origin of a widespread arthropod supertramp

The biologically and geologically extremely diverse archipelagos of Wallacea, Australasia and Oceania have long stimulated ecologists and evolutionary biologists. Yet, few molecular phylogenetic analyses of the terrestrial fauna have been carried out to understand the evolutionary patterns. We use dense taxon and character sampling of more than 7000 bp DNA sequence data for a group of diving beetles ranging from the Holarctic throughout Asia to as far east as French Polynesia. We here show that an ecologically diverse, common and widespread (Portugal to New Zealand) arthropod supertramp species originated in the highlands of New Guinea, ca 6.0–2.7 Myr ago. The approximately 25 closely related species are narrow endemics in Australasia/Oceania. The ancestor of this clade colonized that region from Eurasia ca 9–7 Myr ago. Our finding contradicts the widely held view of local endemism as an evolutionary dead end, as we find multiple peripatric speciation events within the Pleistocene and complex colonization patterns between the Oriental and Australian zoogeographic regions, including the recolonization of Eurasia, jumping across Wallace''s line and colonization of continental Australia out of New Guinea. Our study strongly highlights the importance of dispersal over water gaps in shaping biogeographic patterns.     

Did dinosaurs invent flowers? Dinosaur—angiosperm coevolution revisited

Angiosperms first appeared in northern Gondwana during the Early Cretaceous, approximately 135 million years ago. Several authors have hypothesised that the origin of angiosperms, and the tempo and pattern of their subsequent radiation, was mediated by changes in the browsing behaviour of large herbivorous dinosaurs (sauropods and ornithischians). Moreover, the taxonomic and ecological radiation of angiosperms has been associated with the evolution of complex jaw mechanisms among ornithischian dinosaurs. Here, we review critically the evidence for dinosaur-angiosperm interactions during the Cretaceous Period, providing explicit spatiotemporal comparisons between evolutionary and palaeoecological events in both the dinosaur and angiosperm fossil records and an assessment of the direct and indirect evidence for dinosaur diets. We conclude that there are no strong spatiotemporal correlations in support of the hypothesis that dinosaurs were causative agents in the origin of angiosperms; however, dinosaur-angiosperm interactions in the Late Cretaceous may have resulted in some coevolutionary interactions, although direct evidence of such interactions is scanty at present. It is likely that other animal groups (insects, arboreal mammals) had a greater impact on angiosperm diversity during the Cretaceous than herbivorous dinosaurs. Elevated levels of atmospheric CO2 might have played a critical role in the initial stages of the angiosperm radiation.     

Historical contingency and behavioural divergence in territorial Anolis lizards

The extent that evolution - including adaptation - is historically contingent (dependent on past events) has often been hotly debated, but is still poorly understood. In particular, there are little data on the degree that behaviour, an aspect of the phenotype that is strongly linked to contemporary environments (social or physical), retains the imprint of evolutionary history. In this study, I examined whether differences in the design of the territorial displays among species of Caribbean Anolis lizards reflect island-specific selection regimes, or historically contingent predispositions associated with different clade histories. Adult males advertise territory ownership using a series of headbobs and dewlap extensions, bouts of which vary in duration among species. When display durations were mapped onto the Anolis phylogeny, prominent differences between species belonging to the Western and Eastern Caribbean radiations were apparent. Statistical analyses confirmed that species differences in the duration of headbob displays, and to some extent the duration of dewlap extensions, were historically contingent. The unique evolutionary histories of each clade have seemingly had a profound effect on the subsequent direction of display evolution among descendent taxa. These results combined with those from previous studies on these lizards show that past history can have an important impact on the type of behaviour exhibited by species today, to the point that adaptive evolution can proceed quite differently in lineages originating from different evolutionary starting points.     

Evolution of the angiosperms: calibrating the family tree.

Growing evidence of morphological diversity in angiosperm flowers, seeds and pollen from the mid Cretaceous and the presence of derived lineages from increasingly older geological deposits both imply that the timing of early angiosperm cladogenesis is older than fossil-based estimates have indicated. An alternative to fossils for calibrating the phylogeny comes from divergence in DNA sequence data. Here, angiosperm divergence times are estimated using non-parametric rate smoothing and a three-gene dataset covering ca. 75% of all angiosperm families recognized in recent classifications. The results provide an initial hypothesis of angiosperm diversification times. Using an internal calibration point, an independent evaluation of angiosperm and eudicot origins is performed. The origin of the crown group of extant angiosperms is indicated to be Early to Middle Jurassic (179-158 Myr), and the origin of eudicots is resolved as Late Jurassic to mid Cretaceous (147-131 Myr). Both estimates, despite a conservative calibration point, are older than current fossil-based estimates.     

Microevolutionary dynamics of a macroevolutionary key innovation in a Lepidopteran herbivore

Background  

A molecular population genetics understanding is central to the study of ecological and evolutionary functional genomics. Population genetics identifies genetic variation and its distribution within and among populations, it reveals the demographic history of the populations studied, and can provide indirect insights into historical selection dynamics. Here we use this approach to examine the demographic and selective dynamics acting of a candidate gene involved in plant-insect interactions. Previous work documents the macroevolutionary and historical ecological importance of the nitrile-specifier protein (Nsp), which facilitated the host shift of Pieridae butterflies onto Brassicales host plants ~80 Myr ago.     

BOOM AND BUST: ANCIENT AND RECENT DIVERSIFICATION IN BICHIRS (POLYPTERIDAE: ACTINOPTERYGII), A RELICTUAL LINEAGE OF RAY‐FINNED FISHES

Understanding the history that underlies patterns of species richness across the Tree of Life requires an investigation of the mechanisms that not only generate young species‐rich clades, but also those that maintain species‐poor lineages over long stretches of evolutionary time. However, diversification dynamics that underlie ancient species‐poor lineages are often hidden due to a lack of fossil evidence. Using information from the fossil record and time calibrated molecular phylogenies, we investigate the history of lineage diversification in Polypteridae, which is the sister lineage of all other ray‐finned fishes (Actinopterygii). Despite originating at least 390 million years (Myr) ago, molecular timetrees support a Neogene origin for the living polypterid species. Our analyses demonstrate polypterids are exceptionally species depauperate with a stem lineage duration that exceeds 380 million years (Ma) and is significantly longer than the stem lineage durations observed in other ray‐finned fish lineages. Analyses of the fossil record show an early Late Cretaceous (100.5–83.6 Ma) peak in polypterid genus richness, followed by 60 Ma of low richness. The Neogene species radiation and evidence for high‐diversity intervals in the geological past suggest a “boom and bust” pattern of diversification that contrasts with common perceptions of relative evolutionary stasis in so‐called “living fossils.”     

Molecular evidence for the early divergence of placental mammals

Paleontological and molecular data suggest quite different patterns for the early evolution of placental mammals. Paleontological evidence indicates a radiation, with most of the extant orders diverging at approximately the same time, close to the Cretaceous-Tertiary boundary, 65 Myr ago. Molecular evidence suggests a branching pattern of evolution that started much earlier. Resolving this discrepancy requires a consideration of the assumptions that underlie both approaches. It is argued here that the pattern indicated by the molecular approach is the most likely to be correct. If it is correct then either: 1) A diversity of placental mammals remains to be sampled from the Cretaceous, or 2) The placental orders diverged phylogenetically long before they diversified morphologically, implying a decoupling of the evolutionary processes associated with speciation and adaptation. The adaptive diversification of placental mammals may have required the demise of the dinosaurs at the end of the Cretaceous, but it occurred in lineages that had a long prior history of independent existence. 1999.     

A multi-locus analysis of phylogenetic relationships within cheilostome bryozoans supports multiple origins of ascophoran frontal shields

Phylogenetic relationships within the bryozoan order Cheilostomata are currently uncertain, with many morphological hypotheses proposed but scarcely tested by independent means of molecular analysis. This research uses DNA sequence data across five loci of both mitochondrial and nuclear origin from 91 species of cheilostome Bryozoa (34 species newly sequenced). This vastly improved the taxonomic coverage and number of loci used in a molecular analysis of this order and allowed a more in-depth look into the evolutionary history of Cheilostomata. Maximum likelihood and Bayesian analyses of individual loci were carried out along with a partitioned multi-locus approach, plus a range of topology tests based on morphological hypotheses. Together, these provide a comprehensive set of phylogenetic analyses of the order Cheilostomata. From these results inferences are made about the evolutionary history of this order and proposed morphological hypotheses are discussed in light of the independent evidence gained from the molecular data.Infraorder Ascophorina was demonstrated to be non-monophyletic, and there appears to be multiple origins of the ascus and associated structures involved in lophophore extension. This was further supported by the lack of monophyly within each of the four ascophoran grades (acanthostegomorph/spinocystal, hippothoomorph/gymnocystal, umbonulomorph/umbonuloid, lepraliomorph/lepralioid) defined by frontal-shield morphology. Chorizopora, currently classified in the ascophoran grade Hippothoomorpha, is phylogenetically distinct from Hippothoidae, providing strong evidence for multiple origins of the gymnocystal frontal shield type. Further evidence is produced to support the morphological hypothesis of multiple umbonuloid origins of lepralioid frontal shields, using a step-wise set of topological hypothesis tests combined with examination of multi-locus phylogenies.     

Nonequilibrium thermodynamics and different axioms of evolution

Proponents of two axioms of biological evolutionary theory have attempted to find justification by reference to nonequilibrium thermodynamics. One states that biological systems and their evolutionary diversification are physically improbable states and transitions, resulting from a selective process; the other asserts that there is an historically constrained inherent directionality in evolutionary dynamics, independent of natural selection, which exerts a self-organizing influence. The first, the Axiom of Improbability, is shown to be nonhistorical and thus, for a theory of change through time, acausal. Its perception of the improbability of living states is at least partially an artifact of closed system thinking. The second, the Axiom of Historically Determined Inherent Directionality, is supported evidentially and has an explicit historical component. Historically constrained dynamic populations are inherently nonequilibrium systems. It is argued that living, evolving systems, when considered to be historically constrained nonequilibrium systems, do not appear improbable at all. Thus, the two axioms are not compatible. Instead, the Axiom of Improbability is considered to result from an unjustified attempt to extend the contingent proximal actions of natural selection into the area of historical, causal explanations. It is thus denied axiomatic status, and the effects of natural selection are subsumed as an additional level of constraint in an evolutionary theory derived from the Axiom of Historically Determined Inherent Directionality.     

Cell evolution and Earth history: stasis and revolution

This synthesis has three main parts. The first discusses the overall tree of life and nature of the last common ancestor (cenancestor). I emphasize key steps in cellular evolution important for ordering and timing the major evolutionary innovations in the history of the biosphere, explaining especially the origins of the eukaryote cell and of bacterial flagella and cell envelope novelties. Second, I map the tree onto the fossil record and discuss dates of key events and their biogeochemical impact. Finally, I present a broad synthesis, discussing evidence for a three-phase history of life. The first phase began perhaps ca 3.5 Gyr ago, when the origin of cells and anoxic photosynthesis generated the arguably most primitive prokaryote phylum, Chlorobacteria (= Chloroflexi), the first negibacteria with cells bounded by two acyl ester phospholipid membranes. After this 'chlorobacterial age' of benthic anaerobic evolution protected from UV radiation by mineral grains, two momentous quantum evolutionary episodes of cellular innovation and microbial radiation dramatically transformed the Earth's surface: the glycobacterial revolution initiated an oxygenic 'age of cyanobacteria' and, as the ozone layer grew, the rise of plankton; immensely later, probably as recently as ca 0.9 Gyr ago, the neomuran revolution ushered in the 'age of eukaryotes', Archaebacteria (arguably the youngest bacterial phylum), and morphological complexity. Diversification of glycobacteria ca 2.8 Gyr ago, predominantly inhabiting stratified benthic mats, I suggest caused serial depletion of 13C by ribulose 1,5-bis-phosphate caboxylase/oxygenase (Rubisco) to yield ultralight late Archaean organic carbon formerly attributed to methanogenesis plus methanotrophy. The late origin of archaebacterial methanogenesis ca 720 Myr ago perhaps triggered snowball Earth episodes by slight global warming increasing weathering and reducing CO2 levels, to yield runaway cooling; the origin of anaerobic methane oxidation ca 570 Myr ago reduced methane flux at source, stabilizing Phanerozoic climates. I argue that the major cellular innovations exhibit a pattern of quantum evolution followed by very rapid radiation and then substantial stasis, as described by Simpson. They yielded organisms that are a mosaic of extremely conservative and radically novel features, as characterized by De Beer's phrase 'mosaic evolution'. Evolution is not evenly paced and there are no real molecular clocks.