Periods Of Stasis Research Articles

The evolution of the G matrix: selection or drift?

The evolution of quantitative characters can be described by the equation Deltaz=GP-1S where Deltaz is the vector of mean responses, G is the matrix of additive genetic variances and covariances, P is the matrix of phenotypic variances and covariances and S is the vector of selection differentials. This equation can be used to predict changes in trait values or to retrospectively estimate the selection gradient and is thus a central equation of evolutionary quantitative genetics. Genetic variances and covariances will be reduced by stabilizing selection, directional selection and drift, and increased by mutation. Changes in trait values resulting from directional selection that are comparable with differences observed among species are readily obtainable in short geological time spans (<5000 generations) with selection intensities so small that they would have an insignificant effect on the G matrix (of course it is possible that such changes came about by strong selection over a few generations, followed by long periods of stasis; there is insufficient evidence to presently distinguish these two possibilities). On the other hand, observed effective population sizes are sufficiently small that considerable changes in G can be expected from drift alone. The action of drift can be distinguished from selection because the former produces a proportional change in G whereas the latter, in general, will not. A survey of studies examining variation in G suggests that the null hypothesis that most of the variation can be attributed to drift rather than selection cannot be rejected. However, more research on the predicted statistical distribution of G as a result of selection and/or drift is required and further development of statistical tests to distinguish these two forces needs to be made.

Self-organized criticality and partial synchronization in an evolving network

We describe an evolving network type model of stock market, and present numerical results. Recent works on Self-organized criticality in pulse coupled relaxation oscillators have shown SOC to be related to frustrated attempts of the system to synchronise. Other works have shown the emergence of a self-organized control parameter which feeds back onto an order parameter, causing avalanches which separate periods of stasis. We define an all-to-all network of connected spins which also have an attached fitness. We find a self-organized fitness threshold given by the mean fitness, which separates a solid type state from a gaseous type state. We also find a self-organized control parameter given by the fitness deviation. Approach of this parameter to zero causes `avalanches' of partial synchronizations which occur on all sizes, separated by periods of stasis, and re-order the system.

Lactoferrin secretion into milk: Comparison between ruminant, murine, and human milk

Lactoferrin is an iron-binding protein found in external secretions such as tears, saliva, and milk and the secondary granules of granulocytes. As part of the innate nonspecific immune system it seems to play a wide variety of roles in infection and inflammation. The functions of milk lactoferrin have been postulated to include iron transfer to the neonate, iron scavenging from the neonatal gut, stimulation of intestinal growth, and defense against infection. The concentration of lactoferrin in milk varies widely from one species to another. It is highest in human milk (2 g/L, 25 × 10-6M), moderate in murine milk (.28 mg/L, 3.5 × 10-6M), and very low in ruminant milk (-.01 mg/L in bovine milk,.12 × 10-6M). Unlike other milk proteins, lactoferrin has been found at high levels in the prepartum secretions of the cow and human and at very high levels in postweaning mammary secretions from the cow, sheep, guinea pig, mouse, and human. It is also elevated in mastitic mammary glands. Data accumulated over the past three decades suggest that the protein may play an important role in defending the mammary gland against infection, particularly during periods of milk stasis.

How environmental factors regulate mutagenesis and gene transfer in microorganisms

This review is focused on the physiological and evolutionary strategies of the processes occurring during the entry of microbial cells into stationary phase and the subsequent period of stasis. The molecular mechanisms adapting microorganisms from exponential growth to a static state involve activation and complex regulation of the stationary factor Sigma-S, which directs RNA polymerase to the specific promoters. As a result the static cells acquire general resistance (simultaneous tolerances) to different environmental stresses. In parallel with the physiological adaptation to stasis, diverse genetical processes are aimed towards resuming the growth of the static cells. Different types of mutagenesis occur: (i) in cells entering stasis and (ii) in static cells (adaptive mutagenesis). Cessation of growth induces the transient hypermutator state resulting in the accumulation of random mutations in the subpopulation of the static cells. If by chance, one or a few of such mutations lead to resumption of division, the growing cell will return to a normal mechanism of spontaneous mutagenesis. Another mechanism for generating genetical variability in stressed cells involves transposons and conjugative plasmids. Stresses can stimulate the excision of some transposons, which, in turn, can generate chromosomal mutations and activate intracellular mechanisms of mutagenesis. Under stress some conjugative plasmids activate genes encoding antirestriction proteins that repress restriction-modification systems of the recipient cells. Moreover, under stress special cellular mechanisms decrease (alleviate) the activity of restriction-modification systems which, in turn, enhance the probability of gene transfer into the stressed cells. Under stress, the efficiency of inter-species genetical barriers also decreases. This, stimulates inter-species gene transfer and may lead to a burst of incipient speciation in the population of non-growing cells. After resumption of growth the genetical barriers leading to isolation will be restored. In general, the cessation of growth “switches on”, and resumption of growth “switches off”, a set of special processes that are responsible for generating bursts of genetical variability in populations of microorganisms.

Statistical Dynamics of the Royal Road Genetic Algorithm

Metastability is a common phenomenon. Many evolutionary processes, both natural and artificial, alternate between periods of stasis and brief periods of rapid change in their behavior. In this paper an analytical model for the dynamics of a mutation-only genetic algorithm (GA) is introduced that identifies a new and general mechanism causing metastability in evolutionary dynamics. The GA's population dynamics is described in terms of flows in the space of fitness distributions. The trajectories through fitness distribution space are derived in closed form in the limit of infinite populations. We then show how finite populations induce metastability, even in regions where fitness does not exhibit a local optimum. In particular, the model predicts the occurrence of “fitness epochs” — periods of stasis in population fitness distributions — at finite population size and identifies the locations of these fitness epochs with the flow's hyperbolic fixed points. This enables exact predictions of the metastable fitness distributions during the fitness epochs, as well as giving insight into the nature of the periods of stasis and the innovations between them. All these results are obtained as closed-form expressions in terms of the GA's parameters. An analysis of the Jacobian matrices in the neighborhood of an epoch's metastable fitness distribution allows for the calculation of its stable and unstable manifold dimensions and so reveals the state space's topological structure. More general quantitative features of the dynamics — fitness fluctuation amplitudes, epoch stability, and speed of the innovations — are also determined from the Jacobian eigenvalues. The analysis shows how quantitative predictions for a range of dynamical behaviors, that are specific to the finite population dynamics, can be derived from the solution of the infinite population dynamics. The theoretical predictions are shown to agree very well with statistics from GA simulations. We also discuss the connections of our results with those from population genetics and molecular evolution theory.

Stage transitions and the transtheoretical “stages of change” model of smoking cessation 1Joseph P. Carbonari, Ed.D., Kenneth B. Sewell, M.S., M.A. are in the Psychology Department at the University of Houston. Carlo C. DiClemente, Ph.D., is Chair of the Psychology Department at the University of Maryland, Baltimore County. This research was partially supported by Grant CA 27821 from the National

This study examined the application of a Markov chain analysis for modeling transitions in the process of smoking cessation from the perspective of the Transtheoretical “Stages of Change” model. A convenience sample of 308 individuals, who were followed at 6-month intervals for a 30-month period, answered questionnaires regarding their past and current smoking behavior, and their quitting attempts and intentions. A first order, single transition, Markov model was found to fit the data well, indicating that stage information from a single previous 6 month time point was sufficient to predict current stage membership. A composite transition probability matrix indicated that significant stability occurred in Precontemplation, Contemplation, and Maintenance stages with greater transitions among stages for smokers in Preparation and Action stages. In addition, more people tended to progress in stage status toward successfully maintained cessation than to either regress or make no change in stage membership over the 2 1/2 years of the study. These results support the utility of stage based models of change in understanding the process of smoking cessation. Smoking cessation is marked by significant periods of stasis as well as recycling through the stages. For a significant number of this cohort of participants there was progress and movement toward successful change that seemed to support the spiral movement through the stages of change proposed by the Transtheoretical “stages of change” model.

Coevolution of the Earth’s environment and life: Goldilocks, Gaia and the anthropic principle

Abstract Observation suggests that the Earth’s surface environment is maintained by processes in which non-living and living causes are linked inextricably. Once established on Earth, life rapidly became a dominant influence on the evolution of the planetary environment. But life was also shaped by that evolution, constrained and directed by the physical and chemical processes that moulded the planet’s surface. Life and the planetary environment form a closely coupled entity, a view of the Earth as a complex system which is prefigured in the writings of James Hutton. Hutton compared the workings of the Earth’s surface to the body of an animal, being both wasted and repaired continually. In modern times, James Lovelock has argued that it is a property of this system that it acts to maintain the planet in a habitable condition. Alternatively, perhaps it is pure chance that the planet has always remained hospitable for life — it could just as easily have followed an infinity of different evolutionary paths, many and perhaps most of which would lead rapidly to global extinction. Consideration of the fates of our near-neighbour planets, Mars and Venus, and the dangerous nature of the inner solar system, leads to the conclusion that there is indeed a substantial element of luck involved in the Earth’s biosphere having survived as long as it has. The fact that our own existence is dependent on it having survived makes it nearly impossible to accurately assess a priori the probability of survival. Abstract models such as ‘Daisyworld’ can capture some of the complex behaviour of the Earth-life system. This may include periods of stasis and sudden changes to new states, the stasis being an example of regulatory behaviour where the system is dominated by negative feedback, and the sudden changes being essentially the opposite — brief but traumatic periods where the dominant feedbacks are positive. GEOCARB, a biogeochemical model for the Phanerozoic which links changes in the long-term carbon cycle to planetary temperature, shows examples of both regulatory and destabilizing behaviour in a less abstract system, and suggests that such responses have indeed characterized Earth history. I conclude that the properties of the Earth-life system are complex and not easily predictable. The longevity of Hutton’s animal (nearly 4 Ga) is no guide to its future life expectancy, and even if the system as a whole lasts many aeons into the future, any given species (such as humans) is most unlikely to.

Persistence and intermittency in sliding of blocks

An extensive Hurst analysis to quantify long-term statistical dependence is performed on time series associated with sliding of blocks on an inclined plane induced by small perturbations. The analysis reveals the existence of a fluctuation phenomenon exhibiting two phases, depending on the angle of inclination, both obeying a Hurst scaling . The first (second) phase has H = 0.70 (0.50) and refers to large (small) inclinations irrespective of the material of the block. These results indicate that sliding of blocks in the first regime exhibits persistence and indicate that there are long-term memory effects on the storage of shear stress at the block-plane interface. In the second regime, the long-term memory effects disappear and the sliding activity is intermittent; that is, it is distributed in bursts separated by variable periods of stasis.

Multi-trichomous cyanobacterial microfossils from the Mesoproterozoic Gaoyuzhuang Formation, China: paleoecological and taxonomic implications.

Populations of the multi-trichomous microbial fossil Eoschizothrix composita n.gen. et sp. are preserved in growth position in silicified stratiform stromatolites of the Gaoyuzhuang Formation, Hebei Province, northern China. The microbial fossils consist predominantly of preserved sheaths, although several specimens retain shriveled remains of trichomes within sheaths. Comparisons with modern morphological counterparts, including shape, growth habit and orientation, degradational sequences, and habitat, support the interpretation of the multi-trichomous microfossils as cyanobacteria, which acted as frame-builders of ancient stromatolites. The distribution and orientation of multi-trichomous microfossils within a synsedimentary context reveal their behavioral responses to sedimentation regime. Horizontally spread, interwoven mats formed during periods of sedimentary stasis. During periods of rapid sediment influx, the filaments assumed an upright orientation, possibly to avoid accumulating particles. This is the first record of fossil stromatolite-building multi-trichomous cyanobacterial which underscores early morphological and functional diversification in cyanobacterial evolution.

Effects of Demographic Change on Group Kin Structure and Gene Dynamics of Populations of Red Howling Monkeys

Four populations of red howling monkeys (Alouatta seniculus) in different states of pop­ ulation growth were studied to examine how changes in intrasexual competition and kin availability influence mating patterns and population genetic structure. The demography of each population was monitored for 5 years. Genetic variance within and between groups and coefficients of relatedness (r) were estimated from allozyme loci. Advantages of co­ operating with kin over non-kin changed with different demographic circumstances for each sex, such that social groups were more likely to be kin groups when population density was approaching or at carrying capacity than when it was low. In populations growing rapidly through new social group formation, genetic differentiation between groups was low. Genetic differentiation increased as populations approached carrying capacity and groups matured socially (i.e., r increased). That trend was reversed as r within group breed­ ing subsets (males and females, respectively) increased, thereby decreasing r between males and females and reducing group r. This was enhanced when inbreeding within groups was followed by transfer of pods of related males, which occurred in a saturated habitat and in a declining population experiencing high mortality of adult females. Heterozygosity within groups in excess of predicted values was increased due to lineage effects as groups matured and as r within breeding subsets increased. It also increased when population growth oc­ curred through immigration from genetically differentiated demes, and when groups fused during population decline. There were large intergroup differences in mortality during pop­ ulation decline, which was accompanied by social instability that resulted in increased intergroup transfer of both sexes, thereby resorting survivors throughout remaining breeding groups. Conditions favoring intergroup selection thus decreased during periods of popula­ tion growth when selection was relaxed and group r was low, increased during periods of stasis when habitats were saturated and group r was highest, and were most prominent during periods of popUlation decline when differences in group fitness should have the most pronounced consequences.

Biometrical trends in the Tetraconodontinae, a subfamily of pigs

AbstractThe Tetraconodontinae Lydekker, 1876 are a subfamily of pigs (Suidae) with a great variety of dental adaptations, such as greatly enlarged or reduced premolars, enlarged posterior molars, hypsodonty, enlarged incisors etc. The essence of most of these adaptations can be described very well by simple biometry. Size increase of the last molars has proved to be of importance in African biostratigraphy. However, it appears that members of different lineages have been mixed up.Tendencies in relative premolar size indicate that the 21 species known can be grouped in three tribes: (1) the Tetraconodontini, with very large or even extremely enlarged premolars and, in one of the lineages, a tendency to enlarge the posterior molars; (2) the Nyanzachoerini new tribe, characterised by moderately enlarged or even reduced premolars and with a tendency to increase the length of the posterior molars by the addition of distal cusps; and (3) the Parachleuastochoerini new tribe, with relatively long but narrower premolars. Both Nyanzachoerini and Tetraconodontini include species that were placed previously inConohyusandNyanzachoerus.A very long gradually evolving lineage in the Nyanzachoerini, spanning 13 Ma, is documented. In the last 5 Ma of this lineage, M3 length increased at such a rate that a (sub)species can be recognised about every 0·7 Ma. Slightly lesser rates of evolution are found in some other lineages, but still others experienced apparently long periods of stasis or very slow evolution. In a single case there is a period of stasis combined with a sudden evolutionary jump as in the punctuated equilibria model.In the cases of intercontinental dispersal events combined with cladogenesis, where the ancestor in the continent of origin could be identified, the cause of cladogenesis seems to have been geographical isolation. Two tetraconodont lineages were restricted to Eurasia for about 5 Ma. During the Late Miocene, members of these lineages dispersed repeatedly to Africa. The Vallesian is a period of profound climatic changes which are reflected in changes in biogeography of many taxa. It is assumed that these and later climatic changes enabled the dispersal of Tetraconodontinae to Africa.

James B. Hartman. The Organ in Manitoba: A History of the Instruments, the Builders, and the Players. Winnipeg: University of Manitoba Press, 1997. xii, 292 pp. ISBN 0-88755-644-2 (paperback)

James B. Hartman. The Organ in Manitoba: A History of Instruments, Builders, and Players. Winnipeg: University of Manitoba Press, 1997. xii, 292 pp. ISBN 0-88755-644-2 (paperback). Unlike United States, France, United Kingdom, and other countries,1 Canada has seen only sporadic attempts to trace history of its church, concert, and theatre organs. The literature in both official languages is sparse, and becomes even more limited if one seeks a scholarly account.2 Precisely why this should be case is unclear, especially when one considers national contributions to organ building and playing. For more than a century Canada has been home to a thriving community of organ builders with a reputation for quality and innovation that extends well beyond country's borders. Though their principal markets remain North American, Canadian firms export organs as far afield as Asia and Australia in face of strong international competition. Canadian organists, too, have enjoyed recognition at home and abroad: one need only cite names of Healey Willan, Lynnwood Farnam, Bernard Lagace, and Raymond Daveluy, among others, to demonstrate a significant Canadian presence in world of organ performance. Other than invoking national reticence about cultural accomplishments, therefore, it is difficult to account for such sparse historical treatment of Canadian organs, organ builders, and organists. James Hartman' s volume addresses problem in part by supplying a thorough, well-documented account of organ in Manitoba extending from mid-nineteenth-century introduction into St. Boniface Cathedral of a reed organ to uncertainties of present day, when future of organ is, for many observers, difficult to discern. Hartman begins by surveying early manifestations of musical life in area, including singers and instrumentalists who were part of Red River Settlement, and provides a brief account of establishment of various churches and their divergent approaches to issue of music in worship. Overviews of nature and construction of reed and pipe organs are provided for non-specialist readers. With this background set out, Hartman begins his historical account by admitting that, given settlement patterns and subsequent gradual shrinkage of rural population, the history of organs in Manitoba is largely a chronicle of events in (p. 29). Organs in capital city outnumber those in smaller centres and rural areas of province by a ratio of more than four to one; all but a few significant Manitoba organs are to be found in Winnipeg. Hartman identifies period from 1875 to 1919, when more than forty per cent of province's organs were installed, as a age for instrument. Though pace of installations slowed in 1920s, decade witnessed arrival of several important instruments as well as formation of Winnipeg centre of Canadian College of Organists. The coming of Great Depression halved number of organs built for Manitoba churches, and introduced a period of relative inactivity that lasted well beyond end of World War II. Not until 1950s would Manitoba organ community experience a dramatic resurgence of activity (p. 169) during which purchase and installation of new instruments occurred at a rate close to that of pre- 1920 golden age. This mid-century boom has been followed by a period of relative stasis wherein relatively few new organs have been installed. Hartman' s account concludes with a brief chapter on future of organ. Appendices provide a list of all known Manitoba organs from 1875 to present and specifications of their tonal resources. Though his focus is on a single province, Hartman' s discussion has wider applicability. The prominence of organ in Canadian cultural life prior to Depression is perhaps difficult for us now to grasp. …

Self-organized criticality and punctuated equilibria

Many natural phenomena evolve intermittently, with periods of tranquillity interrupted by bursts of activity, rather than following a smooth gradual path. Examples include earthquakes, volcanic eruptions, solar flares, gamma-ray bursts, and biological evolution. Stephen Jay Gould and Niles Eldredge have coined the term “punctuated equilibria” for this behavior. We argue that punctuated equilibria reflects the tendency of dynamical systems to evolve towards a critical state, and review recent work on simple models. A good metaphoric picture is one where the systems are temporarily trapped in valleys of deformable, interacting landscapes. Similarities with spin glasses are pointed out. Punctuated equilibria are essential for the emergence of complex phenomena. The periods of stasis allow the system to remember its past history; yet the intermittent events permit further change.

Lack of stasis in late Cenozoic marine faunas and communities, central California

Pliocene strata in the Kettleman Hills of west-central California were deposited in the broad San Joaquin embayment as a cyclic succession of parasequences during approximately three million years. Depositional environments within each cycle ranged from relatively open marine to brackish and non-marine. Although the strata were deposited in similar, recurrent environments, the fauna changed gradually rather than during brief intervals separating periods of stasis. Although environmental gradients and community structure in the Pliocene San Joaquin Embayment and in the present-day San Francisco Bay are similar, species compositions of the faunas and of parallel communities at the two sites are markedly different. In addition, times of origination and extinction of species in the Pliocene strata of the Kettleman Hills and in San Francisco Bay do not document coordinated stasis within the Late Cenozoic. In contrast, Silurian and Devonian faunas and communities of the Appalachian Basin persisted with little change in ecological-evolutionary units that lasted for up to eight million years. Relatively brief intervals of great biotic change separate these intervals of stasis. One intriguing explanation of this pattern of coordinated stasis within an ecologic-evolutionary unit is ecological locking, in which interaction between species within the community is sufficiently strong that only major changes in the environment are able to change community and faunal composition. Probably the late Cenozoic fauna underwent rapid evolution as a result of rapidly changing environmental conditions within a complex and changing shallow, inshore marine paleogeography. In contrast, coordinated stasis in the lower Paleozoic probably resulted from negligible evolution during long periods of stable to gradually changing environments in an outer shelf setting, punctuated by brief episodes of abrupt environmental change and large-scale turnover. The independent assortment of species in late Cenozoic parallel communities indicates that ecological locking did not exist.

Finite populations induce metastability in evolutionary search

We introduce an analytical model that predicts the dynamics of a simple evolutionary algorithm in terms of the flow in the space of fitness distributions. In the limit of infinite populations the dynamics is derived in closed form. We show how finite populations induce periods of stasis - “fitness epochs” - and rapid jumps - “innovations”. The analysis identifies the epochs with the flow's metastable fixed points and gives exact predictions of epoch fitness level and population distribution.

Body mass and encephalization in Pleistocene Homo.

Many dramatic changes in morphology within the genus Homo have occurred over the past 2 million years or more, including large increases in absolute brain size and decreases in postcanine dental size and skeletal robusticity. Body mass, as the 'size' variable against which other morphological features are usually judged, has been important for assessing these changes. Yet past body mass estimates for Pleistocene Homo have varied greatly, sometimes by as much as 50% for the same individuals. Here we show that two independent methods of body-mass estimation yield concordant results when applied to Pleistocene Homo specimens. On the basis of an analysis of 163 individuals, body mass in Pleistocene Homo averaged significantly (about 10%) larger than a representative sample of living humans. Relative to body mass, brain mass in late archaic H. sapiens (Neanderthals) was slightly smaller than in early 'anatomically modern' humans, but the major increase in encephalization within Homo occurred earlier during the Middle Pleistocene (600-150 thousand years before present (kyr BP)), preceded by a long period of stasis extending through the Early Pleistocene (1,800 kyr BP).

2.5-million-year-old stone tools from Gona, Ethiopia.

The Oldowan Stone tool industry was named for 1.8-million-year-old (Myr) artefacts found near the bottom of Olduvai Gorge, Tanzania. Subsequent archaeological research in the Omo (Ethiopia) and Turkana (Kenya) also yielded stone tools dated to 2.3 Myr. Palaeoanthropological investigations in the Hadar region of the Awash Valley of Ethiopia, revealed Oldowan assemblages in the adjacent Gona River drainage. We conducted field work in the Gona study area of Ethiopia between 1992 and 1994 which resulted in additional archaeological discoveries as well as radioisotopic age control and a magnetic polarity stratigraphy of the Gona sequence. These occurrences are now securely dated between 2.6-2.5 Myr. The stone tools are thus the oldest known artefacts from anywhere in the world. The artefacts show surprisingly sophisticated control of stone fracture mechanics, equivalent to much younger Oldowan assemblages of Early Pleistocene age. This indicates an unexpectedly long period of technological stasis in the Oldowan.

Speciation patterns and trends in the fossil record

Phyletic trends in the fossil record can be explained by alternative models involving anagenesis andcladogenesis, the latter being favoured by the propounders of punctuated equilibrium theory. To render fossil material suitable for evolutionary analysis of morphospecies it is necessary to make allowance for a full range of populational variation. A number of studies suggest that fossil morphospecies are sufficiently representative of biological species to allow valid generalisation about speciation patterns. Punctuated equilibrium, with long periods of stasis interrupted by relatively brief episodes of change, appears to be the dominant mode, but the evidence for producing trends by species selection or sorting is weak. Instead, most of the trends described can be readily attributable either to a combination of anagenesis and heterochrony, or to phyletic size increase. Examples are given from Jurassic ammonites and bivalves, and an attempt made to relate key changes to an ecological model involving changing adaptive strategies related to environmental fluctuations.

Model of normal prepubertal growth.

Growth over the short term is a highly complex non-linear process. Contrasting models of short term growth have been proposed which include periodic growth cycles versus abrupt growth spurts with intervening growth arrest ('saltation and stasis'). The variability of short term growth has been characterised from a study of 46 healthy prepubertal children measured three times a week over one academic year using a combination of descriptive statistical approaches and regression modelling. Growth in childhood over one year is represented by a biphasic process comprising three to six unpredictable growth spurts, each of mean length 56 days (range 13-155 days), separated by periods of stasis (less than or equal to 0.05 cm height increment over more than seven days), each lasting a mean of 18 days (range 8-52 days) and accounting for at least 20% of the period of observation. This is superimposed on strong seasonal trends in growth with a declining growth rate over the autumn months reaching a nadir in midwinter, followed by a growth spurt in the spring. Human growth over short periods is therefore a discontinuous, irregular, and unpredictable process.

Can Llobateres: the pattern and timing of the Vallesian hominoid radiation reconsidered

Abstract New biostratigraphic data and paleomagnetic analysis of the site of Can Llobateres (Northeastern Spain) allows an age-estimation of 9.6 Ma for the recently discoveredDryopithecus laietanusfacial remains (CLl-18000) and 9.7 Ma for the earlier hominoid finds from this locality. This is an unexpectedly young age for such a primitive form, which can be accurately correlated with some of the finds of youngestSivapithecusin the Siwaliks. According to the phylogenetic model proposed here,Dryopithecusdisplayed a long period (some 5 Ma) of morphological stasis in Europe, which could be related to the preservation of subtropical forest conditions in the Western Mediterranean. In other parts of Southern Eurasia (Eastern Mediterranean and Siwaliks), quick vicarious evolution led to the diversification of thick-enamelled hominoids between 14 Ma and 12 Ma.