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At least 19 recordsLinked to original sources

PubMed1990

The fossil record and estimating divergence times between lineages: maximum divergence times and the importance of reliable phylogenies.

Bounded estimates on divergence times between lineages are crucial to the calculation of absolute rates of molecular evolution. Upper (minimum) bounds on divergence times are easily estimated based on earliest fossil finds. Lower (maximum) bounds are more difficult to estimate; the age of putative ancestors may be used, though in practice it is virtually impossible to distinguish ancestors from primitive sister groups, which do not, of logical necessity, constitute lower bounds on divergence times. Two relatively new approaches to estimating lower bounds directly assess the incompleteness of the fossil record. The first uses taphonomic control groups to distinguish real absences from nonpreservation, while the second, and probably more powerful, uses the quality of the fossil record to estimate confidence intervals on the bases of stratigraphic ranges. For some groups, especially vertebrates, the inclusion or exclusion of problematic fossils can dramatically affect estimated lower bounds on divergence times, often swamping the uncertainties due to the incompleteness of the fossil record and/or correlation and dating errors. When datable paleogeographic events reflect ancient divisions of faunas, a lower bound on the divergence time of species within a fauna can be established based on the geologic, rather than fossil, record. The fossil records of hominids, eutherian mammals, echinoids, and geese are used as examples.

Animals↗

PubMed2003

Estimation of divergence times for major lineages of primate species.

Although the phylogenetic relationships of major lineages of primate species are relatively well established, the times of divergence of these lineages as estimated by molecular data are still controversial. This controversy has been generated in part because different authors have used different types of molecular data, different statistical methods, and different calibration points. We have therefore examined the effects of these factors on the estimates of divergence times and reached the following conclusions: (1) It is advisable to concatenate many gene sequences and use a multigene gamma distance for estimating divergence times rather than using the individual gene approach. (2) When sequence data from many nuclear genes are available, protein sequences appear to give more robust estimates than DNA sequences. (3) Nuclear proteins are generally more suitable than mitochondrial proteins for time estimation. (4) It is important first to construct a phylogenetic tree for a group of species using some outgroups and then estimate the branch lengths. (5) It appears to be better to use a few reliable calibration points rather than many unreliable ones. Considering all these factors and using two calibration points, we estimated that the human lineage diverged from the chimpanzee, gorilla, orangutan, Old World monkey, and New World monkey lineages approximately 6 MYA (with a range of 5-7), 7 MYA (range, 6-8), 13 MYA (range, 12-15), 23 MYA (range, 21-25), and 33 MYA (range 32-36).

Animals↗

PubMed2001

Estimation of divergence times from multiprotein sequences for a few mammalian species and several distantly related organisms.

When many protein sequences are available for estimating the time of divergence between two species, it is customary to estimate the time for each protein separately and then use the average for all proteins as the final estimate. However, it can be shown that this estimate generally has an upward bias, and that an unbiased estimate is obtained by using distances based on concatenated sequences. We have shown that two concatenation-based distances, i.e., average gamma distance weighted with sequence length (d(2)) and multiprotein gamma distance (d(3)), generally give more satisfactory results than other concatenation-based distances. Using these two distance measures for 104 protein sequences, we estimated the time of divergence between mice and rats to be approximately 33 million years ago. Similarly, the time of divergence between humans and rodents was estimated to be approximately 96 million years ago. We also investigated the dependency of time estimates on statistical methods and various assumptions made by using sequence data from eubacteria, protists, plants, fungi, and animals. Our best estimates of the times of divergence between eubacteria and eukaryotes, between protists and other eukaryotes, and between plants, fungi, and animals were 3, 1.7, and 1.3 billion years ago, respectively. However, estimates of ancient divergence times are subject to a substantial amount of error caused by uncertainty of the molecular clock, horizontal gene transfer, errors in sequence alignments, etc.

Animals↗

PubMed2002

Time to the most recent common ancestor and divergence times of populations of common chaffinches (Fringilla coelebs) in Europe and North Africa: insights into Pleistocene refugia and current levels of migration.

We analyzed sequences from a 275-bp hypervariable region in the 5' end of the mitochondrial DNA control region in 190 common chaffinches (Fringilla coelebs) from 19 populations in Europe and North Africa, including new samples from Greece and Morocco. Coalescent techniques were applied to estimate the time to the most recent common ancestor (TMRCA) and divergence times of these populations. The first objective of this study was to infer the locations of refugia where chaffinches survived the last glacial episode, and this was achieved by estimating the TMRCA of populations in regions surrounding the Mediterranean that were unglaciated in the late Pleistocene. Although extant populations in Iberia, Corsica, Greece, and North Africa harbor haplotypes that are basal in a phylogenetic tree, this information alone cannot be used to infer that these localities served as refugia, because it is impossible to infer the ages of populations and their divergence times without also considering the population genetic processes of mutation, migration, and drift. Provided we assume the TMRCAs of populations are a reasonable estimate of a population's age, coalescent-based methods place resident populations in Iberia, Corsica, Greece, and North Africa during the time of the last glacial maximum, suggesting these regions served as refugia for the common chaffinch. The second objective was to determine when populations began diverging from each other and to use this as a baseline to estimate current levels of gene flow. Divergence time estimates suggest that European populations began diverging about 60,000 years before present. The relatively recent divergence of populations in North Africa, Italy, and Iberia may explain why classic migration estimates based on equilibrium assumptions are high for these populations. We compare these estimates with nonequilibrium-based estimates and show that the nonequilibrium estimates are consistently lower than the equilibrium estimates.

Africa, Northern↗

PubMed2004

Precision and accuracy of divergence time estimates from STR and SNPSTR variation.

Inference of intraspecific population divergence patterns typically requires genetic data for molecular markers with relatively high mutation rates. Microsatellites, or short tandem repeat (STR) polymorphisms, have proven informative in many such investigations. These markers are characterized, however, by high levels of homoplasy and varying mutational properties, often leading to inaccurate inference of population divergence. A SNPSTR is a genetic system that consists of an STR polymorphism closely linked (typically < 500 bp) to one or more single-nucleotide polymorphisms (SNPs). SNPSTR systems are characterized by lower levels of homoplasy than are STR loci. Divergence time estimates based on STR variation (on the derived SNP allele background) should, therefore, be more accurate and precise. We use coalescent-based simulations in the context of several models of demographic history to compare divergence time estimates based on SNPSTR haplotype frequencies and STR allele frequencies. We demonstrate that estimates of divergence time based on STR variation on the background of a derived SNP allele are more accurate (3% to 7% bias for SNPSTR versus 11% to 20% bias for STR) and more precise than STR-based estimates, conditional on a recent SNP mutation. These results hold even for models involving complex demographic scenarios with gene flow, population expansion, and population bottlenecks. Varying the timing of the mutation event generating the SNP revealed that estimates of divergence time are sensitive to SNP age, with more recent SNPs giving more accurate and precise estimates of divergence time. However, varying both mutational properties of STR loci and SNP age demonstrated that multiple independent SNPSTR systems provide less biased estimates of divergence time. Furthermore, the combination of estimates based separately on STR and SNPSTR variation provides insight into the age of the derived SNP alleles. In light of our simulations, we interpret estimates from data for human populations.

Data Interpretation, Statistical↗

PubMed1991

Molecular distance and divergence time in carnivores and primates.

Numerous studies have used indices of genetic distance between species to reconstruct evolutionary relationships and to estimate divergence time. However, the empirical relationship between molecular-based indices of genetic divergence and divergence time based on the fossil record is poorly known. To date, the results of empirical studies conflict and are difficult to compare because they differ widely in their choice of taxa, genetic techniques, or methods for calibrating rates of molecular evolution. We use a single methodology to analyze the relationship of molecular distance and divergence time in 86 taxa (72 carnivores and 14 primates). These taxa have divergence times of 0.01-55 Myr and provide a graded series of phylogenetic divergences such that the shape of the curve relating genetic distance and divergence time is often well defined. The techniques used to obtain genetic distance estimates include one- and two-dimensional protein electrophoresis, DNA hybridization, and microcomplement fixation. Our results suggest that estimates of molecular distance and divergence time are highly correlated. However, rates of molecular evolution are not constant; rather, in general they decline with increasing divergence time in a linear fashion. The rate of decline may differ according to technique and taxa. Moreover, in some cases the variability in evolutionary rates changes with increasing divergence time such that the accuracy of nodes in a phylogenetic tree varies predictably with time.

Animals↗

PubMed2001

Performance of a divergence time estimation method under a probabilistic model of rate evolution.

Rates of molecular evolution vary over time and, hence, among lineages. In contrast, widely used methods for estimating divergence times from molecular sequence data assume constancy of rates. Therefore, methods for estimation of divergence times that incorporate rate variation are attractive. Improvements on a previously proposed Bayesian technique for divergence time estimation are described. New parameterization more effectively captures the phylogenetic structure of rate evolution on a tree. Fossil information and other evidence can now be included in Bayesian analyses in the form of constraints on divergence times. Simulation results demonstrate that the accuracy of divergence time estimation is substantially enhanced when constraints are included.

Bayes Theorem↗

PubMed2005

Bayesian estimation of species divergence times under a molecular clock using multiple fossil calibrations with soft bounds.

We implement a Bayesian Markov chain Monte Carlo algorithm for estimating species divergence times that uses heterogeneous data from multiple gene loci and accommodates multiple fossil calibration nodes. A birth-death process with species sampling is used to specify a prior for divergence times, which allows easy assessment of the effects of that prior on posterior time estimates. We propose a new approach for specifying calibration points on the phylogeny, which allows the use of arbitrary and flexible statistical distributions to describe uncertainties in fossil dates. In particular, we use soft bounds, so that the probability that the true divergence time is outside the bounds is small but nonzero. A strict molecular clock is assumed in the current implementation, although this assumption may be relaxed. We apply our new algorithm to two data sets concerning divergences of several primate species, to examine the effects of the substitution model and of the prior for divergence times on Bayesian time estimation. We also conduct computer simulation to examine the differences between soft and hard bounds. We demonstrate that divergence time estimation is intrinsically hampered by uncertainties in fossil calibrations, and the error in Bayesian time estimates will not go to zero with increased amounts of sequence data. Our analyses of both real and simulated data demonstrate potentially large differences between divergence time estimates obtained using soft versus hard bounds and a general superiority of soft bounds. Our main findings are as follows. (1) When the fossils are consistent with each other and with the molecular data, and the posterior time estimates are well within the prior bounds, soft and hard bounds produce similar results. (2) When the fossils are in conflict with each other or with the molecules, soft and hard bounds behave very differently; soft bounds allow sequence data to correct poor calibrations, while poor hard bounds are impossible to overcome by any amount of data. (3) Soft bounds eliminate the need for "safe" but unrealistically high upper bounds, which may bias posterior time estimates. (4) Soft bounds allow more reliable assessment of estimation errors, while hard bounds generate misleadingly high precisions when fossils and molecules are in conflict.

Animals↗

PubMedDate not supplied

The Wilhelmine E. Key 2001 Invitational Lecture. Estimation of divergence times for a few mammalian and several primate species.

Statistical methods for estimating divergence times by using multiprotein gamma distances are discussed. When a large number of proteins are used, even a small degree of deviation from the molecular clock hypothesis can be detected. In this case, one may use the stem-lineage method for estimating divergence times. However, the estimates obtained by this method are often similar to those obtained by the linearized tree method. Application of these methods to a dataset of 104 proteins from several vertebrate species indicated that the divergence times between humans and mice and between mice and rats are about 96 and 33 million years (MY) ago, respectively. These estimates were obtained by assuming that birds and mammals diverged 310 MY ago. Similarly application of the methods to the protein sequence data from primate species indicated that the human lineage separated from the chimpanzee, gorilla, Old World monkeys, and New World monkeys about 6.0, 7.0, 23.0, and 33.0 MY ago, respectively. In this case the use of two calibration points, that is, the divergence time (13 MY ago) between humans and orangutans and between primates and artiodactyls (90 MY ago) gave essentially the same estimates.

Animals↗

PubMed2003

Comparison of likelihood and Bayesian methods for estimating divergence times using multiple gene Loci and calibration points, with application to a radiation of cute-looking mouse lemur species.

Divergence time and substitution rate are seriously confounded in phylogenetic analysis, making it difficult to estimate divergence times when the molecular clock (rate constancy among lineages) is violated. This problem can be alleviated to some extent by analyzing multiple gene loci simultaneously and by using multiple calibration points. While different genes may have different patterns of evolutionary rate change, they share the same divergence times. Indeed, the fact that each gene may violate the molecular clock differently leads to the advantage of simultaneous analysis of multiple loci. Multiple calibration points provide the means for characterizing the local evolutionary rates on the phylogeny. In this paper, we extend previous likelihood models of local molecular clock for estimating species divergence times to accommodate multiple calibration points and multiple genes. Heterogeneity among different genes in evolutionary rate and in substitution process is accounted for by the models. We apply the likelihood models to analyze two mitochondrial protein-coding genes, cytochrome oxidase II and cytochrome b, to estimate divergence times of Malagasy mouse lemurs and related outgroups. The likelihood method is compared with the Bayes method of Thorne et al. (1998, Mol. Biol. Evol. 15:1647-1657), which uses a probabilistic model to describe the change in evolutionary rate over time and uses the Markov chain Monte Carlo procedure to derive the posterior distribution of rates and times. Our likelihood implementation has the drawbacks of failing to accommodate uncertainties in fossil calibrations and of requiring the researcher to classify branches on the tree into different rate groups. Both problems are avoided in the Bayes method. Despite the differences in the two methods, however, data partitions and model assumptions had the greatest impact on date estimation. The three codon positions have very different substitution rates and evolutionary dynamics, and assumptions in the substitution model affect date estimation in both likelihood and Bayes analyses. The results demonstrate that the separate analysis is unreliable, with dates variable among codon positions and between methods, and that the combined analysis is much more reliable. When the three codon positions were analyzed simultaneously under the most realistic models using all available calibration information, the two methods produced similar results. The divergence of the mouse lemurs is dated to be around 7-10 million years ago, indicating a surprisingly early species radiation for such a morphologically uniform group of primates.

Animals↗

PubMedDate not supplied

Mean d(2) and divergence time: transformations and standardizations.

The fitness consequences of inbreeding and outbreeding have intrigued biologists for a long time. Recently a measure of relatedness of parental haplotypes has been proposed called mean d(2). This measure is based on a stepwise mutational process and therefore is tailored to microsatellite genetic markers. Theoretical work suggests that mean d(2) typically is less suited for measuring fitness consequences due to close inbreeding rather than heterozygosity. However, mean d(2) may be more appropriate than heterozygosity for measuring divergence times over longer time scales and thus for detecting outbreeding depression. Here, simulations are used to (1). identify appropriate standardization coefficients and transformations for mean d(2), and (2). evaluate mean d(2) as a measure of divergence time of parental lineages over time scales up to 10000 generations. Results show that mean d(2) is a linear predictor of divergence time. The coefficient of variation of mean d(2) approaches a constant value with increasing divergence time and therefore logarithm transformation is appropriate to restore homoscedasticity. When mutation rates and sizes are known for each locus they can be incorporated into a standardization coefficient to increase the precision of mean d(2). As few as 10 loci can explain more than 70% of the variation in divergence time between lineages. While heterozygosity outperforms mean d(2) at detecting differences in divergence time over relative short time periods ( or=1000 generations). However, gene flow of as little as 1% per generation can significantly reduce the ability of either mean d(2) or heterozygosity to estimate divergence time.

Computer Simulation↗

PubMed2001

Statistics of divergence times.

Given the number of nucleotide substitutions between two species (K) and the substitution rate nu, the expectation of the corresponding divergence time is usually calculated as K/(2 nu). This is strictly true only if nu is regarded as a constant because the ratio of two random variables, such as K/(2 nu), has distributional properties different from those of the distribution of K. Therefore, both the mean and any confidence interval for divergence times are unknown in this situation. We model the distribution of K and nu using the Gamma distribution and calculate the mean and 95% confidence interval for the corresponding divergence time. These calculations are compared with results obtained by bootstrapping sequence data from the model plant Arabidopsis thaliana and its relatives. We show that for nonoverlapping pairs of phylogenetic distances, our method approaches the bootstrap results very closely. In contrast, regarding the mutation rate as a constant leads to strong underestimation of the confidence interval. An implementation of our method of computing divergence times is accessible through a web interface at http://www.soft.ice.mpg.de/cite.

Arabidopsis↗

PubMed1999

Assessing the Cretaceous superordinal divergence times within birds and placental mammals by using whole mitochondrial protein sequences and an extended statistical framework.

Using the set of all vertebrate mtDNA protein sequences published as of May 1998, plus unpublished examples for elephant and birds, we examined divergence times in Placentalia and Aves. Using a parsimony-based test, we identified a subset of slower evolutionary rate placental sequences that do not appear to violate the clock assumption. Analyzing just these sequences decreases support for Marsupionta and the carnivore + perissodactyl group but increases support for armadillo diverging earlier than rabbit (which may represent the whole Glires group). A major theme of the paper is to use more comprehensive estimates of divergence time standard error (SE). From the well-studied horse/rhino split, estimated to be 55 million years before present (mybp), the splitting time within carnivores is confidently shown to be older than 50 million years. Some of our estimates of divergence times within placentals are relatively old, at up to 169 million years, but are within 2 SE of other published estimates. The whale/cow split at 65 mybp may be older than commonly assumed. All the sampled splits between the main groups of fereuungulates (the clade of carnivores, cetartiodactyls, perissodactyls, and pholidotes) seem to be distinctly before the Cretaceous/Tertiary boundary. Analyses suggest a close relationship between elephants (representing Afrotheria) and armadillos (Xenarthra), and our timing of this splitting is coincident with the opening of the South Atlantic, a major vicariant event. Recalibrating with this event (at 100 mybp), we obtain younger estimates for the earliest splits among placentals. Divergence times within birds are also assessed by using previously unpublished sequences. We fail to reject a clock for all bird taxa available. Unfortunately, available deep calibration points for birds are questionable, so a new calibration based on the age of the Anseriform stem lineage is estimated. The divergence time of rhea and ostrich may be much more recent than commonly assumed, while that of passerines may be older. Our major concern is the rooting point of the bird subtree, as the nearest outgroup (alligator) is very distant.

Animals↗

PubMed2006

Testing the molecular clock: molecular and paleontological estimates of divergence times in the Echinoidea (Echinodermata).

The phylogenetic relationships of 46 echinoids, with representatives from 13 of the 14 ordinal-level clades and about 70% of extant families commonly recognized, have been established from 3 genes (3,226 alignable bases) and 119 morphological characters. Morphological and molecular estimates are similar enough to be considered suboptimal estimates of one another, and the combined data provide a tree that, when calibrated against the fossil record, provides paleontological estimates of divergence times and completeness of their fossil record. The order of branching on the cladogram largely agrees with the stratigraphic order of first occurrences and implies that their fossil record is more than 85% complete at family level and at a resolution of 5-Myr time intervals. Molecular estimates of divergence times derived from applying both molecular clock and relaxed molecular clock models are concordant with estimates based on the fossil record in up to 70% of cases, with most concordant results obtained using Sanderson's semiparametric penalized likelihood method and a logarithmic-penalty function. There are 3 regions of the tree where molecular and fossil estimates of divergence time consistently disagree. Comparison with results obtained when molecular divergence dates are estimated from the combined (morphology + gene) tree suggests that errors in phylogenetic reconstruction explain only one of these. In another region the error most likely lies with the paleontological estimates because taxa in this region are demonstrated to have a very poor fossil record. In the third case, morphological and paleontological evidence is much stronger, and the topology for this part of the molecular tree differs from that derived from the combined data. Here the cause of the mismatch is unclear but could be methodological, arising from marked inequality of molecular rates. Overall, the level of agreement reached between these different data and methodological approaches leads us to believe that careful application of likelihood and Bayesian methods to molecular data provides realistic divergence time estimates in the majority of cases (almost 80% in this specific example), thus providing a remarkably well-calibrated phylogeny of a character-rich clade of ubiquitous marine benthic invertebrates.

Animals↗

PubMed2001

Estimating divergence time with the use of microsatellite genetic distances: impacts of population growth and gene flow.

Genetic distances play an important role in estimating divergence time of bifurcated populations. However, they can be greatly affected by demographic processes, such as migration and population dynamics, which complicate their interpretation. For example, the widely used distance for microsatellite loci, (deltamu)2, assumes constant population size, no gene flow, and mutation-drift equilibrium. It is shown here that (deltamu)2 strongly underestimates divergence time if populations are growing and/or connected by gene flow. In recent publications, the average estimate of divergence time between African and non-African populations obtained by using (deltamu)2 is about 34,000 years, although archaeological data show a much earlier presence of modern humans out of Africa. I introduce a different estimator of population separation time based on microsatellite statistics, T(D), that does not assume mutation-drift equilibrium, is independent of population dynamics in the absence of gene flow, and is robust to weak migration flow for growing populations. However, it requires a knowledge of the variance in the number of repeats at the beginning of population separation, V(0). One way to overcome this problem is to find minimal and maximal bounds for the variance and thus obtain the earliest and latest bounds for divergence time (this is not a confidence interval, and it simply reflects an uncertainty about the value of V(0) in an ancestral population). Another way to avoid the uncertainty is to choose from among present populations a reference whose variation is presumably close to what it might have been in an ancestral population. A different approach for using T(D) is to estimate the time difference between adjacent nodes on a phylogenetic population tree. Using data on variation at autosomal short tandem repeat loci with di-, tri-, and tetranucleotide repeats in worldwide populations, T(D) gives an estimate of 57,000 years for the separation of the out-of-Africa branch of modern humans from Africans based on the value of V(0) in the Southern American Indian populations; the earliest bound for this event has been estimated to be about 135,000 years. The data also suggest that the Asian and European populations diverged from each other about 20,000 years, after the occurrence of the out-of-Africa branch.

Animals↗

PubMed2004

Assessing the quality of molecular divergence time estimates by fossil calibrations and fossil-based model selection.

Estimates of species divergence times using DNA sequence data are playing an increasingly important role in studies of evolution, ecology and biogeography. Most work has centred on obtaining appropriate kinds of data and developing optimal estimation procedures, whereas somewhat less attention has focused on the calibration of divergences using fossils. Case studies with multiple fossil calibration points provide important opportunities to examine the divergence time estimation problem in new ways. We discuss two cross-validation procedures that address different aspects of inference in divergence time estimation. 'Fossil cross-validation' is a procedure used to identify the impact of different individual calibrations on overall estimation. This can identify fossils that have an exceptionally large error effect and may warrant further scrutiny. 'Fossil-based model cross-validation' is an entirely different procedure that uses fossils to identify the optimal model of molecular evolution in the context of rate smoothing or other inference methods. Both procedures were applied to two recent studies: an analysis of monocot angiosperms with eight fossil calibrations and an analysis of placental mammals with nine fossil calibrations. In each case, fossil calibrations could be ranked from most to least influential, and in one of the two studies, the fossils provided decisive evidence about the optimal molecular evolutionary model.

Animals↗

PubMed2006

TimeTree: a public knowledge-base of divergence times among organisms.

UNLABELLED: Biologists and other scientists routinely need to know times of divergence between species and to construct phylogenies calibrated to time (timetrees). Published studies reporting time estimates from molecular data have been increasing rapidly, but the data have been largely inaccessible to the greater community of scientists because of their complexity. TimeTree brings these data together in a consistent format and uses a hierarchical structure, corresponding to the tree of life, to maximize their utility. Results are presented and summarized, allowing users to quickly determine the range and robustness of time estimates and the degree of consensus from the published literature. AVAILABILITY: TimeTree is available at http://www.timetree.net

Biological Evolution↗

PubMed1999

Evolutionary and preservational constraints on origins of biologic groups: divergence times of eutherian mammals.

Some molecular clock estimates of divergence times of taxonomic groups undergoing evolutionary radiation are much older than the groups' first observed fossil record. Mathematical models of branching evolution are used to estimate the maximal rate of fossil preservation consistent with a postulated missing history, given the sum of species durations implied by early origins under a range of species origination and extinction rates. The plausibility of postulated divergence times depends on origination, extinction, and preservation rates estimated from the fossil record. For eutherian mammals, this approach suggests that it is unlikely that many modern orders arose much earlier than their oldest fossil records.

Animals↗