Ancestral Species (ancestral + species)

Distribution by Scientific Domains


Selected Abstracts


PERSPECTIVE: GENE DIVERGENCE, POPULATION DIVERGENCE, AND THE VARIANCE IN COALESCENCE TIME IN PHYLOGEOGRAPHIC STUDIES

EVOLUTION, Issue 6 2000
ScottV.
Abstract Molecular methods as applied to the biogeography of single species (phylogeography) or multiple codistributed species (comparative phylogeography) have been productively and extensively used to elucidate common historical features in the diversification of the Earth's biota. However, only recently have methods for estimating population divergence times or their confidence limits while taking into account the critical effects of genetic polymorphism in ancestral species become available, and earlier methods for doing so are underutilized. We review models that address the crucial distinction between the gene divergence, the parameter that is typically recovered in molecular phylogeographic studies, and the population divergence, which is in most cases the parameter of interest and will almost always postdate the gene divergence. Assuming that population sizes of ancestral species are distributed similarly to those of extant species, we show that phylogeographic studies in vertebrates suggest that divergence of alleles in ancestral species can comprise from less than 10% to over 50% of the total divergence between sister species, suggesting that the problem of ancestral polymorphism in dating population divergence can be substantial. The variance in the number of substitutions (among loci for a given species or among species for a given gene) resulting from the stochastic nature of DNA change is generally smaller than the variance due to substitutions along allelic lines whose coalescence times vary due to genetic drift in the ancestral population. Whereas the former variance can be reduced by further DNA sequencing at a single locus, the latter cannot. Contrary to phylogeographic intuition, dating population divergence times when allelic lines have achieved reciprocal monophyly is in some ways more challenging than when allelic lines have not achieved monophyly, because in the former case critical data on ancestral population size provided by residual ancestral polymorphism is lost. In the former case differences in coalescence time between species pairs can in principle be explained entirely by differences in ancestral population size without resorting to explanations involving differences in divergence time. Furthermore, the confidence limits on population divergence times are severely underestimated when those for number of substitutions per site in the DNA sequences examined are used as a proxy. This uncertainty highlights the importance of multilocus data in estimating population divergence times; multilocus data can in principle distinguish differences in coalescence time (T) resulting from differences in population divergence time and differences in T due to differences in ancestral population sizes and will reduce the confidence limits on the estimates. We analyze the contribution of ancestral population size (,) to T and the effect of uncertainty in , on estimates of population divergence (,) for single loci under reciprocal monophyly using a simple Bayesian extension of Takahata and Satta's and Yang's recent coalescent methods. The confidence limits on , decrease when the range over which ancestral population size , is assumed to be distributed decreases and when increases; they generally exclude zero when /(4Ne) > 1. We also apply a maximum-likelihood method to several single and multilocus data sets. With multilocus data, the criterion for excluding = 0 is roughly that l/(4Ne)> 1, where l is the number of loci. Our analyses corroborate recent suggestions that increasing the number of loci is critical to decreasing the uncertainty in estimates of population divergence time. [source]


Adapting wheat cultivars to resource conserving farming practices and human nutritional needs

ANNALS OF APPLIED BIOLOGY, Issue 4 2005
R M TRETHOWAN
Summary As farmers increasingly adopt resource conserving farming practices, there is a need for wheat cultivars that better adapt to the changing environment and the nutritional needs of people, particularly those living in developing countries. Improved adaptation to zero and minimum tillage, better water use efficiency, improved root health, durable resistance to foliar diseases and enhanced nutritional value of the grain are key selection criteria for plant breeders. Significant responses to selection for these constraints have been achieved at the International Maize and Wheat Improvement Center (CIMMYT), by selecting segregating populations and advanced lines in carefully managed tillage, moisture deficit and heat stressed environments, that correlate with key spring wheat growing environments globally. Root health has been improved through a combination of marker assisted selection and disease bioassays, and the nutritional value of wheat grain has been enhanced using genetic variation for high Fe and Zn grain content found among tetraploid wheat ancestral species. [source]


Speciation and inversions: Chimps and humans

BIOESSAYS, Issue 9 2003
Jody Hey
A new set of models has resurrected a role for chromosomal inversions in the formation of new species.1,3 Traditional models, which are generally considered to be unlikely in most cases, had imagined that inversions might aid speciation by directly causing low hybrid fitness. In contrast, the newer models focus on the effect that inversions have on local recombination rates. A test of these models found a strikingly high rate of amino-acid substitution within regions where humans and chimpanzees differ by inversions, suggesting perhaps that our ancestral species underwent a divergence process in which gene flow and inversions played a key role.4 However, it remains uncertain whether this interesting finding is actually consistent with the proposed model. BioEssays 25:825,828, 2003. 2003 Wiley Periodicals, Inc. [source]


Pattern and process in the distribution of North American freshwater fish

BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 1 2010
DAVID GRIFFITHS
Published species lists were analysed to determine the contributions of dispersal, habitat preference, river channel size, body size, and glacial history to large-scale patterns in freshwater fish species richness in North America, north of central Mexico. Total species richness declines to the north and west but the pattern for endemics differs from that of widespread species. Mississippi Basin regions are more species rich than more isolated, coastal, regions. Richness declines more rapidly with increasing latitude in riverine specialist than in habitat generalist species. Levels of endemism are greatest in species found in small- to medium-sized river channels. The strong Rapoport effect, more marked in migratory than resident species, is correlated with habitat preference, channel size, and glacial history. Body size increases with latitude, largely as a result of a trend from small resident to large migrant species. In unglaciated regions, ancestral species survived in large habitats because these are longer-lived, more extensive, less isolated and more stable than headwaters, permitting larger populations and lower extinction levels. Reduced levels of gene flow in small, peripheral, channels isolated by larger downstream habitats have resulted in the production of many, small range, small-bodied species. The latitudinal richness gradient is a consequence of speciation and extinction events in unglaciated faunas and an increasing domination of faunas by generalist, large bodied, large channel, recolonizing species in more northern regions. 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 46,61. [source]


Heterochronic differences in fin development between latitudinal populations of the medaka Oryzias latipes (Actinopterygii: Adrianichthyidae)

BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2009
MAIKO KAWAJIRI
Heterochrony is believed to have played important roles in macroevolutionary morphological changes. However, few studies have focused on intraspecific heterochrony, although interspecific differences ultimately originated from variation within ancestral species. We have demonstrated heterochrony in fin development between two latitudinal populations of the medaka, Oryzias latipes. Comparisons of fin length (anal and dorsal) among wild individuals revealed that fins are shorter with respect to body length in the northern population, indicating that they are ,paedomorphic' compared with the southern population. Observations of fin ray formation and subsequent fin growth in the laboratory revealed that the timing of pterygiophore development occurs later, and that fins start to elongate later with respect to body length in the northern fish, indicating that fin growth is ,post-displaced' compared with the southern population. In addition, the rate of fin growth with respect to body length was lower in the northern males, indicating ,neoteny'. Given that all Oryzias except O. latipes are distributed in the tropics, it is likely that higher-latitude fish have evolved post-displacement and neoteny during northern extension of their geographic range. The delayed development in higher-latitude fish is probably a trade-off for faster body growth, which has evolved as an adaptation to seasonally time-constrained environments. 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 571,580. [source]


Distinguishing "or" from "and" and the case for historical identification

CLADISTICS, Issue 6 2002
Arnold G Kluge
The adequacy of a probabilistic interpretation must be judged according to the nature of the event, or thing, being inferred. For example, conditional (frequency) probability is not admissible in the inference of phylogeny, because basic statements of common ancestry do not fulfill the requirements of the relations specified by the probability calculus. The probabilities of the situation peculiar to the time and place of origin of species are unique. Moreover, according to evolutionary theory, an event of species diversification is necessarily unique, because species are parts of a replicator continuum,species arise from ancestral species. Also, these probabilities cannot be ascertained, because the relevant situation cannot be repeated,it is unique. Finally, the applicability of conditional (frequency) probability is denied, because events of common ancestry have already occurred,there is nothing to predict. However, hypotheses of species relationships can be identified objectively according to the degree to which they have survived simultaneous testing with critical evidence, not with generally confirming evidence. The most parsimonious hypothesis of species relationships represents the least disconfirmed, best supported, proposition among the alternatives being compared. That hypothesis does not, however, deserve any special epistemological status beyond serving as the focus of the next round of testing. [source]