The water of life (Griffith REVIEW Selections)

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Although we regard this as being rather a strong claim considering the relatively small size of the database available at that time and the correlational nature of all comparative studies, we would agree with Stutchbury b that the breeding synchrony hypothesis has held up better in comparative tests than has the breeding density hypothesis.

Indeed, only two intraspecific studies have provided significant support for such a relationship, both within a single population. This is, however, relatively weak evidence for a causal relationship between synchrony and rate of EPP due to the potential influence of uncontrolled confounding variables and the small number of independent comparisons.

The observational evidence on the empirical link between breeding synchrony and EPP is at best mixed, therefore. In a population of the great tit, the whole, or part, of the first clutch was removed provoking a second, more asynchronous breeding attempt. The only experimental evidence available does not, therefore, support the breeding synchrony hypothesis. Overall, we suggest that, despite considerable empirical effort and much heated debate, it remains difficult to assess the role of variation in breeding synchrony in determining interspecific variation in EPP.

Although Stutchbury a,b comparative analyses appear to provide phylogenetically robust correlational evidence for a link between these variables, it remains unclear whether this link is causal. We say this for three reasons. First, the key supportive comparative tests Stutchbury a,b were performed on relatively small databases and the relative contribution of potentially confounding factors were not examined in detail.

Finally, the empirical evidence for a causal link between breeding synchrony is not straightforward. Given the lack of experimental studies of the influence of breeding synchrony, this contradiction is difficult to interpret biologically. We conclude that the breeding density hypothesis has not been falsified and could plausibly play a role in determining interspecific variation in EPP.

To go further than this we need further comparative tests on the relative role of breeding density vs. Without these forms of evidence we feel it is too early to say that the breeding synchrony hypothesis is either important or trivial. EPP and genetic diversity. The difficulties in finding support for the traditional ecological hypotheses based on breeding density and breeding synchrony has led some authors to suggest that the key factor in determining interspecific variation in EPP may be genetic rather than demographic.

Thus, if it is assumed that island populations are genetically depauperate compared to their mainland counterparts, this observation is consistent with the genetic diversity hypothesis. Of course, for most of the species used in these island—mainland comparisons there is no quantitative evidence that the insular population are indeed genetically depauperate, but such an effect has been widely reported in birds as well as other organisms Frankham Our main conclusion from these comparative studies is that the genetic diversity hypothesis deserves more study.

Although the interspecific studies are difficult to interpret because they are correlational and based on indirect indices of additive genetic variation in male fitness, they do show much stronger correlations than have ever been demonstrated for either breeding density and breeding synchrony.

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We predict that the greatest potential of the genetic diversity hypothesis will be in explaining differences in the level of EPP among very closely related species and among populations of the same species. EPP and the need for paternal care. Another response to the limited explanatory success of the two traditional ecological explanations for interspecific variation in EPP breeding density and breeding synchrony is the hypothesis that high rates of EPP should be associated with little need for paternal care.


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As predicted, both studies found that interspecific variation in the rate of EPP was significantly negatively associated with variation in the direct effect of paternal care in terms of reproductive success Fig. There is therefore strong correlative evidence from several research groups for a link between interspecific variation in the need for paternal care and interspecific variation in the rate of EPP. As far as we are aware, only a single empirical study has investigated experimentally the link between the need for paternal care and the incidence of EPP.

Thus, although this is only a single study, there is experimental support for a causal link between differences in parental care can lead to differences in the rate of EPP. We conclude that there is relatively good evidence for a link between the need for paternal care and the rate of EPP.

The correlational evidence is particularly strong, being based on phylogenetically robust tests on large data sets and controlling for several other factors, and is consistent with the observation that much of the interspecific variation in both EPP and the form of parental care occurs at high taxonomic levels.

From this comparative evidence we suggest that ancient changes in the form of parental care may have influenced the large differences in EPP between major lineages of birds. It is more difficult to know the role that variation in parental care may play in explaining variation among more closely related species, or among populations of the same species, or even among individuals within the same population.

EPP and the rate of adult mortality. Another variable that has been suggested recently to explain interspecific variation in the rate of EPP is the rate of adult mortality. In consequence, high rates of EPP will only be evolutionarily stable in species with short reproductive lifespans.


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Indeed, it is very striking even from a visual inspection of the data in Fig. In the case of the mortality hypothesis, to our knowledge there have been no attempts to test experimentally for a causal relationship between the rate of mortality and the rate of EPP.

Extra pair paternity in birds: a review of interspecific variation and adaptive function

Indeed, because the logic of this argument is based on changes over an evolutionary timespan, rather than facultative changes within an individual, such tests would not be straightforward. We therefore conclude that, as with the parental care hypothesis, there is strong correlative evidence in support of of a link between adult mortality and EPP but a lack of experimental evidence for the causal nature of this relationship.

Again, given that both adult mortality and the rate of EPP vary most extensively among ancient avian lineages, it seems most probable that changes in adult mortality played a role in the ancient diversification of sexual mating systems but that other factors may be more important in determining contemporary variation among populations and among individuals.


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  8. We suggest that the major conclusion of our preceding review of the various explanations for variation in the rate of EPP is that there is no single explanation for this phenomenon. For none of these hypotheses do we feel that there is overwhelming evidence that the proposed factor explains the majority of variation across all levels of organization: that is, among major avian lineages, among closely related species, among populations of the same species and among individuals within a single population.

    Instead, each factor appears to work best at one or two of these levels. Here, therefore, we will only review briefly the competing hypothesis and concentrate instead on empirical tests of the predictions arising from these hypotheses. In many respects these explanations mirror the hypotheses that have been proposed to explain the evolution of secondary sexual ornaments in birds, with an early emphasis on fertility and genetic diversity gradually being augmented by theories on genetic quality and compatibility. Although some may seem more probable than others, none of these explanations can be excluded on logical grounds alone.

    Despite the large number of theoretically plausible explanations for EPP, there have been few direct empirical tests that have provided unambiguous support for only one type of explanation even assuming that there will be a unitary explanation. The reason for this shortfall is twofold: 1 failure to gather sufficient types of data to differentiate between different hypotheses; or 2 failure to use an experimental approach to control for potentially confounding factors.

    In the majority of studies that discuss the function of EPP, however, only one type of data is available. Even in those cases where all these types of data are available, it is usually impossible to tell whether the ecological correlates are part of causal mechanism. However, because of a lack of other types of evidence, neither of these studies could rule out the possibility that the relationships between EPP and hatchability were due to confounding effects such as female quality. Data on this question are extremely limited. The interpretation of all of the studies mentioned above is problematic for three reasons.

    For example, females have been shown to invest differentially in eggs by either their sex e. As pointed out by Sheldon , however, differential investment by females, based on offspring paternity, would be adaptive only if there were differences in paternal genetic contributions to offspring fitness. To date, no empirical study has dealt with all these problems simultaneously.

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    It now seems probable that differences between species in the rate of EPP are due to a combination of differences in life history, pattern of parental care and local opportunities for promiscuity. Revealing the function of EPP, however, remains the most conspicuous ongoing challenge. It remains to be shown whether these are the general explanation for EPP in birds. Species estimates reported in the literature that have been excluded from the unambiguous database Appendix I for the reasons given.

    Appendix I Table A1. The sample sizes on which the estimates of offspring No. Where more than one estimate exists for a single species a single estimate has been calculated using the mean of the different estimates, weighted by their sample size. Breeding system is defined; 1 social monogamy or polygyny, 2 Social polyandry, 3 social polygynandry, 4 cooperative breeder, 5 no social association. Cooperative breeders are often difficult to assign to a particular breeding system and therefore we have not distinguished between them.

    References show the sources for data on the rate of EPP. Missing data. Appendix II Table A2. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article. Simon Griffith is a NERC research follow based at Oxford University currently working on sexual selection, and ornamental signaling in birds, particularly in monogamous mating systems.

    And she beams as she explains her decision — after three decades of beginning tours in the United States — to start a tour in Ireland and the United Kingdom where she will play 17 dates in March. Ireland was that for me. Her music soon caught on in England, Scotland and Wales, where she has toured every year since The imagery of her music may be as much the allure as the politics, whether in her native country or for those outside who are fascinated by it. With roots in the blue-sky vastness of West Texas, Ms.

    Griffith has always sung candidly about the history of the United States and the simple and often heartbreaking stories that shaped it: Dust Bowl farmers, a Crow Indian boy sweeping up a saloon amid the white men who conquered his people, young lovers dancing in an empty Woolworth store, loneliness — Garbo-style — inside a hotel late at night, life as it fades away on Death Row.

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    And her concerts have always connected her music to the moment and to the land, wherever she may be. And after being a lifelong Texan, she sold her house in Austin last year and moved to Nashville. The same could be said for many of Ms. View all New York Times newsletters. She also works each year at the festival with Protestant and Catholic schoolchildren in poetry classes she helped start seven years ago. No fan of many Texans for her provocative lyrics about everything from politics to capital punishment, Ms. When Griffith heard of this a moment later he dashed off to find Lippman, and stood talking with him in College Green.

    It is the case that Griffith was not as militant as some other advanced nationalists, believing that armed revolt could be counter-productive and hurt the poor.

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    That did not save him from being roughed up along with his son and threatened with execution in And he never disavowed those who went out in Easter Week. Widely read and intelligent, Griffith distrusted ideologues. He collapsed during the Civil War in , dying aged No diary or memoir survives. His exact relationship with the rebels in Easter Week, and his precise strategy on other crucial matters such as his signing of final Treaty terms without reference back to de Valera remain matters of speculation.

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