Study Of Evolutionary Rates Research Articles

RRmorph—a new R package to map phenotypic evolutionary rates and patterns on 3D meshes

The study of evolutionary rates and patterns is the key to understand how natural selection shaped the current and past diversity of phenotypes. Phylogenetic comparative methods offer an array of solutions to undertake this challenging task, and help understanding phenotypic variation in full in most circumstances. However, complex, three-dimensional structures such as the skull and the brain serve disparate goals, and different portions of these phenotypes often fulfil different functions, making it hard to understand which parts truly were recruited by natural selection. In the recent past, we developed tools apt to chart evolutionary rate and patterns directly on three-dimensional shapes, according to their magnitude and direction. Here, we present further developments of these tools, which now allow to restitute the mapping of rates and patterns with full biological realism. The tools are condensed in a new R software package.

Neutrale Mutationen und evolutionäre Fortentwicklung

Neutral mutation and evolutionary progress The process and causes of regressive evolution are still under debate. Contrary to DARWIN'S original assumption, Neo-Darwinian proponents make selection responsible for reduction. Biologically functionless structures like eye and pigmentation in cave animals deliver excellent material to study this problem. Comparison of regressive (eye, pigmentation, aggression, dorsal light reaction) and constructive traits (gustatory equipment, egg yolk content, feeding behavior) in epigean and cave fish (Astyanax fasciatus, Characidae) reveal a high variability of the regressive features in the cave forms. Contrary to this, the constructive traits are characterized by a low variability in epigean and cave fish. This difference is attributed to the lack of selection on regressive structures. The existence of an intermediate cave population between epigean and true cave fish of A. fasciatus makes possible the study of evolutionary rates. It is shown that the regressive traits do not evolve more quickly than the constructive ones do. On the contrary, constructive traits like egg yolk content are even more rapid because they are of great biological value in the cave biotope. Especially energy economy is claimed by Neo-Darwinists to play a decisive role as a selective force. Comparison of the development of epi- and hypogean larvae of A. fasciatus shows that the formation of a smaller and less differentiated eye in the cave specimens has no effect on body growth. Furthermore, even behavioral traits like aggressiveness, schooling, dorsal light reaction, or negative phototaxis, which all are not performed in darkness by the epigean ancestor, become genetically reduced in the cave fish. The principles of regressive evolution, loss of selection and increase in variability, play a central role in evolution in general. When biota with empty niches are colonized, stabilizing selection relaxes from the special adaptations to the niche inhabited before by the invading species. Variability may arise in these and is permitted as long as fitness is guaranteed. Such processes characterize adaptive radiation. Examples are given by the species flocks on isolated islands or in chemically abnormal lakes like those of the East African Rift Valley. Only secondarily, on the basis of the arisen variability, does directional selection promote the newly developing species into different niches. The loss of stabilizing selection is an important factor for the evolutionary process to be open for evolutionary progress.

Spurious or Island Effect? A Response to J. A. Pérez-Claros and J. C. Aledo's Comment on “Morphological Evolution Is Accelerated among Island Mammals”

In their comment [1], Perez-Claros and Aledo claimed that the main result of accelerated evolution in island mammals, compared with their mainland relatives, is not well supported by the data and analyses presented in [2]. They raise two points: the first refers to a long-standing methodological debate regarding the study of evolutionary rates and the second refers to the quality of the dataset in [2]. Haldane [3] first proposed the unit darwin to calculate rates of morphological evolution in fossil taxa. Since then, there has been much controversy on the relevance of the darwin as a measure of evolutionary rates [4]. First, evolutionary rates in darwins that are calculated over the largest time intervals are necessarily lowered, because they average periods of rapid change with periods of slow change or stasis, and they can also include fluctuations in the direction of change [4]. Second, because evolutionary rates are dependent on the time interval over which they are measured, rates measured over different time intervals cannot be compared without taking into account the time interval over which they were calculated (i.e., temporal scaling), which is often done by plotting rates against time intervals on a log-log graph. This is equivalent to plotting a ratio (rate) against its denominator (time interval), which produces a negative correlation and may be a mathematical artifact that has been dubbed “the spurious effect” [5,6]. Despite these two important caveats, rates in darwins are still useful to compare similar or different lineages evolving at similar or different times and places [7]. More importantly, these concerns simply do not affect the results presented in [2], because the conclusions were solely based on the comparison of the elevation of the two regression lines. The extent to which the “spurious effect” influences the slopes of the regression is not of direct relevance to the question of whether mammals evolve faster on islands [2]. In fact, there is no reason to believe that such a bias will introduce a systematic difference among the mainland and island groups compared in [2]. Given this, it is unlikely that the “spurious effect” can be evoked to explain the differences in the island–mainland comparison. Another more-specific point raised by Perez-Claros and Aledo concerns the data that was assembled to test the theory and upon which the conclusions were based in [2]. Perez-Claros and Aledo indicate that the analysis is no longer significant when re-performed on a subset of the data. This is true, but it is easy to reduce the range of data to a point where the relation is no longer significant. Is it relevant to test a theory on a subset of data that represents less than half of the data and only 16% on a log scale (or 0.15% on a natural scale) of the original range of time intervals? I contend that their analysis reduces the dataset to such an extent that it is no longer meaningful to address the evolutionary question at hand. The data made available in [2] represent, to date, the largest dataset ever assembled on morphological rates of evolution in mammal species, fossil and recent. Yet, the data are unbalanced, with an over-representation of island taxa for the smallest time intervals, and an over-representation of mainland data for the largest time intervals. This issue was already addressed [2, page 0002], and readers are referred to the original paper for more details. In the future, as more data become available, the conclusion in [2] may be either validated or challenged, but for the time being, the difference in elevation between the two regression lines is statistically significant and is the most robust for time intervals below 20,000 y. Morphological evolution is faster in island mammals over these time intervals, when compared with that of mammals evolving on the mainland.

Paleobotany: Perspectives in 1980

In the past twenty years, both knowledge of fossil plants and ways of looking at the paleobotanical record have expanded significantly. Studies of Precambrian microfossils have provided insight on early photoautotrophs, while fundamental changes in perceptions of Silurian and Devonian plant remains have clarified our understanding of the colonization of land, as well as the subsequent evolution of complex stems, leaves, and the seed habit. A recurring question concerns the phylogenetic relationships of gymnosperms: is the gymnospermy found in various living and extinct taxa the result of evolutionary convergence or of common descent? Rapidly accumulating data on the reproductive and developmental biology of Paleozoic seed ferns is reopening debate on this issue. The mystery of angiosperm origins remains “abominable”, but the past two decades of research have seen convincing documentation of the initial radiation and rise to taxonomic and ecological dominance of the flowering plants.While phylogenetic and systematic botany remain at the core of paleobotanical research, increased attention is being devoted to studies of ontogeny and reproductive biology, paleoecology, taphonomy, paleophytochemistry, paleoclimatology, and paleobiogeography. Studies of evolutionary rates and diversity are in their infancy, but initial results permit an optimistic appraisal of the contributions that paleobotanists can make to evolutionary paleontology.

Impact of the Lognormal Distribution on Studies of Phenotypic Variation and Evolutionary Rates

Kerfoot, W. Charles and Arnold G. Kluge (Museum of Zoology and Department of Zoology, University of Michigan, Ann Arbor, Michigan 48104). 1971. Impact of the Lognormal Distribution on Studies of Phenotypic Variation and Evolutionary Rates. Syst. Zool., 20:459-464.-Both normal and lognormal statistics describe the distribution of vertebral numbers in local samples of lower vertebrates. When samples from populations possessing different means are compared, however, standard deviations of vertebral counts are correlated with mean counts, a relationship predicted by the lognormal, but not the normal distribution. Fit to the lognormal requires recognition in significance testing, neoessitates log transformation of variables in studies of evolutionary rates, and predicts exponential rates of character divergence under constant selective pressure. Numerous papers discuss the variability of vertebral number in the context of systematics, embryology, and genetics. Studies show that vertebral number is heritable and polygenic both in natural populations and in laboratory strains (Green and Russell, 1951; McLaren and Michie, 1954), and that vertebral number is susceptible to temperature shock during embryonic development (Fox, Gordon and Fox, 1961; Kubota, 1967; Lindsey, 1962; Taning, 1944). Other studies suggest the selective advantage of the modal number found in natural populations (Ford and Bull, 1926). Most of these works implicitly assume that vertebral numbers are normally distributed. The distributions appear bell-shaped, and slight departures from normality have little effect on the results of the above mentioned studies. Comparison of vertebral distributions between organisms indicates a higher order relationship between the variance and the mean, which suggests that vertebral distributions more closely fit a lognormal distribution, and thus may provide additional insights into the development and evolution of vertebral number.