Saturday, March 18, 2006
Popper, A.N. and Fay, R.R. (1997) Evolution of the ear and hearing: Issues and questions. Brain, Behav, Evol, 50:213-221.
The ear appears to have arisen early in the evolution of the vertebrates. While there are significant interspecific differences in ear structure, it appears that receptor cell structure and the basic function of the ear and auditory system are similar among all vertebrate groups. In this paper we present the evolution of the sensory hair cells of the ear, the origins of the ear itself, and selected functions of the sense of hearing. We argue that there have been strong selective pressures in most vertebrate groups for the sorts of sound encoding and processing abilities that result in the efficient detection, localization, and identification of sound sources in noisy environments. Many of the encoding and processing strategies underlying these functions are shared as well.
Wednesday, March 15, 2006
Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered
Stephen Jay Gould, Niles Eldredge
Paleobiology, Vol. 3, No. 2 (Spring, 1977) , pp. 115-151
We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade - speciation may be random with respect to the direction of a trend (Wright's rule).
As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation - not phyletic evolution - must be the dominant mode of evolution.
We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation.
Phyletic gradualism was an a priori assertion from the start - it was never "seen" in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an "unnecessary difficulty." We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.
Sunday, March 12, 2006
Inverse Correlation between an Organ's Cancer Rate and Its Evolutionary Antiquity
Jamie A. Davies
View Article Vol: 1 | Issue: 2 | oct/nov/dec 2004 | pgs: 60-63 | Brief Communication
Human cancer rates vary dramatically across the range of internal organs in the body, but there is no single model to explain the variation and there is also no obvious overall pattern to it. Theories have been proposed to account for high rates in particularly cancer-prone organs, and they usually concentrate on the peculiar vulnerability of certain cells to mutation (eg lung cells' direct exposure to airborne carcinogens). These explanations are valuable for understanding mechanisms of disease and also for cancer prevention, but they neither address the overall distribution of cancers nor the possibility that some states of differentiation may be intrinsically less stable than others to the effects of random mutation, a possibility predicted on purely theoretical grounds many years ago. This brief report describes an overall pattern to human organ-specific cancer incidence data and shows that organ-specific cancer rates correlate negatively with an organ's evolutionary antiquity. Although the relationship may just be coincidental, it suggests the possibility that recently-evolved differentiation states may be intrinsically more vulnerable to neoplastic change. Extrapolation of the regression line to a cancer incidence of zero equates to a level of tissue organization typical of 660Myr ago; the inferred beginning of neoplasia therefore coincides with the rise of complex multicellular animals.