They are correct that punctuated equilibria apply to sexually reproducing organisms and that morphological evolutionary change is regarded as largely (if not exclusively) correlated with speciation events. However, they err in suggesting that we attribute stasis strictly to "developmental constraints," which represent only one of a set of possible mechanisms that we have suggested for the causes of stasis. Others include habitat tracking and the internal structure of species themselves [for example, (2)].

In eukaryotic cells the nucleus and its contents are separated from the cytoplasm by the nuclear envelope. Macromolecules, as well as smaller molecules and ions, can cross the nuclear envelope through the nuclear pore complex. Molecules greater than approx. 60 kDa and containing a nuclear localization signal are actively transported across the nuclear membranes, but there has been little evidence for regulatory mechanisms for smaller molecules and ions. Recently, diffusion across the nuclear envelope has been observed to be regulated by (...) nuclear cisternal Ca2+ concentrations. Following depletion of Ca2+ from the nuclear store by inositol 1,4,5‐trisphosphate or Ca2+ chelators, a fluorescent 10 kDa marker molecule was no longer able to enter the nucleus. Distinct conformational states of the nuclear pore complexes depended on the Ca2+ filling state of the nuclear envelope, supporting the assumption that a switch in the conformation of the nuclear pore complex may control the transport of intermediate‐sized molecules across the nuclear envelope. Thus nuclear Ca2+ stores may regulate the conformational state of the nuclear pore complex, and thereby passive diffusion of molecules between the cytosol and the nucleoplasm. The physiological significance of this finding is currently unknown. (shrink)

ORIGIN OF SPECIES. INTRODUCTION. When on board HMS 'Beagle,' as naturalist, I was ranch struck with certain facts in the distribution of the organic beings ...

There is increasing evidence for the wide‐spread existence of functionally redundant genetic pathways in developmental processes. However, both their significance and manner of evolution are still matters of debate. I will argue here that redundancy of gene actions may, in fact, be a necessary requirement for the development and evolution of complex life forms. One can view development as a process that transmits information from the egg to the adult organism. Transmission of information is, however, always an error‐prone process, which (...) can only be safeguarded by including redundancies in the message. Molecular examples for well analysed redundant processes indicate that redundancies may best be understood within a conceptual framework of overlaps between different gene functions. (shrink)

... Difficulty of distinguishing between Varieties and Species — Origin of Domestic ... and Origin— Principle of Selection anciently followed, its Effects— ...