What is macroevolution?

Definitions of macroevolution fall into three categories: (1) evolution of taxa of supraspecific rank; (2) evolution on the grand time‐scale; and (3) evolution that is guided by sorting of interspecific variation (as opposed to sorting of intraspecific variation in microevolution). Here, it is argued that only definition 3 allows for a consistent separation of macroevolution and microevolution. Using this definition, speciation has both microevolutionary and macroevolutionary aspects: the process of morphological transformation is microevolutionary, but the variation among species that it produces is macroevolutionary, as is the rate at which speciation occurs. Selective agents may have differential effects on intraspecific and interspecific variation, with three possible situations: effect at one level only, effect at both levels with the same polarity but potentially different intensity, and effects that oppose between levels. Whereas the impact of all selective agents is direct in macroevolution, microevolution requires intraspecific competition as a mediator between selective agents and evolutionary responses. This mediating role of intraspecific competition occurs in the presence of sexual reproduction and has therefore no analogue at the macroevolutionary level where species are the evolutionary units. Competition between species manifests both on the microevolutionary and macroevolutionary level, but with different effects. In microevolution, interspecific competition spurs evolutionary divergence, whereas it is a potential driver of extinction at the macroevolutionary level. Recasting the Red Queen hypothesis in a macroevolutionary framework suggests that the effects of interspecific competition result in a positive correlation between origination and extinction rates, confirming empirical observations herein referred to as Stanley's rule

La Formazione dei Foglietti Embrionali, L'origine Dell'Epitelio Intestinale e la Determinazione Della Linea Germinale Femminile Nell' Haploembia Solieri

RIASSUNTOIn Haploembia solieri e negli altri Embiotteri esaminati, il foglietto interno si forma per migrazione diffusa di cellule dalla piastra germinativa. E assente un solco gastrulare. Non si hanno abbozzi bipolari entodermici. Costituitosi il mesoderma splancnico, l'epitelio mesenterico si forma per un processo di epibolia, e non di proliferazione, delle cellule della parete ectodermica stomodeale e proctodeale; il processo e piu intenso nel tratto mesenterico proctodeale che non in quello stomodeale. Successivamente le cellule vitelline, tranne alcune, migrano alla periferia assiepandosi sulle pareti. L'assiepamento e assai piu intenso nel quarto anteriore dove le cellule epiteliali di origine stomodeale sono piu rade. Sulla parete i nuclei dei vitellofagi subiscono ripetute divisioni amitotiche disponendosi quindi in un'assisa uniforme tra le preesistenti cellule di origine ectodermica. Al momento della schiusa, l'epitelio mesenterico e quindi di costituzione mista e di derivazione prevalentemente ..