Human population dynamics revisited with the logistic model: How much can be modeled and predicted?

Decrease or growth of population comes from the interplay of death and birth (and locally, migration). We revive the logistic model, which was tested and found wanting in early-20th-century studies of aggregate human populations, and apply it instead to life expectancy (death) and fertility (birth), the key factors totaling population. For death, once an individual has legally entered society, the logistic portrays the situation crisply. Human life expectancy is reaching the culmination of a two-hundred year-process that forestalls death until about 80 for men and the mid-80's for women. No breakthroughs in longevity are in sight unless genetic engineering comes to help. For birth, the logistic covers quantitatively its actual morphology. However, because we have not been able to model this essential parameter in a predictive way over long periods, we cannot say whether the future of human population is runaway growth or slow implosion. Thus, we revisit the logistic analysis of aggregate human numbers. From a niche point of view, resources are the limits to numbers, and access to resources depends on technologies. The logistic makes clear that for homo faber, the limits to numbers keep shifting. These moving edges may most confound forecasting the long-run size of humanity.     

A complex species complex: The controversial role of ecology and biogeography in the evolutionary history of Syllis gracilis Grube, 1840 (Annelida,Syllidae)

The cryptic diversity in the polychaete Syllis gracilis Grube, 1840, in the Mediterranean Sea was examined with an integrative morpho-molecular approach. Individuals of S. gracilis were collected at eleven Mediterranean localities to provide an insight into the role of brackish environments in inducing cryptic speciation. The examination of morphological features combined with a molecular genetic analysis based on a partial sequence of the 16S rRNA gene highlighted discrepancies between morphological and molecular diversity. Morphological data allowed to identify a morphotype with short appendages occurring in coralline algae communities and another one with long appendages observed in brackish-water environments and Sabellaria reefs. Multivariate analyses showed that sampling localities were the greatest source of morphological divergence, suggesting that phenotypic plasticity may play a role in local adaptations of S. gracilis populations. Molecular data showed the occurrence of four divergent lineages not corresponding to morphological clusters. Different species delimitation tests gave conflicting results, retrieving, however, at least four separated entities. Some lineages occurred in sympatry and were equally distributed in marine and brackish-water environments, excluding a biogeographic or ecological explanation of the observed pattern and suggesting instead ancient separation between lineages and secondary contact. The co-occurrence of different lineages hindered the identification of the lineage corresponding to S. gracilis sensu stricto. The discrepancy between morphological and molecular diversity suggests that different environmental and biogeographic features may interact in a complex and unpredictable way in shaping diversity patterns. An integrative approach is needed to provide a satisfactory insight on evolutionary processes in marine invertebrates.     

Orthosteric Binding of ρ-Da1a,a Natural Peptide of Snake Venom Interacting Selectively with the α1A-Adrenoceptor

ρ-Da1a is a three-finger fold toxin from green mamba venom that is highly selective for the α_1A-adrenoceptor. This toxin has atypical pharmacological properties, including incomplete inhibition of ³H-prazosin or ¹²⁵I-HEAT binding and insurmountable antagonist action. We aimed to clarify its mode of action at the α_1A-adrenoceptor. The affinity (pKi 9.26) and selectivity of ρ-Da1a for the α_1A-adrenoceptor were confirmed by comparing binding to human adrenoceptors expressed in eukaryotic cells. Equilibrium and kinetic binding experiments were used to demonstrate that ρ-Da1a, prazosin and HEAT compete at the α_1A-adrenoceptor. ρ-Da1a did not affect the dissociation kinetics of ³H-prazosin or ¹²⁵I-HEAT, and the IC₅₀ of ρ-Da1a, determined by competition experiments, increased linearly with the concentration of radioligands used, while the residual binding by ρ-Da1a remained stable. The effect of ρ-Da1a on agonist-stimulated Ca²⁺ release was insurmountable in the presence of phenethylamine- or imidazoline-type agonists. Ten mutations in the orthosteric binding pocket of the α_1A-adrenoceptor were evaluated for alterations in ρ-Da1a affinity. The D106^3.32A and the S188^5.42A/S192^5.46A receptor mutations reduced toxin affinity moderately (6 and 7.6 times, respectively), while the F86^2.64A, F288^6.51A and F312^7.39A mutations diminished it dramatically by 18- to 93-fold. In addition, residue F86^2.64 was identified as a key interaction point for ¹²⁵I-HEAT, as the variant F86^2.64A induced a 23-fold reduction in HEAT affinity. Unlike the M1 muscarinic acetylcholine receptor toxin MT7, ρ-Da1a interacts with the human α_1A-adrenoceptor orthosteric pocket and shares receptor interaction points with antagonist (F86^2.64, F288^6.51 and F312^7.39) and agonist (F288^6.51 and F312^7.39) ligands. Its selectivity for the α_1A-adrenoceptor may result, at least partly, from its interaction with the residue F86^2.64, which appears to be important also for HEAT binding.