Why are arthropods segmented?

SUMMARY Segmentation as an attribute of organisms is being increasingly discussed in the recent literature because (1) new phylogenies suggest that organisms classically considered to be segmented may lie in separate clades; (2) the molecular basis of segmental development has been much studied; (3) various theories of bilaterian origins place weight on segmentation as a primitive character; (4) there has been recent stress on the importance of modularity as an evolutionary topic. However, the definition and extent of segmentation are highly ambiguous and usually typological. Here, segmentation is regarded as an attribute of organs, not organisms. The evolution of just one system, the arthropod epidermis, is examined on the basis of the fossil record and the extant euarthropods, tardigrades, and onychophorans. It may be seen to have become segmented in a complex pathway that necessitated shifts in function, redundancy, and changes in associated organs. This complexity must inevitably reflect on, and to an extent have primacy over, the genetic basis for the changes involved. Evolutionary functional morphology has been relatively little considered in the context of the evolution of development, but may play an important role in defining the framework within which this evolution occurs.     

Why are most mutations recessive?

It is a long-standing observation that most mutations are recessive. That is, they do not lead to visible phenotypic effects when in heterozygous combination with the wild-type allele. The reason for this has long been debated. Fisher (1930) attributed the observed dominance of the wild type to the action of natural selection at modifier loci. Wright (1929) on the other hand asserted that dominance did not have a selective functionper se, but was a more-or-less automatic offshoot of genetic regulatory mechanisms. The present essay discusses these explanations from a contemporary standpoint and suggests that neither is likely to be valid exclusively. In particular, even when physiology appears to offer a sufficient explanation, evolution of dominance cannot be ruled out.     

Why are some birds polyterritorial?

Whether a male bird is mono- or polyterritorial may be determined by several factors, but a polyterritorial system tends to be found when territories are easy to defend, male competition is not great and the period of defence is short.     

Why are Hox genes clustered?

The evolutionarily conserved genomic organization of the Hox genes has been a puzzle ever since it was discovered that their order along the chromosome is similar to the order of their functional domains along the antero-posterior axis. Why has this colinearity been maintained throughout evolution? A close look at regulatory sequences from the mouse Hox clusters^(1,2) suggests that enhancer sharing between adjacent Hox genes may be one reason. Moreover, characterizing the activity of one of these mouse enhancers in Drosophila⁽²⁾ illustrates that despite many similarities, not all Hox clusters are built in the same way.     

Why are proteins marginally stable?

Most globular proteins are marginally stable regardless of size or activity. The most common interpretation is that proteins must be marginally stable in order to function, and so marginal stability represents the results of positive selection. We consider the issue of marginal stability directly using model proteins and the dynamical aspects of protein evolution in populations. We find that the marginal stability of proteins is an inherent property of proteins due to the high dimensionality of the sequence space, without regard to protein function. In this way, marginal stability can result from neutral, non-adaptive evolution. By allowing evolving protein sub-populations with different stability requirements for functionality to complete, we find that marginally stable populations of proteins tend to dominate. Our results show that functionalities consistent with marginal stability have a strong evolutionary advantage, and might arise because of the natural tendency of proteins towards marginal stability.     

Why environmental scientists are becoming Bayesians

Advances in computational statistics provide a general framework for the high‐dimensional models typically needed for ecological inference and prediction. Hierarchical Bayes (HB) represents a modelling structure with capacity to exploit diverse sources of information, to accommodate influences that are unknown (or unknowable), and to draw inference on large numbers of latent variables and parameters that describe complex relationships. Here I summarize the structure of HB and provide examples for common spatiotemporal problems. The flexible framework means that parameters, variables and latent variables can represent broader classes of model elements than are treated in traditional models. Inference and prediction depend on two types of stochasticity, including (1) uncertainty, which describes our knowledge of fixed quantities, it applies to all ‘unobservables’ (latent variables and parameters), and it declines asymptotically with sample size, and (2) variability, which applies to fluctuations that are not explained by deterministic processes and does not decline asymptotically with sample size. Examples demonstrate how different sources of stochasticity impact inference and prediction and how allowance for stochastic influences can guide research.     

Why metabolic systems are rarely chaotic

One of the mysteries surrounding the phenomenon of chaos is that it can rarely be found in biological systems. This has led to many discussions of the possible presence and interpretation of chaos in biological signals. It has caused empirical biologists to be very sceptical of models that have chaotic properties or even employ chaos for problem solving tasks. In this paper, it is demonstrated that there exists a possible mechanism that is part of the catalytical reaction mechanisms which may be responsible for controlling enzymatic reactions such that they do not become chaotic. It is proposed that where these mechanisms are not present or not effective, chaos may still occur in biological systems.