Prokaryotic genomes of endosymbionts and parasites are examples of naturally evolved minimal cells, the study of which can shed light on life in its minimum form. Their diverse biology, their lack of a large set of orthologous genes and the existence of essential linage (and environmentally) specific genes all illustrate the diversity of genes building up naturally evolved minimal cells. This conclusion is reinforced by the fact that sometimes the same essential function is performed by genes from different evolutionary origins. (...) Nevertheless, all cells perform a set of life‐essential functions however different their cell machinery and environment in which they thrive. An upcoming challenge for biologists will be to discern, by studying differences and similarities in current biodiversity, how cells with reduced genomes have adapted while retaining all basic life‐supporting functions. (shrink)

Notwithstanding the many strengths of the dynamic developmental theory, there remain challenges to be overcome before it can be incorporated into a true causal model of attention-deficit/hyperactivity disorder (ADHD). These include the development of reliable measures of reinforcement delay gradients, the validation of shortened reinforcement delay as an endophenotype, and the integration of this pathway with other potential pathways.

Sagvolden, Johansen, Aase, and Russell (Sagvolden et al.) examine attention-deficit/hyperactivity disorder (ADHD) at levels of analysis ranging from neurotransmitters to behavior. At the behavioral level they attribute aspects of ADHD to anomalies of delay-of-reinforcement gradients. With a normal gradient, responses followed after a long delay by a reinforcer may share in the effects of that reinforcer; with a diminished or steepened gradient they may fail to do so. Steepened gradients differentially select rapidly emitted responses (hyperactivity), and they limit the (...) effectiveness with which extended stimuli become conditioned reinforcers, so that observing behavior is less well maintained (attention deficit). Impulsiveness also follows from steepened gradients, which increase the effectiveness of smaller, more immediate consequences relative to larger, more delayed ones. Individuals who vary in the degree to which their delay gradients are steepened will show different balances between hyperactivity and attention deficit. Given the range of ADHD phenomena addressed, it may be unnecessary to appeal to additional behavioral processes such as extinction deficit. Extinction deficit is more likely a derivative of attention deficit, in that failure to attend to stimuli differentially correlated with extinction should slow its progress. The account suggests how relatively small differences in delay gradients early in development might engender behavioral interactions leading to very large differences later on. The steepened gradients presumably originate in properties of neurotransmitter function, but behavioral interventions that use consistently short delays of reinforcement to build higher-order behavioral units as a scaffolding to support complex cognitive and social skills may nonetheless be feasible. Key Words: ADHD; attention deficit; delay gradient; exponential decay; extinction deficit; hyperactivity; impulsiveness; intervention; observing responses; self-control. Footnotes1000 Jeffrey Gray was the BBS Editor for this treatment. When he accepted Sagvolden et al. for publication, he invited Catania, who was one of the reviewers of the original submission, to prepare a precommentary. Com-mentators were then invited to respond to the Sagvolden et al. article, to the Catania precommentary, or to both. (shrink)