This phenomenon has been attributed to a release from constraints on form and function driven by the high ratio of O2 supply to metabolic demand for these organisms [37], [38]; [ but see 39].
The hypothesis posits that a number of innovations to the vascular system provided the release from constraints which allowed the implementation of an AIS.
These innovations provided the evolutionary release from constraints that allowed the implementation of an AIS: whereas adipocytes buffer against the sudden metabolic demand associated with the activation of an AIS, vascular innovations decreased the cost of mobilising an antibody-based immune response and augmented the performance of an AIS (e.g., promoting affinity maturation).
When constructing the LRTs, the null hypothesis fixes ω2 = 1, allowing sites to evolve under the negative selection of the background lineages being released from constraint, and to evolve neutrally on the foreground lineage.
When constructing the LRTs, the null hypothesis fixes ω2 = 1, allowing sites evolving under negative selection in the background lineages to be released from constraint and to evolve neutrally on the foreground lineage; the alternative hypothesis constrains ω2 ≥ 1 [ 64].
Instead these data suggest a release from evolutionary constraint model, followed by aquatic diversifications within crown families.
This pattern of sequence divergence may be explained by the release from selective constraint for these redundant copies.
Taken together, these tests indicate that the recruitment of Sxl to the Drosophila sex determining cascade coincided with release from selective constraint and adaptive changes in the protein sequence.
So, activation of Rac1 removes an inhibitory effect of VIF on mitochondrial motility (Nekrasova et al., 2011) that might indicate their release from the constraint caused by their interaction with VIF.
However, the fact the branch lengths of both of the D. pulex NOS genes (especially that of Dappu-NOS2) are long relative to other branches within the tree suggests that both of these genes are either experiencing release from selective constraint or are subject to positive selection that has accelerated their evolution.
