0 a part of a machine that allows movement in one direction only. It is usually a wheel with teeth-like parts that either slide over or lock against the free end of a bar: --
1 a wooden device that when turned around and around produces a noise like a series of knocks --
2 something that makes a situation change or develop in one direction only, for example by being able to rise but not fall or the other way round: --
4 an arrangement by which managers are given increasing numbers of shares in a company in relation to increases in its profits or success: --
5 a part of a machine that allows controlled movement in one direction only --
The figure shows large populations needing a mutation rate only slightly larger than the mean incoming s to ensure a drive to the ratchet.
Medical cost control has followed a ratchet-like pattern quite different from the zigzag of change and reversal seen in some other areas of health policy.
This is described as a ' ratchet ' process because only back-mutation can reverse it.
All other parameters act only as scaling factors and do not influence the operation of the ratchet.
If this least-loaded class is lost in the whole population, a click of the ratchet is recorded.
We have introduced in our random-network model these events in order to assess how important extinction is for the speed of the ratchet.
The same will be true if the ratchet has advanced sufficiently far that a substantial proportion of loci are fixed within haploid sets.
Although increasing migration does slow the ratchet down, its speed is always higher in the structured population, for all values of m tested.
