Where does fixation collide?

The point at which one allele becomes the only variant of its gene in a population, so its frequency reaches 1 and all competing variants are lost: once an allele is fixed, variation at that locus is gone until new mutation or migration restores it, and the chance that a given mutant ever reaches this state is its fixation probability. Kimura's 1962 On the probability of fixation of mutant genes in a population derived that probability for a mutant under genetic drift and selection. The referent is a change in the genetic makeup of a whole population across generations, measured as an allele frequency, not a held gaze, a chemical conversion, or a preserved tissue.[1]

Unit

An allele frequency reaching 1 in a population

Recovery means

Once an allele is lost or fixed, that variation returns only through new mutation or migration

Measured by

Estimated from allele frequencies across generations, and modelled as a fixation probability under drift and selection

The interval during which the eye holds its gaze on a single point, keeping the image roughly still on the retina between the rapid jumps called saccades: most visual information is taken in during these pauses, and their number and duration index how the eye samples a scene or a line of text. Rayner's 1998 Eye movements in reading and information processing: 20 years of research reviewed how fixation duration and location track the moment-to-moment work of reading. The referent is a brief pause of the eyes on a target, timed in milliseconds, not an allele frequency, a chemical conversion, or a preserved specimen.[2]

Unit

A pause of the gaze on one point, in milliseconds

Recovery means

Not applicable

Measured by

Recorded by an eye tracker as fixation duration, location, and count between saccades

The conversion of a chemically inert, diffuse form of an element into a reactive compound that living systems or reactions can use: in nitrogen fixation, atmospheric N2 is turned into ammonia and related forms, whether by microbes or by industry, making the element available to organisms. Vitousek and colleagues' 2013 Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems reviewed how much nitrogen is fixed biologically and what limits the rate. The referent is a chemical transformation that fixes an element into a usable form, measured as a rate or a flux, not an allele frequency, a held gaze, or a preserved tissue.[3]

Unit

A rate of an element converted into a fixed compound

Recovery means

The fixed compound can be broken down again as the element cycles back toward its diffuse form

Measured by

Estimated as fixation rates or fluxes, by isotope tracing, mass balance, or enzyme assay

The chemical preservation of biological tissue so that its structure is held close to the living state for later study: a fixative such as formaldehyde cross-links proteins and halts decay, stabilising cells and molecules against autolysis before the specimen is sectioned and stained. Fox, Johnson, Whiting, and Roller's 1985 Formaldehyde fixation examined how that agent reacts with tissue and what it preserves. The referent is a bench procedure that stabilises a specimen for microscopy, judged by how well structure is preserved, not an allele frequency, a held gaze, or a converted element.[4]

Unit

A specimen preserved for study, by preservation quality

Recovery means

Not applicable

Measured by

Judged by how well tissue structure, antigens, and molecules survive across fixative and timing