Where does drift collide?

Random change in the frequency of an allele from one generation to the next, arising because a finite population passes on only a sample of its genes. Wright set the idea out in his 1931 Evolution in Mendelian Populations, where it works alongside selection, mutation, and migration. The referent is an allele frequency in a population; the change has no direction and no adaptive purpose, and its strength scales inversely with population size, so that in small populations frequencies wander fast and variation is lost. It differs from selection, which pushes frequencies in a direction: genetic drift is pure sampling error, a change caused by chance rather than by any advantage.[1]

Unit

The frequency of an allele in a finite population

Recovery means

Not applicable

Measured by

Change in allele frequencies across generations; the per-generation variance is about 1 over twice the effective population size

The change over time in the statistical relationship between a model's inputs and its target, which degrades a model that was fit to the old relationship. Gama, Žliobaitė, Bifet, Pechenizkiy, and Bouchachia surveyed it in 2014 in A survey on concept drift adaptation, building on Widmer and Kubat, who in 1996 studied learning under hidden contexts that change. The referent is the joint distribution of inputs and target, or the conditional distribution of the target given the inputs, that a deployed model depends on. Nothing is copied across generations; what moves is the world the model was trained on. Unlike genetic drift, the shift is often driven by an outside cause and is not undirected sampling error.[2][3]

Unit

The conditional relation of target to inputs that a model relies on

Recovery means

Relevant: an adaptive learner detects the change and updates or retrains the model to recover accuracy

Measured by

Falling predictive accuracy on incoming data, and statistical tests comparing the input or error distributions across time windows

Wegener's hypothesis that the continents are not fixed but move across the surface of the Earth, once joined in a single landmass and since parted. He proposed it in 1912 and developed it in The Origin of Continents and Oceans. The referent is a continent's position on the globe, changing over millions of years. The mechanism Wegener offered was wrong, and the idea was later subsumed by plate tectonics, but the drift itself, the motion of the continents, is real and is now measured by geodesy in centimetres per year. This is literal movement of matter across space, not a change in a frequency or a probability distribution.[4]

Unit

A continent's position on the Earth's surface

Recovery means

Not applicable

Measured by

Reconstructed past positions from fossils, rock and glacial records; today, plate velocities from satellite geodesy in centimetres per year

A slow, unwanted change in a measuring instrument's output over time while the quantity being measured is held constant. The International Vocabulary of Metrology defines instrumental drift as a continuous or incremental change in indication that comes from changes in the instrument itself, in its temperature or its ageing components, rather than from any change in the measurand. The referent is a sensor's reading under a fixed reference input; the change is a defect to be corrected, and recalibration against a standard removes it. Here drift is a property of an instrument, not of a population, a model, or a planet, and it is unwanted by definition.[5]

Unit

A sensor's indicated output under a constant reference input

Recovery means

Relevant: recalibration against a known reference removes accumulated drift

Measured by

The change in indicated output over time while a stable reference input is applied, expressed per hour, per day, or per year

In safety science, drift refers to drift into failure, the slow erosion of safety margins in complex systems through decisions that each seem reasonable at the time. This meaning comes from Dekker (2010) in Drift into failure.[6]

Unit

Recovery means

Measured by