Drift barrier

The drift barrier is a theoretical limit on the refinement of biological traits that arises when the fitness advantage of further improvement falls below the power of genetic drift to eliminate that improvement by chance. Proposed by Michael Lynch, the drift barrier predicts that organisms with smaller effective population sizes — and therefore stronger drift — should accumulate more mutational load, tolerate lower genomic quality, and maintain less efficient molecular machinery than organisms with larger effective population sizes. The barrier is not a single fixed threshold but a population-size-dependent filter: what selection can 'see' and preserve scales with the ratio of selection coefficient to drift magnitude (approximately 1/N_e). Empirical support includes the observed inverse relationship between effective population size and genome complexity — large genomes with abundant non-coding DNA are found in organisms with small effective populations, where drift is strong enough to fix slightly deleterious insertions that selection would otherwise purge.

The drift barrier matters for understanding evolvability: it implies that evolution's capacity to improve is itself a variable, constrained by demography. A population bottleneck does not merely reduce genetic diversity — it raises the drift barrier, making subsequent adaptation less precise. The relationship between drift barriers and niche construction is underexplored: organisms that construct stable, buffered niches may indirectly lower their own drift barriers by maintaining larger, more connected populations across generations.