Lock and key hypothesis

Definition

An evolutionary explanation for the rapid divergence of male and female genitalia (and associated recognition systems) positing that species-specific mechanical or chemical fit prevents interspecific mating, thereby maintaining reproductive isolation. The 'lock' represents female structures or sensory criteria; the 'key' represents corresponding male structures or signals. Originally formulated for genitalic incompatibility, the concept extends to pheromone-receptor systems and other species-specific recognition mechanisms.

Etymology

Example

In many cryptic species of Drosophila fruit flies, subtle differences in male genital arch shape create mechanical incompatibility with females of sister species, consistent with lock-and-key selection reinforcing reproductive isolation where ranges overlap.

Synonyms

• lock-and-key mechanism
• mechanical isolation hypothesis

Related Terms

• cryptic female choice
• sexual conflict
• reproductive isolation
• genitalic evolution
• species recognition
• mechanical isolation
• pleiotropy

Usage Notes

Historically attributed to Dufour (1844) and popularized by Eberhard (1985), though empirical support remains mixed. Modern usage distinguishes strict mechanical lock-and-key (preventing intromission or sperm transfer) from broader species recognition functions. Often contrasted with sexual selection by cryptic female choice or sexually antagonistic coevolution, which predict genitalic divergence driven by within-species dynamics rather than between-species avoidance. The hypothesis is difficult to test directly because mechanical incompatibility may evolve as a byproduct of other selective pressures (pleiotropy) rather than as an adaptive barrier to hybridization. In arachnology, the concept applies to complex pedipalp-spermathecal fit in spiders and scorpions.