Single-gene changes can drive speciation. Discuss with reference to left–right asymmetry in snails.

A new species forms only when there is reproductive isolation, meaning a barrier that stops genes flowing between populations. In snails, the direction a shell coils (dextral or sinistral) is set by a single maternal-effect gene. Flipping that gene mirrors the snail's whole body [1]. Snails therefore offer a rare natural test of whether one gene can split a species.

How one gene flips the body

• Coiling follows the mother's genotype, not the embryo's own, so a change shows up one generation late [1].
• In the pond snail Lymnaea, a frame-shift in the formin gene Lsdia1 changes early spiral cleavage [2]. In the land snail Euhadra, a 2026 UK–Japan study points to an unconventional myosin [1]. Both proteins act on the actin cytoskeleton.
• Handedness is fixed early: moving cells at the 8-cell stage reverses the adult's chirality [3].

How reversal can drive speciation

• Mechanical isolation: the genital pore moves to the other side, so mirror-image partners struggle to mate [1].
• Phylogenetic evidence: reversal evolved several separate times in Euhadra, which led to the "single-gene speciation" hypothesis [4]. Reversed populations have become fixed repeatedly in land snails [5].
• Ecological push: Pareas snakes have asymmetric jaws and mainly eat dextral snails, so sinistral snails survive better, which makes up for their mating cost [6]. The same gene is both an adaptation and an isolating barrier.

Maternal gene variant → mirrored cleavage → reversed body & genital pore
   → mating mismatch + predation advantage → reduced gene flow → incipient species

Fig: Single-gene route to speciation

Limits of the claim

• Leaky barrier: in a mixed left/right population in north Japan, found in 2023 through hikers' photos, the two forms appear to interbreed [1].
• Counter-adaptation: snakes can recognise which way their prey coils, which weakens the sinistral advantage [7].
• Frequency trap: a rare mutant finds few mates, so a reversal tends to die out unless selection favours it.
• Unconfirmed mechanism: the Euhadra myosin link has not yet been confirmed by genome editing, as was done for Lymnaea [2].

Snail chirality shows that one gene can start speciation by linking body form, mating and survival. Whether the split is completed depends on ecology and time. Genome editing, field genetics and citizen-science data together can measure how strong the barrier really is. Such work explains how biodiversity arises and helps conserve endemic species, in line with SDG 15.