Theory 022/Life & Evolution /Evolutionary theory
Sex Persists Above a Parasite Threshold
Sex is expensive and universal. The advantage must appear only under some conditions, and those conditions may have a threshold.
Sexual reproduction is maintained only when parasite diversity exceeds a threshold, so low-parasite environments should select for asexuality, and the transition should be abrupt rather than gradual.
01The evidence
Asexual lineages occur in many groups and are usually short-lived, while sexual reproduction persists despite its twofold cost, which suggests a strong conditional advantage.
02The tension
Parasite-driven models explain why sex can be favoured on average, and they struggle to explain the persistence of asexuals in some environments and their rarity overall.
03The mechanism
Parasites adapt to common host genotypes, which gives rare genotypes an advantage. Recombination creates rare genotypes, so its benefit scales with parasite diversity and with the speed of parasite adaptation.
Below a threshold diversity the advantage is too small to pay the twofold cost, so asexuality should win; above it, sex should win. The transition should be sharp because the benefit grows nonlinearly with diversity.
04What this is not
Those models compute an average benefit. This makes the benefit conditional on a threshold, which predicts sharp transitions and explains asexual lineages in simple parasite communities.
05Predictions
- P1Asexuals should occur where parasite diversity is low.
- P2The transition should be abrupt across a diversity gradient.
- P3Experimental parasite enrichment should favour sex.
06Refutation
Sexual lineages persisting at low parasite diversity with no other cost explanation would weaken the threshold.
07What it changes
It would make parasite diversity a predictor of reproductive mode.