In a way that is accidental, incidental, or not inherent; by chance or from an external source rather than naturally.
From 'adventitious' (coming from outside, accidental) plus the adverbial suffix '-ly.' The root 'adventitious' comes from Latin 'adventitius' (foreign, extraneous).
When mutations arise 'adventitiously' in organisms, they're accidental changes that happen to have beneficial effects—this is how evolution works, with random changes that just happen to work out.
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