Relating to or involving optional ability rather than strict requirement; in biology, describing organisms that can function under alternative conditions.
From Medieval Latin facultativus, from facultas 'power, ability.' Applied in modern biology to describe organisms with metabolic flexibility, versus obligate specialists.
Facultative parasites can survive without a host, facultative anaerobes can survive without oxygen, and facultative bipeds can walk on two legs OR four—the word describes biological flexibility!
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