1. 生存
    If you refer to the survival of something or someone, you mean that they manage to continue or exist in spite of difficult circumstances.

    e.g. ...companies which have been struggling for survival in the advancing recession...
           在日益萧条的经济环境中挣扎求生的各个公司
    e.g. Ask for the free booklet 'Debt: a Survival Guide'.
           可以索要免费小册子《债务：生存指南》。

2. 幸存;存活
    If you refer to the survival of a person or living thing, you mean that they live through a dangerous situation in which it was possible that they might die.

survival的解释

    e.g. If cancers are spotted early there's a high chance of survival...
           如果癌症在早期发现的话，存活的几率会很高。
    e.g. An animal's sense of smell is still crucial to its survival.
           动物的嗅觉对其生存仍然至关重要。

3. 适者生存
    You can use the survival of the fittest to refer to a situation in which only the strongest people or things continue to live or be successful, while the others die or fail.