Meaning: 'Their' is used to indicate that something belongs to or is associated with the people or things previously mentioned, in this case, the survivors' adaptive traits.

Wrong options:

• that: The word 'that' is a relative pronoun used to introduce a clause, but it does not indicate possession, making it grammatically incorrect in this context.
• its: 'Its' is a possessive pronoun but refers to a singular noun, which does not match the plural 'survivors' in this context.
• they: 'They' is a pronoun used to refer to people or things previously mentioned, but it does not indicate possession, thus it does not fit the grammatical structure needed here.

Correct answer: to evolve

Explanation: The phrase 'to evolve' is an infinitive form that correctly indicates the purpose or intention of the time it may take for complex traits to develop. It fits grammatically as it follows the modal structure of 'may take' and contextually aligns with the idea of evolution as a process.

Meaning: 'To evolve' means to develop gradually, especially from a simple to a more complex form, which is relevant to the context of evolutionary biology.

Wrong options:

• having evolved: This gerund phrase suggests a completed action, which does not fit the context of discussing the ongoing process of evolution over time.
• evolving: This present participle form implies an ongoing action but does not fit the grammatical structure required after 'may take'.
• evolved: This past participle indicates a completed action, which does not align with the context of discussing the future process of evolution.

Correct answer: in action

Explanation: The phrase 'in action' is an idiomatic expression that indicates something is currently happening or being demonstrated. It fits grammatically and contextually, as it describes the process of natural selection being observable.

Meaning: 'In action' means actively occurring or functioning, which is appropriate in the context of observing natural selection processes.

Wrong options:

• despite itself: This phrase suggests something happening against its own nature, which does not fit the context of observing a natural process.
• on demand: This phrase implies something that can be activated or requested, which does not align with the ongoing observation of natural selection.
• in detail: This phrase suggests a thorough examination, which does not fit the context of simply observing the process in its active state.

Correct answer: across

Explanation: The word 'across' is a preposition that indicates movement or distribution over a range or area. It fits grammatically and contextually, as it describes the tracking of mutations over multiple generations.

Meaning: 'Across' means from one side to the other or throughout, which is relevant in the context of examining changes over generations.

Wrong options:

• except: This preposition indicates exclusion, which does not fit the context of tracking changes across generations.
• above: This preposition suggests a position higher than something else, which does not apply to the context of generational tracking.
• upon: This preposition indicates a position on a surface or a condition, which does not fit the context of tracking changes over time.