Which social theorist believed that societies could evolve through time by adapting to changing conditions?

The answer is: Herbert Spencer

He is known as the one that coined the phrase 'Survival of the fittest'. He believed that our environment will currently changing as time progress. That changes will force people in societies to adapt and change their behavior in order to survive in the new condition.

In modern time phenomenon we, One example that we can use is the development of social media. In the past, marketing and political campaigns relied heavily on television and door to door interaction. Social media changes that. The only corporations and politicians that can survive are the one that willing to implement social media into their strategies.