Once cells with nuclei developed, organisms were able to become more complex, which allowed them to adapt to the conditions in more habitats. By which process do organisms adapt and evolve?

Organism tend to adapt and evolve through natural selection.

Explanation:

Natural Selection can be described as a phenomenon by which those organisms are favored and selected by nature which are better adapted to live in an environment. These organism are able to reproduce and pass on their favorable traits to their offspring. Through natural selection, evolution occurs with the passage of time as those traits which are more suited to the ecosystem start existing in organisms.