How is chromatin structurally different from chromosomes?

The main difference between chromatin and chromosome is that chromatin consists of the unravelled condensed structure of DNA for the purpose of packaging into the nucleus whereas chromosome consists of the highest condensed structure of the DNA doublehelix

Darwin describes evolution through the mechanism of natural selection which is an explanation of how populations could evolve so that they became better suited to their environments over time.

Because resources are limited in nature, only those organisms with heritable traits (adaptations to the environment) that favor survival and reproduction will tend to survive and leave more offspring than other organisms.  This will cause the increase in the frequency of those traits over generations. That’s the way natural selection works, it causes populations to become adapted to their environments over time. Natural selection depends on the environment (it favors traits that are beneficial for the specific environment, that trait might be harmful in some other environment)  and requires existing heritable variation in a group (there must be variation, the difference between the individuals).