Gene expression might be altered at the level of post-transcriptional processing in eukaryotes rather than prokaryotes because of

Explanation:

The transcription and translation occurs in different location in eukaryotes, while in prokaryotes, both the processes occurs at same location and can be co-translation.

As, in eukaryotes, the transcription occurs in the nucleus and then this transcript have to move in the cytoplasm to translation, so before reaching in the cytoplasm, various modifications occur in this transcript known as post transcriptional modification or processing ( include 5' capping, 3' poly-adenylation and RNA splicing).

As, in prokaryotes, both transcription and translation occur at same place so there is no time for modification for transcript because translation starts as the transcription is still going on.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.