Step 1: Chromosomal inversion.
Chromosomal inversions can result in tightly linked genes because the genes that are part of the inverted region will tend to be inherited together due to the altered structure of the chromosome. This prevents recombination between the genes in the inverted segment, effectively linking them.
Step 2: Explanation of the other options.
Gene duplication involves the creation of multiple gene copies, but this does not necessarily result in tight linkage. It can lead to paralogs that may diverge over time.
Deletion or insertion of an exon can affect gene structure but does not generate tightly linked genes.
The graph shows the relationship between a variable on the x-axis and genetic diversity on the y-axis. Each point represents a species and the trend line describes the relationship across species.

Select the most appropriate variable for the x-axis.