Question $5($Mandatory$)(0\mathrm{.}625$ points$)$

Why did Kingsley and his team cross marine and freshwater stickleback fish?

To compare the Pitx$1$ protein$-$coding sequence from fish with and without pelvic spines.

To find the location of the gene$($s$)$ causing the differences among stickleback populations with and without spines.

To understand the timing of the expression of gene$($s$)$ for making pelvic spines in embryonic development.

To test whether Pitx$1$ is involved in the formation of pelvic spines.