In ontogeny, TIMING IS EVERYTHING.
In zebras, for example, the striped pattern is determined by a series of genes that "turn on" the production of a dark pigment (melanin) on only certain parts of the body. This can happen at any point in development, and differs between species. This is why different species of zebras have different types of striping.
Plains Zebra has relatively wide stripes...
Picture it this way...
Blow up a balloon, and then paint evenly-spaced stripes on it. Then continue to blow up the balloon, and you'll end up with a striped balloon with the same number of stripes you painted, only wider.
To compare how the different stripe patterns develop in zebras, let's make each species into one of two different balloons.
Now watch them both grow, and compare the finished stripe patterns.
The balloon whose striping was "turned on" earlier developed large, widely spaced stripes. The balloon whose striping was "turned on" later developed thin, narrowly-spaced stripes. This is very similar to what happens in the two species of zebras.
This is another example of HETEROCHRONY: The two species have similar genes, but they "turn on" at different times in development, resulting in completely different looking animals!