The passionate focus on evolutionary relationships in biology papers tends to obscure awe at the wonders in life.
Let’s consider the eye, which is but one of many subsystems (along with the brain, heart, liver, lungs, etc.) in higher animals that coordinate their tasks to keep an organism alive.
One of the central pillars of the standard evolutionary model is the belief that all living species evolved from a common ancestor through a gradually unfolding tree of life.
The changes involved in adapting a generic mammalian template into a whale are certainly not all simple, independent, single-letter changes.
A bottleneck of two, or a first pair at our origin older than 500,000 years, is possible.