Evolution or alteration means change over time. Sameness over time is called permanence or stability. The study of change or the lack of change over time is called history or diachrony.
Change happens. But sameness happens, too. One easily sees that sameness happens in the natural realm much more than change. That is not the result of chance but of law, which is why natural science is able to articulate laws and predict the future. The natural future is like the natural past.
Some changes are unpredictable individually but have predictable distributions or aggregates. Many sciences from statistical mechanics to quantum mechanics to genetics are stochastic in nature.
Similar to the coastline paradox, the amount of change depends on the length of the “ruler” used to measure change. If it is a small ruler, one measures minutiae, and more changes will be found. If it is a large ruler, one measures key features, and fewer changes will be found.
The conceit of evolutionary biology is that very low rates of unpredictable change over very long periods of time can result in all the biological diversity of today. It is an appeal to the imagination more than an appeal to knowledge. Without imagination, the argument becomes an assertion of mere possibility, rather than plausibility, probability, or necessity.
But if very low rates of unpredictable change can determine what happens, how much more can very high rates of predictable stability. One does not need to appeal to the imagination to see that stability is the rule, and exceptions only prove the rule.
Organisms are similar in some respects but not in other respects. If one focuses on minutiae, there are many differences. It is part of the conceit of evolutionary biology to overstate the importance of minor differences such as color and understate the importance of major differences such as body plan.
One might hope that biologists would be working toward finding the optimum characteristics to measure biological change. Alas, they are determined to find the smallest ruler and overstate change as much as possible.
I predict that a more mature biology will seek the optimum measure of change, and will accept that some characteristics are permanent features of a body type.