Apparently, constraint is quite strict:
On one hand, despite dramatic mutations in individuals’ genes and diverse environments in which they grow, members of a species develop into strikingly similar creatures. This robustness ensures that almost all individuals are functional. On the other hand, for evolution to occur, members of a species need diverse traits that natural selection can act upon. Those two forces — robustness and evolvability — tug in opposite directions. One wants less variation, and one wants more.
Around 20 years ago, biologists expected genetics and environmental factors to produce substantial heterogeneity, giving natural selection plenty of choice, said Alex Lancaster, an evolutionary biologist at the Ronin Institute in New Jersey who wasn’t involved in the new study. But, he said, more recent observations have attested to unexpected similarity across populations…
The photos of fly wings offered no clues as to the mechanisms that restrict the possible morphologies that can develop. Rather, the results substantiated the extensive power of these guardrails. Natural selection must mostly act on the significant diversity exhibited in the small number of linked, variable traits, while robustness tightly constrains the rest. Elena Renken, “Mathematical Analysis of Fruit Fly Wings Hints at Evolution’s Limits” at Quanta
Evolution has LIMITS? Isn’t it supposed to account for everything? Put another way, consider the Darwinian claim:
It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, wherever and whenever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.
The claim is doubtful, given the huge constraints on the system.
The paper is open access.