Why some body plans are hard to change
One kind of limit comes from development. In mammals, the number and arrangement of vertebrae (the bones of the spine) vary between species, but the range of possible patterns is not unlimited. A study of vertebral formulae across mammals found that developmental constraints restrict how much this trait can change over evolutionary time [4]. In plain terms, the way an embryo builds its spine makes some arrangements much easier to produce than others, so evolution tends to stay within a narrower set of possibilities.
This does not mean vertebrae can never change. It means the changes that actually appear are biased toward those that development can readily produce. The same logic likely applies to other body plans, though the evidence here comes specifically from the mammalian spine [4].
When most changes are weeded out
A second kind of limit is negative selection, which removes individuals carrying harmful changes. In primates, a family of proteins called classical cadherins, which help cells stick together and signal to one another, shows strong negative selection on some members. Specifically, CDH2, CDH4, and most type II cadherins have been under significantly stronger negative selection than CDH1, CDH3, CDH5, and CDH19 [5]. This means that, for those proteins, most amino-acid changes that arise are likely to be harmful and are removed from the population.
The study suggests that this strong constraint is linked to where those proteins are expressed: cadherins used in the central nervous system show a negative correlation between expression level and evolutionary rate, unlike most cell-surface proteins [5]. The interpretation offered is that because these proteins are important for building and wiring the nervous system, changes to them tend to reduce fitness. That is a plausible explanation, not a proven cause, but it illustrates how functional importance can translate into an evolutionary limit.
How much of the genome is held back
At the level of DNA, evolutionary constraint can be measured by comparing genomes across species. A comparison of 29 mammals found that the amount of evolutionary constraint acting across those species is correlated with constraint within the human genome [2]. In other words, many stretches of DNA are conserved because changes there tend to be harmful, and this pattern is shared across mammals.
Constraint is not uniform across all DNA. A study of the branch point sequence, a short stretch of DNA needed for splicing RNA, found that in 14 species with largely unbiased variant data, the fourth and sixth positions of this sequence are more constrained than coding nucleotides [3]. That means changes at those positions are even more strongly disfavored than changes that alter protein sequence. The same study also warns that public variant databases can contain biases that distort such measurements, so the exact degree of constraint should be read with that caveat [3].
Do limits apply to intelligence too?
The question of limits also applies to cognition. A 2024 paper argues that comparative cognition has focused mostly on what animals can do, and that identifying cognitive limits is equally important for understanding how cognition evolves [1]. The authors point out that inferring a limit is difficult: a negative result could mean a true limit, or it could mean the task was poorly designed, and publication bias favors positive results [1].
This is a different kind of limit from the genetic and developmental ones. It is not yet established which cognitive limits are hard evolutionary ceilings and which are simply gaps in current testing. The paper proposes methods to distinguish these possibilities, but does not claim to have identified specific fixed limits [1]. So for cognition, the honest answer is that limits likely exist, but how tight they are remains an open question.
Sources used in this answer
Understanding the limits to animal cognition
The paper argues that studying cognitive limits in animals is as important as studying their abilities, and proposes methods to identify such limits despite challenges of interpretation and publication bias.
A high-resolution map of human evolutionary constraint using 29 mammals
A comparison of 29 mammal genomes found that evolutionary constraint across those species is correlated with constraint within the human genome.
Exploiting public databases of genomic variation to quantify evolutionary constraint on the branch point sequence in 30 plant and animal species
In 14 species with largely unbiased variant data, the fourth and sixth positions of the branch point sequence are more constrained than coding nucleotides, though public databases can contain biases.
Evolution of the vertebral formulae in mammals: a perspective on developmental constraints
Developmental constraints restrict the range of vertebral formulae that can evolve in mammals.
Classical cadherins evolutionary constraints in primates is associated with their expression in the central nervous system
In primates, CDH2, CDH4, and most type II cadherins have experienced significantly stronger negative selection than several other cadherins, and this strong constraint is associated with their expression in the central nervous system.