Why the same genes can build very different bodies
A large part of what makes a trait specific is not a new gene but a new pattern of gene activity. Research across roughly 600 million years of bilaterian evolution — animals with a front and back, left and right — found that regulated gene expression and alternative splicing, the process that lets one gene produce several different protein versions, both contribute to tissue-level differences within and between species [1]. The study focused on genes conserved across bilaterians and compared eight tissues across twenty species, including vertebrates and insects, identifying ancestral tissue-specific expression patterns as well as convergent and divergent evolutionary paths [1].
This matters because it means specificity can arise from rewiring rather than invention. A gene already present in an ancestor can be deployed in a new tissue, at a new time, or in a new spliced form, producing a characteristic that looks novel without requiring a wholly new gene [1]. The same logic appears at the cellular level: the emergence of specialized tissues with different functions was a pivotal step in animal evolution, and cell specialization arises from a combination of intrinsic factors, such as a cell's developmental history, and extrinsic factors, such as its local environment [4].
How a strong environmental pressure shapes a trait
Specific characteristics often track specific challenges. Fire is one such pressure: it has been described as a pervasive driver of trait evolution in animals, and its influence may grow as fire regimes change rapidly in coming decades [5]. That framing places environmental pressure at the center of the explanation — the trait is specific because the challenge is specific and recurring.
A broader review of adaptive evolution in wild animals and plants points to molecular mechanisms underlying morphological, behavioral, and physiological traits across different taxonomic groups, and notes that specific genes, or specific features of them, are involved [2]. Read together with the fire example, the picture is one in which selection acts on the molecular machinery that builds a trait, and the trait's specificity reflects the specificity of the demand placed on the animal [2][5].
What domestication and feralization reveal about how fixed a trait is
Some specific traits come as a package. Domesticated animals share a set of cognitive, behavioral, and physical features known as the domestication syndrome, which researchers have hypothesized results from sexual selection of individuals with lower aggression in response to changing environmental factors [3]. The human self-domestication hypothesis extends this idea, proposing that some distinctive human characteristics arise partly through a domestication-like process, with one proposed consequence being a cultural niche that allows language to become more complex [3]. These are hypotheses about possible mechanisms, not settled causes.
The reverse experiment is informative. Feralization, in which a domesticated animal returns to a wild-like environment, does not appear to be a complete reversal of domestication; instead, certain physical, behavioral, and neurobiological traits revert to some extent, and only a limited number of genes seem to be involved in both processes [3]. That suggests specific traits are not simply switched on and off by a single master control, but are woven into a broader developmental and genetic context that constrains how far they can change [3].
Sources used in this answer
Evolution of tissue-specific transcriptomes across bilaterian animals
Regulated gene expression and alternative splicing both contribute to tissue-specific transcriptome differences across about 600 million years of bilaterian evolution, with ancestral tissue-specific patterns and convergent, parallel, and divergent trajectories identified among vertebrates and insects.
Molecular mechanisms of adaptive evolution in wild animals and plants
A review of adaptive evolution in wild animals and plants describes molecular mechanisms underlying morphological, behavioral, and physiological traits across taxonomic groups, involving specific genes or specific features of them.
Human Self-domestication and Language Evolution: Focusing on Disorders
The human self-domestication hypothesis proposes that some human characteristics arise partly through a domestication-like process, and feralization appears to reverse only some domestication traits, with a limited number of genes shared between the two processes.
Endothelial ontogeny and the establishment of vascular heterogeneity
Cell specialization, which enabled specialized tissues in animals, arises from a combination of intrinsic factors such as cellular ontogeny and extrinsic factors such as local environment, with lineage history proposed as a factor in establishing endothelial heterogeneity.
Fire-adapted traits in animals
Fire is described as a pervasive driver of trait evolution in animals, and its importance may increase as fire regimes change in coming decades.