Why can non-adaptive traits stick around?
A trait can survive simply because it is not costly enough to be eliminated. In bacteria, experiments with E. coli show that latent traits—abilities that have no immediate benefit—can emerge and later become useful when the environment changes. The study found that environmental complexity is more important than mutation in driving the evolution of these latent traits, and that survival in multiple novel environments depends on genetic variation affecting non-adaptive traits as well [4]. This suggests that non-adaptive traits can persist as a kind of hidden reservoir, ready to be co-opted later.
Another route for non-adaptive traits to persist is through acquisition from other species. Many organisms endure natural selection not with traits they possess intrinsically but with substances and abilities they acquire from other species through ecological interactions—akin to outsourcing, pillaging, or fraud [3]. These acquired traits can be adaptive for the recipient even if they are not encoded in its own genome, showing that adaptation can come from outside as well as from within.
What do clearly adaptive traits look like in extreme habitats?
In some cases, traits are so tightly linked to survival that their adaptive value is unmistakable. The snow alga Sanguina nivaloides, which forms red snowfields in high mountains and polar regions, has cysts with a wrinkled plasma membrane that expands the interface with the environment, a chloroplast with thylakoids oriented in all directions, and cytosolic droplets loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress [1]. These features are described as adaptive traits that account for the alga's capacity to reside in snow, where it survives for months and remains photosynthetically active [1].
Similarly, black mangroves expanding into colder regions show genetically based trait shifts. Range-margin cohorts survived in greater numbers, established more quickly, and were less stressed under winter temperatures compared to range-core cohorts, and they invested more into lateral growth and biomass accumulation [5]. These phenotypic differences are consistent with adaptation to a colder, open-canopy environment and may facilitate further poleward expansion [5]. Here, the adaptive value of the traits is directly tied to survival and establishment in a stressful habitat.
Do microbes show the same pattern?
Pelagic freshwater microbes offer another angle. A review of genomic traits in these organisms identified metabolic differences associated with transport functions, environmental information processing, osmoregulation, and other functions that enable them to thrive in their habitat [2]. The review emphasizes adaptive traits and metabolic differences, but it does not explore whether non-adaptive traits could also contribute to survival. This leaves open the possibility that, even in well-studied microbial systems, some traits may persist without being adaptive, but the evidence presented focuses on traits that are interpreted as adaptive.
Sources used in this answer
Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging
The snow alga Sanguina nivaloides has cyst traits—such as a wrinkled plasma membrane, starch storage, and lipid droplets—that are described as adaptive for surviving in snow, but the paper does not explore whether non-adaptive traits could also contribute to survival.
Adaptive genetic traits in pelagic freshwater microbes
A review of pelagic freshwater microbes identifies genomic traits related to transport, osmoregulation, and metabolism that are interpreted as adaptive for thriving in lake and ocean water, without addressing non-adaptive traits.
Acquisition of Adaptive Traits via Interspecific Association: Ecological Consequences and Applications
Many species survive not only with innate traits but also with traits acquired from other species through ecological interactions, which can be adaptive for the recipient.
Environmental complexity is more important than mutation in driving the evolution of latent novel traits in E. coli
In E. coli, latent non-adaptive traits can emerge and later become useful, and environmental complexity is more important than mutation in driving the evolution of such traits.
Genetically based adaptive trait shifts at an expanding mangrove range margin
Range-margin black mangrove cohorts show genetically based trait shifts—such as greater survival, faster establishment, and cold tolerance—that are consistent with adaptation to colder environments.