Is there any kind of "unintended consequence" in evolutionary systems?
Preamble
Evolutionary systems do not have these exact terms as far as I'm aware (although I'm not a behavioral expert) This mainly is because generally speaking we must stop thinking about traits in terms of &quot;advantageous&quot; or &quot;intentional consequences&quot; when discussing evolution. However there are countless examples where a seemingly detrimental evolutionary trait flourishes, or when a seemingly beneficial trait can be exploited by other species. Here I point out two simple, well known examples that might be what you're looking for.
Physiological disease trait emerges
The most well known is sickle cell anemia (SCA); a disease that also reduces susceptibility to malaria. In areas of the world with high instances of malaria, there are also high instances of SCA (Piel et al 2010).
Behavioral altruistic trait exploited
For behavioral studies I think humans would be too vague... (okay, I know very little about sociology) But there is one story that springs to mind from another animal. Birds are ideal models for behavioral studies.
The cuckoo and warbler are an endless evolutionary arms race in terms of camouflage and behavior. The cuckoo parasitises the nest of the warbler with a single egg in a clutch of 3 or so warbler eggs. The warbler parents will watch the newly born cuckoo kick the other eggs out of the nest, and then continue to feed the cuckoo chick thinking it is their own. The reason is thought to be because the warblers respond to the feeding call of their chicks (altruistic, albeit from parent to chick), however the cuckoo chick can mimic this call much more intensely than an entire brood of warbler chicks (altruism is exploited)(Davies et al 1998). By the end, the tired parents are feeding a chick three times their size.