Final answer:
Commensalism is a symbiotic relationship where one species benefits and the other is unaffected. It can include scenarios where coevolution takes place, and it's hard to definitively prove the unaffected species gains no benefit.
Step-by-step explanation:
Commensalism is a type of symbiotic relationship in which one species benefits and the other is unaffected. An example of a commensal relationship is when birds nest in trees; the birds get a safe place to live and raise their young, while the tree does not suffer any harm – a situation that is neither beneficial nor detrimental to the tree. In commensal relationships, while one species may find the association obligatory for its survival or well-being, it is not necessarily so for the other species that remains unaffected. Additionally, coevolution may occur in commensal relationships, with the benefiting species evolving traits that reinforce the association.
The competitive exclusion principle states that no two species can coexist if they compete for the exact same resources in the same habitat. On the other hand, in a commensal relationship, since one species is unaffected, competitive exclusion does not take place.
While many commensal associations are evident, in certain cases it can be challenging to affirm with certainty that the unaffected species derives no benefits. Hence, it is difficult to prove that one partner does not gain some benefit from the presence of the other in potential commensal relationships.