Final answer:
A police officer can seize evidence without a warrant if it is in plain sight during a lawful presence, a situation defined by the plain view doctrine. Probable cause is necessary for searches and seizures without a warrant. The exclusionary rule safeguards against the use of illegally obtained evidence in court.
Step-by-step explanation:
A police officer can sometimes seize evidence without a warrant when it is clearly visible. This is known as a plain view doctrine. The plain view doctrine allows police to seize evidence and contraband that are found in plain sight during a lawful observation. This could occur if an officer is legitimately on the premises and discovers the incriminating evidence inadvertently.
The requirement for a search warrant can be bypassed under certain conditions. These include situations where there is consent for the search, exigent circumstances, or when the evidence is in plain view. The concept of probable cause is pivotal in these instances, serving as the legal standard for whether a search or seizure can be conducted without a warrant.
The integrity of evidence is protected by the exclusionary rule, which prevents the use of evidence obtained illegally - that is, without a warrant or falling outside of the exceptions (such as the plain view doctrine). This rule extends to any further evidence that is derived from the original illegally obtained evidence, referred to as the 'fruit of the poisonous tree.'