Question

what is reflex arc, why does reflex arc evolved in animals

Answer 1

Hey there !

Reflex arcs are caused dur to sudden environmental stimuli.
Reflex arc is the path of impulses in the reflex action. Reflex actions are sudden, involuntary reactions of the body in response to stimuli. They occur without the involvement of the conscious areas of the brain. Therefore, all reflex actions are unconscious actions. Somatic nervous system is responsible for reflex arcs.
Reflex actions occur both from the brain and spinal cord which are the parts of the central nervous system. Most often these actions are mainly formed from the spinal cord. When sudden responses are required, instructions are given by the spinal cord itself. Impulses reach the brain only after that. Later conscious responses are made. 

Hope it helps you !

Answer 2

A basic reflex arc consist of sensory neurons , motor neuron and the muscle or gland cells. Reflex arc, have evolved in animals, in order to perform quick responses, as the thinking process of brain is not fast enough.