Which are required for natural selection to occur?

three conditions must be met: There must be variation for the particular trait within a population. The variation must be inheritable (that is, it must be capable of being passed on from the parents to their offspring).

Explanation:

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the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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