Correct option:(4)Genetic drift
Solution:

• Change in gene frequency in a small population by chance is called genetic drift.
• Genetic drift has two ramifications, one is bottle neck effect and another is founder’s effect.
• When accidentally a few individuals are dispersed and act as founders of a new isolated population, founder’s effect is known as observed.
• Crossing over which occurs during gamete formation results in genetic recombination.
• Mutations are random and directionless.