Answer:
It means that species are selective when they are on copulation, and try to mix their genes with other individuals of its own population.
Yes, there are some exceptions like when there is not a lot of population and the individuals have to copulate with very close gene individuals to keep the species alive.
Explanation:
Gene flow has 2 important complex states and are: inbreeding and exogamy.
in Inbreeding, very close individuals from an species (Brother and sister, mother and son, father and daughter etc.) coopulate and generate new individuals, this copulation is something that doesn't help the species because stimulates recessive genes, and makes the population more susceptible to diseases.
In exogamy it's the contrary, mix genes from two individuals who are really different genitecally causes that the individuals have no common characteristics and they are more susceptible to the environment.
