Gene flow is :
1. the migration of species populations to new areas
2. the exchange of genes during fertilization
3. mating between close relatives
4. the movement of genes between species populations

Genetic drift is :
1. random changes in gene frequency in a population
2. generational fluctuations in gene frequencies that produce no effect
3. changes due to interbreeding with other species populations
4. the effect of mutations as they spread through neighbouring populations

The type of natural selection that operates to eliminate intermediate phenotypes in a population is known as: