How can natural selection lead to speciation?
Natural selection is a phenomenon in which the traits selected by the nature exist in a particular niche.
Let us assume that a colour variation arises among a beetle population living in a particular area. The variation takes place from red beetle to green beetles. The beetles in the area are eaten by crows. So red beetle on green leaves will be easily visible to the crows. So the crows will continue to eat the red beetles and thus the green beetles will be safe. As this continues, the red beetle population will experience extinction and green remains unaffected.
Thus a day shall come when only green beetles will be seen.