An intervening variable facilitates a better understanding of the relationship between the independent and dependent variables when the variables appear to not have a definite connection. They are studied by means of operational definitions and have no existence apart.
Macroevolution is the process that takes place over several thousands of years and describes how humans have evolved from primates and how reptiles turned into birds. Microevolution leads to small changes in the same species whereas macroevolution leads to the creation of a new species from parent species.
(mÄ«'krÅ-Äv'É-l'shÉn) Evolutionary change below the level of the species, resulting from relatively small genetic variations. Microevolution produces new strains of microorganisms, for example, or the rise of a new subspecies. The accumulation of many microevolutionary changes results in macroevolution.