Adaptive radiations can be a direct consequence of three of the following four factors. select the exception.
a. vacant ecological niches
b. genetic drift
c. colonization of an isolated region that contains suitable habitat and few competitor species
d. evolutionary innovation

The correct answer is option b, that is, genetic drift.

Explanation:

Adaptive radiation refers to a procedure that exists in enhancing the diversity of life in the course of evolution. Various groups of species develop adaptive compositions to get adapt to the new surroundings and perform new ecological functions.  

These give rise to many new species that get amended with various new ecological habitats. The adaptive radiation occupies the vacant ecological niches. It results in the development of new and diversified characteristics within the species that take place in the niches with appropriate habitat. Evolutionary innovation is also a direct outcome of the adaptive radiation.  

Genetic drift is not a direct outcome of adaptive radiation, it refers to a process, which takes place in small populations, and because of certain chance incidences, the allele frequencies of populations modify with time.  

In this condition, either some of the individuals become distinguished from the main population and turn into new species or a huge population suddenly gets diminished to a small population, and therefore, modification in allelic frequency takes place.