A paleontologist identified two distinct species of mollusks in the fossil record of a region. In the layer below a mass extinction that devastated the region, the paleontologist observed that shells of species A was present in small numbers over a widespread area whereas species B was abundant and found in a few restricted areas. Which of the following predictions about the fossil record in the layer above the mass extinction is the MOST likely to be supported by further excavations? a. Species A survived because of its widespread range including some areas that were not as affected by mass extinction.
b. Species A survived because the low number of individuals meant that there was less competition for resources.
c. Species B survived because it was the more abundant and more individuals survived.
d. Species B survived because the population was concentrated in a few areas and had a higher chance of survival.

a. Species A survived because of its widespread range including some areas that were not as affected by mass extinction.

Explanation:

The species distribution might confer extinction resistance during massive extinction events. One of the most important reasons for a species -or any other taxonomic group- to survive massive extinction is its wide distribution area.

The broad geographical range of a group -as Species A in the example- enhances the survival chances during mass extinctions. When the event occurs, it severely affects most individuals of a certain area. If the species is restricted to that area, then the species will probably disappear, no matter the number of individuals living there.

But if the species is widely distributed, there are more survival chances. The extinction event will only affect a few regions, but not all of them, meaning that some of the individuals of the species will be able to survive.

Probably as only a few individuals of species A got to survive, they must have suffered the effects of genetic drift after the disaster, causing a reduction in genetic variability.