A devastating tsunami wipes out most of the population of a frog species that lives on just one tropical island. what is the most likely outcome of this event?

Answer is: inbreeding depression.

Inbreeding depression is the reduced biological fitness in a given population (in this case frogs) as a result of inbreeding or breeding of related individuals. This is result of population bottleneck (tsunami).

Population bottleneck is a sharp reduction in the size of a population due to environmental events.