The Roosevelt Corollary's goal was to

to prevent European forces on the soil of Americas.

Explanation:

Since the 19th Century and Monroe's doctrine, United States tried to prevent European countries to interfere in the internal conflicts and problems on the American continent. Roosevelt believed that United States are the ones that have the obligation to prevent conflicts and to create a peaceful environment especially in the region of the Caribbean. United States in that sense behaved like a police that will prevent problems.

genetics is the study of how dna works, the methodologies defining the new recent technologies allow scientist to have a panoramic view of the entire dna landscape, especially how genomes interact with their environment, including off course, factors outside their organisms.

thus is very possible the existence of both scenarios, where populations of isolated individuals have different genetic traits, and share traits related to genetic mutation, due rats can easily travel overseas using human transport channels. so scientists could get a vast number of data, including human interaction, evolution of rodents, health development, spread of infections, development of new organisms, how genomes evolve in different environments and so on.

small european communities with shared traditions consolidated into large new states (apex)