Final answer:
Migration mainly causes populations to become more similar in allele frequencies (option 3) due to gene flow, although in cases of small populations, genetic drift may lead to differentiation into distinct species over time.
Step-by-step explanation:
Migration affects the genetic composition of populations in various ways. When considering what migration does to populations, one must understand different evolutionary concepts such as gene flow, genetic drift, and the concept of a gene pool.
Gene flow is the movement of alleles in and out of a population due to migrating individuals or gametes, which tends to homogenize the allele frequencies among the populations. Genetic drift, on the other hand, is the effect of chance on a population's gene pool; it is particularly significant in smaller populations where chance events can lead to large changes in allele frequencies.
Migration can lead to genetic drift through the founder effect, where a small group migrates and establishes a new population, carrying with them only a subset of the alleles present in the larger original population. Over time, if isolated, this can lead to the populations differentiating into distinct species through a process called allopatric speciation, where geographically separate populations evolve independently.
Genetic drift can cause dramatic changes in small populations, but gene flow tends to make the populations more similar in allele frequencies. The please provide correct option to the question 'Migration causes populations to:' with regard to the choices given would likely be (3) 'Become more similar in allele frequencies' since migration leads to gene flow between populations, which homogenizes their genetic makeup.