Because the Galapagos Islands cover a small area and have many ecological niches available, the evolution of Darwin's finches is best explained as adaptive radiation. Adaptive radiation refers to growing specialization between different types of the same species until they finally become different species, as with the small vs the large-beaked finches on the Galapagos, which eat different seeds.

Together in the 1960's, MacArthur and Wilson coined the term "Island Biogeography" within the field of Insular Biogeography. This is a theory that attempts to predict the number of species existing on an island, whether a literal island, or simply a habitat isolated from like habitats by an "ocean" of unlike habitats.

Natural selection is the process by which the organisms with the highest fitness survive and reproduce. Not all organisms reproduce at the same rate; some are better suited for the environment in which they live than others. This suitability is often described using the concept of Darwinian fitness, which is the ability of an organism to survive and reproduce, therefore passing on its genes to its progeny.

As an example of natural selection in action, consider the case of the Peppered Moth in England around the time of the Industrial Revolution. Before the Industrial Revolution, Peppered Moth populations were primarily composed of white-colored moths, with dark-colored moths representing only a small fraction of the population. This is because the trees the moths often rested on were white in color, allowing white-colored moths effective camoflauge but making the dark-colored moths obvious to predators. So, the dark-colored moths got eaten more often and did not survive to reproduce and pass on their genes to progeny; the white-colored moths did. During the Industrial Revolution, the smog and air pollution produced turned the bark of certain trees to a darker color. Now, the dark-colored moths were effectively camoflaged, but the white-colored moths stuck out to predators. More dark-colored moths survived (e.g. weren't eaten) than white-colored moths because of this environmental change, and so the moth population over time came to be composed of primarily dark-colored moths, with a small fraction of white-colored moths. This anecdote demonstrates the process of natural selection and how fitness can be influenced by environmental changes.

Individuals that are "fit" in regard to natural selection does not refer to strength or one particular trait. Rather, it refers to an organism’s adaptability to their environment, which allows them to survive and reproduce. Successful reproductive events pass on genes, traits, and adaptations to future generations of the species.

One of the initial steps towards natural selection is environmental change that selects for or against a particular trait in a species (e.g. ash darkening habitats selects for darker colored moths). Over time, as the ash continues to color the moths' habitat, the light moths—being more visible to predators—will die out. As a result their reproductive success will be reduced and the darker moths will be more likely to reproduce and pass on their traits. 

Since voles not only forage in open grassland and meadows where hawks hunt their prey, they also forage during daylight hours, when hawks are most active. Therefore, the vole population is most likely of all the rodent species to see a significant decline. 

Adaptation, modern synthesis, and mutation are all part of evolution through natural process. Adaptation is the process of inheritable traits being carried from one generation to the next. Modern synthesis is the combining of Darwin's theory of evolution with modern principles of genetics. Mutations are changes in genes and are part of the modern synthesis. 

The correct response is survival and reproductive rate. These are the two factors that are used to determine the fitness of an individual. There are other variables that can impact survival and reproduction rate, but these are the two foundational factors that impact fitness.

Fitness = Rate of Survival times Rate of Reproduction

Thus, if an individual lives to be 100 years old but has 0 children, then their overall fitness is 0. (Fitness = 100 x 0 = 0)

The phenotype is described as the observable characteristics of an organism. Natural selection is based on survival of the fittest due to differences in phenotype. This is not to be confused with genotype, which is the genetic (DNA sequence) of a trait.

The modern theory that is natural selection is from the studies of Charles Darwin. His theory of natural selection was an explanation for the adaptation of species. He further explored his ideas in his book, The Origin of Species.