The historical evolution of apple flies widens our perception towards evolution, a concept which most people do not comprehend and accept as a fact. Evolution is not a linear progression but more like diversification. We humans do not directly evolve from apes but shared a common ancestor. And yeah evolution is a theory but there is a huge difference between a theory and a hypothesis. A hypothesis is an assumption that is based on known facts but has not been proven yet but Theory is a collection of hypotheses. It contains a lot of factual evidence supporting the idea of Evolution.
Anyway, let us go back to the main topic. Many scientists believed that all organisms evolved from one single species which eventually splits into two or more and become a different kind of species. This process is called Speciation and there are two ways in splitting an individual into an entirely new species -- Allopatric and Sympatric.
Allopatric speciation is the former evolution theory explaining that a single species is divided into two or more groups by a physical barrier such as glaciers, lava flow from a volcanic eruption, a highway, or Trump's wall. Over time, each of these builds their own evolutionary path by adapting to the environment they are in or through random genetic drift which alters their genetics and physical appearance. Then they become so different and cannot interbreed with the other kind of species.
Whereas Sympatric Speciation changed the way we see the evolution. It is when species split even though the two diverging groups live in the same habitat and one good example is apple flies. Around the mid-1800s in North America, Apple maggot flies split into two very different groups. The fly used to lay eggs on the fruit of Hawthorn trees but when an apple was introduced to North America in the 1600s, some flies were able to switch over to lay eggs on apples, probably, tired of giving birth in Hawthorn tree's fruits. When the eggs hatched in apples grew up they tend to practice assortative mating, meaning they chose mates that were similar to them. Thus, hawthorn flies and apple flies tend to breed with their own species and consequently obtain different genomes.
Another example of sympatric speciation was the fishes in Lake Apoyo, a volcanic lake in Nicaragua. The lake was populated by a single ancestral species of cichlid fish but today, unexpectedly the lake contained two different species. Their body shapes, teeth, and diet were quite different from each other. The one group of fish are vegetarian, eating plant material while the other group tends to eat insects.
Evolution is a non-stop process which means we are still evolving. Probably in the future, humans might evolve into zombies with a bigger thumb for excessively using it in swiping and scrolling on a rectangular screen of mobile devices due to our addiction to looking down on the blue screens of our phones. The upright structure of our backbone curves down and due to our perseverance of watching our favorite drama series or playing violent games at night, our eyebags get bigger making us appear like a zombie. A zombie that is scary as shit which can make your reflection in the mirror scream with fear of looking at your messed up face.