First of all, remember that a theory speaks about facts, that the theory is a set of information about things that are happening and are facts. A theory is also able to make predictions. The theory of Evolution speaks about the fact that organisms evolve over long periods of time and the practical applications can be seen in three areas: shrinking fish, where did the HIV come from and why are grasslands becoming desserts?
The video from above shows us that, when we apply the theory of evolution to certain issues, the solutions it provides are counter intuitive and might seem even totally off, but they aren’t.
In the fish example, we let the small fish go and only eat the big ones, but since some adult fish are small and we leave them back into the oceans to reproduce we see a shrinking population of fish since only small adult fish continue to exist.
HIV was first discovered in the 80ies in USA, but DNA analysis traced the origins back to the 60s immigrant from Cuba and to 1901 HIV infections in West Africa. HIV is the successor of SIV, a similar virus found in simians, monkeys. HIV is hard to track and kill because it evolves.
Grasslands becoming desserts? Get the cattle back in place to make the grassland grow again.