This is some of the research that contributed to the recently released Darwin’s Paradox: An international science mystery. http://amzn.to/2k8qJgi. Bats have a major role in this books, also the people who try to protect them when they become a target of fearful people during the pandemic.
An important principle of evolution is that evolution does not plan. There is no such thing as a species evolving in a direction that is not superior to the status quo just to be better prepared for some eventual benefit later.
This is important when thinking about flight, or any evolutionary change that makes use of several systems (such as the skeletal system, cardiovascular system, and the respiratory system), or which requires significant anatomical modifications. Both of these are true for flight.
While speciation, based on size and color (canis and felis) or beaks (Darwin’s finches), can be understood by a few, small genetic changes, the evolution of flight is more complex. However, in spite of the complexity, flight has evolved at least four independent times: insects, pterosaurs, bats, and birds.
Flight requires both the development of wings and changes to cardiorespiratory systems to support the short-term effort required for aerial takeoff.
Now, from the no planning principle, we know that in the millennia before flight is achieved, the intermediate changes must be an improvement at every step of the way. In the case of flight, there are two major theories for an evolutionary path to flight.
The first path to flight (ground up) assumes an animal that lives on the ground and evolves to run faster building up their cadriorespiratory endurance and uses their hands to catch prey expanding the area of their hands. Over time this animal benefits from hopping and gliding and finally true flight. This seems to be a good theory for birds.
The second path to flight (height down) assumes an arboreal animal that lives in the trees and first jumps down on prey, and eventually glides, and finally develops true flight. This seems to be a good theory for pterosaurs.
In the case of chiroptera (bats), it could have gone either way. There are bats that run along the ground suggesting ground up and those that climb trees suggesting height down.
In either case, bats have successfully populated all continents except Antarctica. They represent 20% of all mammal species, second only to rodents.