Biological evolution is a change in the frequency of alleles in population of organisms. It has been defined that way since about 1940, based on mathematical analysis of genetic evidence collected since 1900 or so. Since changes in allele frequency have been observed in the wild, in captivity and in experimental organisms, biological evolution is a fact, not a theory. It does not involve fossils, and it does not involve the origin of life.
A species of organism is a population that either will not or cannot interbreed with a closely similar population (no matter how similar they appear) and produce fertile offspring. On these grounds, horses and donkeys are separate species since they can produce offspring but these are not fertile. No, this is not a "theory" but a working definition of "species".
Drosphila melanogaster has long been used for genetic experiments. Using specimens that had been used before would invalidate the experiments, therefore for each set of experiments new specimens are used. This means that biology laboratories, public and private maintain or subscribe to farms where these flies are raised. There are several of these farms. Many of them were stocked from the same source, and some have maintained separate populations for 70 to 90 years.
When attempts have been made to interbreed specimens of D. melanogaster from long separated populations, they have refused to breed together. So by definition, they are separate species. No, this is not a "theory" and it demonstrates evolution of a new species in less than a century in captivity.
North America has a wild population of insects known as Rhagoletis pomonella or the hawthorn maggot fly. These insects lay eggs on the flowers or young fruit of the American hawthorn. About 1606 the apple was introduced to North America. Apples are distantly related to hawthorns but flower and fruit at a different time. Thus in most cases if specimens of R. pomonella mated at the "wrong" time and laid eggs on the newly introduced apple, the eggs and larvae would not survive. However by 1861 apple growers in North America noticed the emergence of a new pest of apples, which was superficially identical to the hawthorn maggot fly.
Subsequent work has shown that the original hawthorn maggot fly has diversified into a variety which infests apples and some other introduced fruits. Attempts to cross breed the apple version and the hawthorn version fail in 19 cases out of 20, which indicates that the two populations have almost (but not quite) formed two different species. In the wild, crossbreeding would rarely happen and the two varieties are virtually distinct species and will diverge more rapidly now as the populations genetically drift apart. Further, there is evidence that mites which are parasites on these flies are becoming different too. No, this is not a "theory" and it demonstrates the appearance of what are almost two different species in the wild in less than 300 years. Since apples are an economic crop, rather a lot is known about the pests that infest them.
The genomes of humans and chimpanzees are almost identical and contain about 3 billion base pairs each. This is by direct chemical analysis, not by "theory". In both genomes the remains of retroviruses that once infected germ line cells but did not kill the cells may be found. These are inherited. In the case of humans and chimps, there are seven (or perhaps more by now) identical retroviral sequences in identical locations in both genomes. This is not a "theory" but observed facts. The chances that a retrovirus will insert itself in any particular spot are one in three billion. The chances that it will insert itself in identical spots in both species are even smaller. something like 1 in 9 billion. The chance that seven of them will do it independently are something like 1 in 10 to the 80th power. That number approaches the number of atoms in the known Universe. That is apart from any anatomical, biochemical or behavioral similarities between chimpanzees and humans. Therefore the chance that humans and chimpanzees did not have a common ancestor are less than 1 in 10 to the 80th power.
You will not find this material in your "science books" because (a) creationists on educational boards take care to remove any direct evidence of evolution from school level textbooks and (b) these results come from ongoing science and such material does not appear in school level textbooks, which are there to provide the utter basics of fact and theory and merely repeat material that has been known for decades.