The theory of evolution is the hypothesis that all life on earth originated from a single source via genetic mutation and natural selection of beneficial mutations. Ever since Charles Darwin first proposed the idea of evolution in mid-19th century, a growing body of evidence is emerging to support it. One of the reasons evolution is difficult to study in laboratory is the large time frames that are involved. Modern human beings first appeared around 200,000 years ago, while the family containing humans and great apes has been evolving over millions of years.
This difficulty, however, can be overcome by studying the evolution of micro-level organisms under specifically designed conditions in a laboratory. The tiny size of these organisms and the quickness with which they reproduce make the study of evolution accessible. Also, the development of DNA sequencing techniques allows the scientists to study the genetic modifications behind the evolutionary changes they observe. A new class of evidence for the theory of evolution is being established using these methods.
One of the pioneers has been Dr Richard Lenski. He, along with his associates, is conducting a 20 year experiment using the E.coli bacteria. He started with a single E.coli and began to observe its behavior under adverse conditions. After hundreds of generations, he saw that they began to reproduce more efficiently, meaning that they adopted to the new conditions. He also observed that, after about 33,000 generations, the bacteria exhibited a behavior trait (feeding on citrate) that is absent among E.coli in natural conditions. This trait developed solely due to its advantage in the circumstances created by Dr Lenski. A very accessible summary of Dr Lenski's research is here.
Finally, here is an interesting exchange between Dr Lenski and Andrew Schlafly. It tells you a little about the ridiculousness of the quack intelligent design movement which is just creationism, in a harmful disguise.