Sexual reproduction, aka meiosis, is possible for yeast. In fact, there are hundreds of labs that force yeast into sexual reproduction every day. The difference is that they are using special environments (high-stress) not found in wort. The yeast "generations" we use are indeed clones of the original culture, and the difference in "flavour" after several generations is due to the overgrowth of one variant in the yeast population of the original starter culture. In any culture, there are always slight differences between the yeast cells, and after time one will accumulate enough differences to outcompete the other variants. These will be slight differences, but noticable if you pay enough attention.
Wikipedia on Mating of Yeast, note difference between a-MAT strains, and alpha-MAT strains.
This mitotic process is practically the same as in bacteria, which do not undergo meiosis, and have used mitosis exclusively to evolve into the numerous species found today, which have evolved into organisms that can tolerate all kinds of environments. The Lanski Lab has been running a famous experiment in bacterial evolution for the last 25 years.
If you are looking for more information, I would suggest a college level biology book (Campbell and Reece is very good) or something like Carl Zimmer's Evolution: Making Sense of Life.