In addition to founder effects, the new population is often a very small population and so shows increased sensitivity to genetic drift, an increase in inbreeding, and relatively low genetic variation.
The short answer for how long does it take to become a lawyer is that in the United States it takes a minimum of three years. In order to practice law anywhere in the western world, one must pass an examination in the geographic jurisdiction that the proposed lawyer wants to practice law.
There is a lot involved in order to become a criminal lawyer. There are seven to eight years of schooling depending on the law degree. Then there is the time it takes to take the bar exam, wait for results, and possibly retake the exam.