Has the law of karma evolved/is it evolving? As I understand it, Buddhism is not inconsistent with Darwin's theory of evolution. Since the law of karma is central to Buddhism and applies to sentient beings, is the law of karma something that evolved, too?
AFAIK, changing is probably the only thing common to both Buddhism and Darwin's theory of evolution. The law of Karma is an important teaching in Buddhism that explains a certain causes and effect process. 

The Buddha Dhamma is called Akaliko for 2 reasons:


Gives effect immediately
True for all three times (past, present and future)


But we must not confuse Buddha Dhamma with the word 'Dhamma' used in "Sabbe Dhamma Anatta". Here the word 'Dhamma' means phenomena. All phenomena except Nibbana are conditioned and subjected to change. Buddha Dhamma is the teachings of those phenomena and their behavior. That is true for all times. 

Science keep changing as it is based on hypothesis. Buddha Dhamma does not change as it is based on direct knowledge.

If the required causes are present, the effect will come to past. Regarding the teaching of Karma, time itself is part of the equation. Read about Kala Sampatti &amp; Kala Vipatti