Thanks for your question. A book could be written on the answer but I will give you a few avenues to pursue if you wish to further research and study this issue.
Like all languages ASL is constantly evolving and changing/growing. Languages are living, organic systems of communication and they constantly change.
Change occurs on several levels
The way individual signs are produced. Historically some signs were originally two handed and have evolved to become one handed. For example signs for COW, HORSE, RABBIT etc. These signs began as more transparent representations of the horns or ears of the animals they represented. In this since like many signs they began as more iconic, transparent representations and gradually became less transparent/iconic. So one way that ASL signs evolve is to move from more iconic gestural representations to more opaque/abstract sign-words. There are scholarly papers you will discover by researching on historical change in American Sign Language.
Other ways that ASL evolves like all languages is to constantly be adding to the vocabulary items in the language. As new concepts emerge in a culture and their is need to talk about these new concepts ASL users will develop new signs to represent these new ideas, concepts, things, gadgets, inventions etc. Sometimes more than one sign will be in use initially but gradually will give way one of the signs becoming standard among users. Although some of the signs will be favored in one region and others in another region of the country and the different signs will become part of the regional dialects of ASL. Vocabulary gets added to ASL in two ways traditionally. One is by using the rich capability of ASL to represent reality through its classifier handshape system. (Look up information on the classifier system in ASL. It is a topic too big to explain here.) Another productive way that new signs find their way into ASL (although less favored today) is through the process of initialization. This means using a "manual alphabet handshape" to create sign words that more directly match their English equivalents. An example of this is the sign TRY (using a "T" handshape instead of the traditional closed fist handshape). The traditional sign that meant "to try or put forth effort" was performed with a closed fist with both hands moving forward at the same time in the space at each side of the signer's torso. In the 1970s and 80s there was an effort by some in education to develop a signing code of English so that deaf students (it was thought) could more readily see the relationship directly between English vocabulary and the signs that were used to communicate with them in educational settings. These systems can be generically called Manual Codes for English systems. At this time the traditional sign for "to put forth effort" was changed by making the same movement gesture with a T handshape to mean specifically TRY, an E handshape to mean specifically Effort, an A handshape to mean Attempt and an S handshape to mean specifically Strive. Most all of these have fallen into disfavor and disuse but many signers will still use the sign marked with a T handshape to mean TRY. This is just one example of how initialization works in ASL and many modern day ASL signs are "marked with manual alphabet handshapes" and are fully accepted into ASL.
