A chart of the development of Signal. Sources: TextSecure and Signal. Android apps are above and iOS app is below.
Events: 
a) Open Whisper Systems updated their protocol to version 2, adding group chat and instant messaging capabilities. 
b) Open Whisper Systems ended support for encrypted SMS/MMS messaging on Android. This prompted some users to create a fork which is named SMSSecure and is meant solely for encrypted SMS/MMS messaging. 
c) Open Whisper Systems merged RedPhone into TextSecure and renamed it as Signal. 
d) Open Whisper Systems launched Signal as a RedPhone counterpart for iOS. 
e) Open Whisper Systems added support for encrypted group chat and instant messaging. 
Colours:

  Red: Encrypted voice calling capabilities.
  Green: Encrypted SMS/MMS messaging capabilities in TextSecure and subsequently in SMSSecure, a third-party fork of TextSecure.
  Blue: Encrypted group chat and instant messaging capabilities.
  Purple: Signal for Android and iOS after both had encrypted group chat, instant messaging and voice calling capabilities.
Own work
Dodi 8238