One of the arguments given by people (incorrectly) to show oppression in Hinduism is that the Brahmins were elites and unfairly discriminated against the lower caste people, also known as anti-Brahminism.
The counter argument given is that there have been many Rishis who had parents of lower caste (Sudras or Dalits). Although, due to their knowledge they came to be known as Rishis. Hence, caste was determined back then by merit and not by birth.
This website lists a number of such examples of Rishis with lower caste parents, without any references. Rishi Vyasa, Rishi Valmiki, Rishi Aitareya, Rishi Parashara, Rishi Vashishtha and Rishi Vidura are mentioned.
Can all or any of these examples be traced back in the scriptures?