There is one reference in Mahabharata that clearly states that Varna is independent of birth and specifies conduct to be the only ground.

Maheshwara said, ‘..Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status.Verily, the conduct is the only ground. All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of a Brahma, O auspicious lady [Uma], is equal wherever it exists. Even this is my opinion. He, indeed, is a Brahmana in which the status of Brahma exists – that condition which is bereft of attributes and which has no stain attached to it. of human beings in four orders dependent on birth is only for purposes of classification.The boon giving Brahma, when he created all creatures, himself said that the distribution of human beings in four orders dependent on birth is only for purposes of classification.'

Mahabharata Anusasana Parva Section CXLIII

There is one reference in Mahabharata that clearly states that Varna is independent of birth and specifies conduct to be the only ground.

Maheshwara said, ‘..Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status. Verily, the conduct is the only ground. All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of a Brahma, O auspicious lady [Uma], is equal wherever it exists. Even this is my opinion. He, indeed, is a Brahmana in which the status of Brahma exists – that condition which is bereft of attributes and which has no stain attached to it. of human beings in four orders dependent on birth is only for purposes of classification.The boon giving Brahma, when he created all creatures, himself said that the distribution of human beings in four orders dependent on birth is only for purposes of classification.'

Mahabharata Anusasana Parva Section CXLIII

Moreover the birth based caste system is an invalid system since it violates the guidelines given in Hindu scriptures.

Guidelines

Everything written in Hindu scripture is not valid. A scriptural statement can be invalid if it does not satisfy reason.

Primacy of reason

Bhishma Said:

Even the words heard from an ignorant person, if in themselves they be fraught with sense, come to be regarded as pious and wise. In days of old, Usanas said unto the Daityas this truth, which should remove all doubts, that scriptures are no scriptures if they cannot stand the test of reason.

Mahabharata Shanti Parva Section CXLII

Acharya Shankara says:

The appeal to the infallibility of the Vedic injunction is misconceived. The infallibility in question refers only to the unseen forces or apurva, and is admissible only in regards to matters not confined to the sphere of direct perceptions, etc ... Even a hundred statements of sruti to the effect that fire is cold and non-luminous won't prove valid. If it does make such a statement, its import will have to be interpreted differently. Otherwise, validity won't attach to it. Nothing in conflict with the means of valid cognition or with its own statements may be imputed to sruti.

REF: Srimad Bhagavad Gita Bhasya 18.66 of Sri Sankaracarya translation by Dr. A. G. Krishna Warrier, p. 629.

Rejection of discriminatory verses

Any discriminatory verse in Hindu scripture is also automatically invalid according to Hindu scripture and should not be followed. This is necessary because Hinduism is not an organized dharma and does not have any central body to oversee the content of Hindu spiritual texts.

If a holy act is against the interest of other members of the society, it should not be practiced. It is Dharma which is the source of Artha and even of Kama.

Kurma Purana I.2.54

The implication of the Kurma Purana verse is that the hereditary caste system practiced by Hindus is an invalid system on the grounds of discrimination.