Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 1
श्रीबादरायणिरुवाचपूरोर्वंशं प्रवक्ष्यामि यत्र जातोऽसि भारत ।यत्र राजर्षयो वंश्या ब्रह्मवंश्याश्च जज्ञिरे ॥ १ ॥
śrī-bādarāyaṇir uvācapūror vaṁśaṁ pravakṣyāmiyatra jāto ’si bhāratayatra rājarṣayo vaṁśyābrahma-vaṁśyāś ca jajñire
Śukadeva Gosvāmī said: O Mahārāja Parīkṣit, descendant of Mahārāja Bharata, I shall now describe the dynasty of Pūru, in which you were born, in which many saintly kings appeared, and from which many dynasties of brāhmaṇas began.
There are many historical instances by which we can understand that from kṣatriyas many brāhmaṇas have been born and that from brāhmaṇas many kṣatriyas have been born. The Lord Himself says in Bhagavad-gītā (4.13), cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ: “According to the three modes of material nature and the work ascribed to them, the four divisions of human society were created by Me.” Therefore, regardless of the family in which one takes birth, when one is qualified with the symptoms of a particular section, he is to be described accordingly. Yal-lakṣaṇaṁ proktam. One’s place in the varṇa divisions of society is determined according to one’s symptoms or qualities. This is maintained everywhere in the śāstra. Birth is a secondary consideration; the first consideration is one’s qualities and activities.