The castes in the erstwhile South Indian society might not have indulged in an exclusive occupation. However, I would consider the dominant stereotypical view of their occupation as the defining identifiers of their group. The discussion is completely restricted to caste and tribal population for obvious reasons.
The biggest groups in Dravidian societies are cultivators. They are either numerically the biggest or the second biggest in all the states. They may form monolithic group as in Karnataka and Kerala or diverse groups as in Tamil Nadu and Andhra Pradesh. Generally, chieftains came from this groups. They may form 15-30% of total population in any state.
In Malayalam and Tulu region they are the biggest group. In Tamil Nadu and Andhra Pradesh they make up sizable part of the population. I believe in Kannada region their proportion in the population is not significant compared to other Dravidian regions(I may be wrong). Though originally made up of diverse groups now overall a monolithic group.
I don't think in any of the states they make up even 10% of total population. Mostly, a single caste but there are exceptions, I suppose.
I am restricting to only sea fishermen. May not form more than 10% in any state population. May or may not be single caste depending upon the region.
Except weavers numerically insignificant. Even weavers' distribution is uneven across the states and in no state form 10% of the population. Weavers are diverse groups but other artisans are generally a single caste group. Though it could be argued that all these smiths(goldsmith, blacksmith, carpenter etc...) were originally a single endogamous unit that turned into multiple castes in the course of time. Tanners are relatively big in Andhra region I suppose. However, in other regions their numbers are insignificant.
Agricultural labourers may form 10-15% of the population.
Other Occupational Groups:
Among hundreds of other numerically insignificant occupational groups priests may form the biggest group at around 3-4% of population followed by merchants.
Tribes may form 3-4% the South Indian population. They are unevenly distributed with the highest concentration in Andhra Pradesh(6%) and the lowest in Kerala/Tamilnadu (1%).
Phase 1: Sea fishermen
In the first phase few people moved away from tribal life and became fishermen. It is possible that tribes and sea fishermen might have come to South India independently. The land fishermen were part of tribes. At the end of first phase we have;
1. nomadic land tribes 2. sea fishermen
Phase 2: Broken tribals Around : 3500 BCE -3000 BCE
This is sedentary civilizational phase for a section of population. A section of population took up farming. These are independent farmers cultivating small strip of land. A section within this took up toddy tapping because of unique South Indian geogrpahy. These are interdependent groups but cut off from organized tribal society. The civilized society of sedentary life did not form fully due to uncertainty of their occupations because of unique South Indian conditions. These are broken tribals without any organization. At the end of second phase we have;
1. nomadic land tribes 2. fishermen 3. farmers 4. toddy tappers
Phase 3: Feudal farmers Around : 2000 BCE- 1500 BCE
In this phase farmers from IVC started moving to South India. These highly organized advanced farmers found easy serfs in unorganized independent farmers. For protection these feudal farmers employed the service of nomadic but tightly knit land tribes. This phase also saw sedentary life of many land tribes. The contact with IVC farmers allowed them to appreciate the sedentary life of farming. This phase saw development of village organization with tribal chiefs as the heads.
This is a significant phase in South Indian society as the society experienced master-slave divisions. The serfs were mainly from independent farmers and to a lesser extent from toddy tappers.
I have discussed about the origin of the name Holeya previously. I need to make certain corrections here. In an earlier post I had predicted that erstwhile bonded labourers in Andhra region, Mala, might have got their name because of purity-pollution concepts. My argument was that if Holeya is from a Dravidian root meaning impure then 'Mala' may also follow the same logic. Then I found Stephen Fuchs' "At the bottom of Indian society" deriving the name in the same fashion.
But I could feel my logic for these groups might have worked in other way round too. I have noticed that only Brahmins in Kannada region derive the name from the root 'impure'. All other people derive it from the root 'field'(the one who works in a field). Again, Mala may have other roots. In Karnataka it is considered as a name of a tribe. Also, Tamil equivalent Pallar were supposedly called 'Mallar' (few centuries ago) showing their affinity with Mala in Andhra region. The place where 'Mala' meaning impure is attested is an old Telugu dictionary. That probably means identification of these two names with 'impure' was the handiwork of brahmins. Since other communities associate Holeya to field (hola in Kannada) I believe they were the earliest broken tribes who ventured into sedentary farming.
At the end of phase 3 we have;
1. feudal farmers from IVC and from land tribes(chiefs) 2. tribal soldiers 3. slave farmers 4. fishermen 5. toddy tappers 6. land tribes
Phase 4: Entry of IE herders Around: 1000 BCE
This period saw IE herders moving to South India. These herders assimilated many local tribes. However, many local tribes independently took up herding.
At the end of this phase we have;
1. feudal farmers 2. tribal soldiers 3. slave farmers 4. fishermen 5. toddy tappers 6. herders 7. land tribes
Phase 5: Entry IE guilds Around: 500 BCE - 0
By this time rudimentary civilization was taking shape in South India. This allowed entry of craftsmen and merchants to this region. Many tribals joined them or started their own guilds. The society until now only class based added one more divisive philosophy, the caste system, to it.
End of this period also saw entry of priests from eastern regions.
At the end of this phase we have;
1. feudal farmers 2. tribal soldiers 3. slave farmers 4. fishermen 5. toddy tappers 6. herders 7. craftsmen 8. priests 9. merchants 10. other occupational groups
Phase 6: 0 - 1900 CE
This period saw reorganizations of various groups according to caste rules. The tribal soldiers became part of various groups. The ruling classes saw north-Indian and non-Indian additions to their ranks.
At the end of this phase we have;
1. feudal farmers 2. independent farmers 2. slave farmers 3. fishermen 4. toddy tappers 5. herders 6. craftsmen 7. priests 8. merchants 9. other occupational castes