Why would some species evolve towards social behavior?
There are costs and benefits to living and working in a group for survival.  Theoretically, individuals will evolve to live in a group if the benefits outweigh the costs:

costs:
-individuals clumped together are conspicuous to predators
-easier transmission of disease and parasites between colony memberes
-more food competition between members
-time and energy expended by subordinates in dealing with more dominant companions
-greater risk of reproductive interference by other individuals

benefits:
-defense against predators via the dilution effect or mutual defense
-opportunities to receive assistance from others in dealing with pathogens
-improved forging with a centralized information center and cooperation
-opportunities for reproductive interference

Ancestral Ants:
Ants that exhibit a relatively large proportion of ancestral morphological traits are commonly believed to exhibit less social complexity and a smaller colony size. In other words, the more morphological traits shared by workers and queens, the smaller and less socially complex is the colony and therefore the more "ancestral" the species of ant.


morphological differences between castes in the ant colonies

 

morphological variations between queens and workers include:

size difference: More socially complex ants will have a larger queen than the workers because the queens job is to disperse and found new colonies, which requires large fat reserves. and large flight muscles
a more primitive, nonfunctiona, or complete lack of the spermatheca reproductive organ: The evolution towards the non-functionality and non-existence of the spermatheca in the workers is more obvious: the indirect fitness gains they get from colony management and aiding the queen are greater than the gains from their own individual reproductive fitness, hence their reproductive organs have evolved towards non-existence.
queens have wings, workers do not: Queens need wings for aerial dispersal and colonization of new habitats. Ants with less socially complex colonies tend to have wingless queens (i.e. queens that more closely resemble the workers)

- Less socially complex ant colonies exhibit:
-lower fecundity in the queens (only 1-5 eggs per day)
-smaller colony size
-more predation (i.e. more insect prey as food over other forms of food)
and less altruistic behavior between workers trophallaxis, or example, following the appropriate tactile signals, a solicited worker will regurgitate a drop of food to another worker who begs for it. This doesn't occur in more "primitive" species with lesser morphological distinction between workers and queens
a lesser degree of coordination among food foragers (more solitary foraging rather than cooperative group foraging)
a lesser degree of role specialization among castes
a lesser degree of morphological differences between castes (size, wing structure, etc)

-"The existence of phenotypically distinct female castes is a hallmark of ant eusociality. The reproductive division of labor is characteristically based on the production of two distinct female phenotypes: queens and workers. Morphological caste specialization is arguably a key to the ecological success of the Formicidae