In the September '99 issue of Nature, Dr. Paul Sherman of Department of Neurobiology and Behavior at Cornell University describes the behavior of groups of male birds called "leks".  To understand the evolutionary basis of college fraternities, we need only take his article, "Birds of a Feather Lek Together", pick through the text, and replace the words "lek" with "frat" or "fraternity", "male" with "frat boy" or some synonym thereof, and, although it is not absolutely necessary, "female" with "drunk chick".  In the interest of readibility, several other word substitutions have also been made-- these  are footnoted with the original text.  Any addition of my own text will be italicized.  Without further delay, I give you the evotionary basis of the college fraternity:

    "A group of frat boys aggregates at a traditional site, where they perform intricate vocal, visual or chemical displays to attract drunk chicks.  Frat hangouts¹ contain no resources valuable to females,...so the drunk chicks choose mates by comparing frat boysâ physiques and displays, or by copying choosy drunk chicks.
    Three hypotheses have been proposed to explain why frat boys aggregate:  the hotspot model (clusters form near places drunk chicks frequently visit), the hotshot model (individuals cluster around attractive frat boys to increase their chances of being noticed), and the female-preference model (frat boys cluster because drunk chicks like to visit groups, where they can choose a mate quickly and safely).  Tests of each hypothesis have found support at some universities² but not others.
    All three hypotheses predict that males should join fratenities to increase their mating opportunities.  Indeed, at some universities², larger fraternities do attract proportionally more drunk chicks.  However, at many universities², mating success per fraternity brother declines as the frat size increases, raising the question of why pledges³ keep joining up.  Either they have no better options, or they are gaining reproductive benefits in some other way.
    Fraternity members that are apparently unsuccessful may gain an evolutionary advantage by boosting the mating success of family members.  For example, ...frat boys might increase their inclusive fitness (the number of genes their behavior enables their kin to pass down to the next generation) by joining fraternities dominated by their relatives.  For example, [if] a pledge⁴ is the brother of the fraternity president⁵, the fraternity presidentâs⁵ matings will conceive his nieces and nephews.
    Petrie et. al. show that frat boys⁶ recognize each other by phenotype matching (comparing other males with themselves).  Frat boys might learn to recognize their own physical features and...associate with other phenotypically similar students⁷.  Such self-referent phenotype-matching could allow frat brothers⁸ to recognize one another.  However, confirmation [of this hypothesis] requires identifying what cues frat boys use [to identify each other] (for example music⁹, clothing¹⁰,or odor), altering them experimentally, and observing whether manipulated frat boys indeed prefer frat brothers similar to themselves."

In other words, the modern-day college fraternity itself, along with the frat boy customs of dressing alike and referring to each other as "brothers" are most likely remnants of an evolutionary period when self-referent phenotype-matching (looking for people who look like you) was used by individuals to identify close genetic relatives so that they could band together in the interest of propagating the family genes.

Footnoted word substitutions:
1.  lekking arena --> frat hangout
2.  in some species --> at some universities
3.  males --> pledges;
4.  subordinates --> pledges
5.  alpha male --> fraternity president
6.  peacocks --> frat boys
7.  birds --> students
8.  males with the same father --> frat brothers
9.  calls --> music
10.  plumage --> clothing