When the Greek philosopher Heraclitus said "the only thing that is constant is change," he could have been talking about musical theatre. Then again, perhaps a more apt understanding of the nature of musical theatre's evolution can be found in Darwin. As our culture changes over the years, musicals have adapted to fill the different roles they play in our lives, from times when they've offered respite from national depression to serving as a well for pop music to the great variety of offerings today, which range from challenging social commentary to comfort food "theme park" shows (so-called not for their similarity to shows at theme parks, but for their actual theme-park like experience of a mindless ride).

Click though to read my selections for the benchmark Broadway musicals that broke new ground and shaped the evolution of the art form.

Show Boat (1927)

Jerome Kern and Oscar Hammerstein II's classic musical broke new ground in at least two major ways. First of all, it was a serious musical, marking the first time a musical was something akin to a play, where there were actual characters and plot, rather than the vaudevillian revues that were the norm on Broadway at the time. Even more significantly, Show Boat, an adaptation of Edna Ferber's bestselling novel of the same name concerning racism and social evolution, was the first time an integrated cast of black and white performers appeared together in a Broadway musical.

Oklahoma! (1943)

Oklahoma! expanded on the precedent set by Show Boat, by not only being a serious piece of storytelling, but by using all its songs to do so. Every number serves a specific function in advancing the plot or developing the characters. There are no throw-aways or "cigarette songs" (as used to be the case where you could go out for smoke without missing anything important).

Carousel (1945)

Carousel took the advancement of Oklahoma! a step further by blurring the lines between songs and dialogue. While still working (as in Oklahoma!) in an American folk style, Rodgers and Hammerstein's score enters the realm of the operatic with its extended sequences incorporating music and lyrics and spoken text, most notably in the 12-minute "bench scene." Not only are the songs not interchangeable, but they function as the key dramatic moments in the show.

West Side Story (1957)

The innovation of West Side Story was its use of dance in the new form of musical play. Where Oklahoma! had its set piece, the "Dream Ballet," the choreography in West Side Story was part and parcel of the entire production. Indeed, there was no separate dancing chorus in West Side Story; the lead actors were required to act, sing and dance.

Cabaret (1966)

Once Oklahoma! had brought on an entire generation of "book musicals" where the songs advanced the plot, Cabaret was the first post-book musical, marking the beginning of the era of "concept musicals" built around a central metaphor. There were earlier examples, such as Threepenny Opera and Lady in the Dark, but Cabaret was the beginning of a new age when this became the dominant form on Broadway. Some of the songs in Cabaret are in the Hammerstein storytelling mold, but the numbers set in the Kit Kat Club are just performance pieces — or are they? These songs offer commentary in counterpoint to the action of Cabaret and amplify the impact of show's complicated, controversial themes.

Hair (1968)

The cultural significance of Hair is so major that the lyrics of its opening song, "This is the dawning of the Age of Aquarius," have come to describe the impact of Hair itself. The changes in our society with the Civil Rights Movement and the Sexual Revolution and the Vietnam War constituted such a major upheaval that Hair's impact can be traced not only to its position in Broadway history, but American history as well.

Company (1970)

Company is the ultimate concept musical. It's not that the songs don't advance the plot; it's that there is no plot to advance. Company begins with Robert celebrating his 35th birthday and ends with the same birthday. You could say the entire piece is set, quantum-physics style, in one moment in space-time. The genius of Company is the depth of character exploration that the framework affords Stephen Sondheim and George Furth as their songs and scenes delve so richly into Robert and the people in his life, and the great style with which they do so. 

A Chorus Line (1975)

Another development of the concept musical, A Chorus Line deconstructs the Broadway musical, removing virtually all scenery and costumes, and providing a look at the form inside-out. Created from a groundbreaking workshop incorporating dancers' actual experiences, A Chorus Line offers a glimpse at the audition process, exploring who performers are and why they perform. This bare-bones and highly theatrical exploration of the humanity (and inhumanity) of Broadway resonates for a much wider population than show people.

Ain't Misbehavin' (1978)

The jukebox musical has become a much maligned genre, with many people balking at its bringing Vegas-style revues to Broadway, although when a jukebox show is a hit, like Jersey Boys or Mamma Mia!, the jeers are drowned out by the cheers. Still, it's hard to believe this mega-genre started with the intimate Ain't Misbehavin', which began life, pre-Broadway, in Manhattan Theatre Club's former cabaret space on the Upper East Side. Just five brilliant, versatile actor-singers breathing life into timeless Fats Waller tunes carried more than enough story, character, comedy and drama inherent in each song. There was no need for a real plot, or much connective tissue. Of course, there were revues before and Off-Broadway's long-running, Jacques Brel is Alive and Well and Living in Paris, which had a briefly played Broadway in 1972, but Ain't Misbehavin' ushered in the trend that is still going strong today.

Evita (1979)

Although it has tapered off in recent years, the era of the British mega-musical was dominant on Broadway throughout the 1980s and much of the 1990s. Evita was really the first of these shows; the original 1978 West End production was essentially copied — and branded — for Broadway and subsequent reproduction all over the world for years to come. This formula would be repeated with major blockbusters Cats, Les Misérables and The Phantom of The Opera, and has become the standard for how hit Broadway musicals are produced and marketed on a global scale today, as in the case of Wicked or Chicago.

Beauty and the Beast (1994)

You might be surprised to see Disney's Beauty and the Beast on a list of groundbreaking new musicals as it was basically a traditional stage musical. Even among Disney shows, The Lion King is clearly the more theatrically innovative, but Beauty and the Beast was the first of many Broadway musicals produced by juggernaut Disney Theatrical and marked the beginning of an era of corporate producing. Necessary in the economics of Broadway today, many of the top producers are corporations, including other film companies besides Disney and other presenting organizations.

The Producers (2001)

After years of Broadway being dominated by the aforementioned British mega-musicals — a category consisting mostly of pop operas, dramatic sagas laced with soaring melodies, but short on laughs — there was a tremendous amount of good will for the resurgence of good, old-fashioned American musical comedy in The Producers. Hits like Hairspray and The Book of Mormon have continued this renaissance.

In the Heights (2008)

2008's Tony Award-winning "Best Musical" In the Heights was successful on Broadway, playing for three years, but as the first hip-hop Broadway musical, its impact on the genre remains to be seen. While rock purists may debate the rock-and-roll legitimacy of Hair, Jesus Christ Superstar or Rent (I think most would at least cop to Hedwig and the Angry Inch), Broadway gets at least partial credit for giving rock musicals more than a fair try. Actually, though, a hip-hop musical is an even better idea. Hip-hop, more than rock and perhaps even more than the Great American Songbook, is a style devoted to storytelling. Time will tell whether this form, which has become so prevalent in contemporary pop music, finds a new home in Broadway musicals.

(Ben Rimalower is the author and original star of the critically acclaimed Patti Issues. Read Playbill.com's coverage of the solo show here. Visit him at benrimalower.com and follow @benrimalower on Twitter.)