JLet me pause to ask the question: what is such a habit made of? What gets passed from individual to individual when a habit is copied? Not stuff, not packets of material, but pure information, the information that generates the pattern of behavior that replicates. A cultural virus, unlike a biological virus, is not tethered to any particular physical medium of transmission. 
Unconscious selection of memes.
On with our Just-so Story. Some of the drummers begin to hum, and of all the different hums, a few are more infectious than the restx, and those hominids who happen to start the humming in these ways become the focus of attention, as sources of humming. A competition between different humming patterns emerges. Here we can begin to see the gradual transition to unconscious selection. Suppose that being such a focus of humming happens to feel good--whether or not it enhances one's genetic fitness slightly (it might, of course; perhaps the females tend to be more receptive to those who start the winning hums). The same transition to unconscious selection can be seen among viruses and other pathogens, by the way. If scratching an itch feels good, and also has the side effect of keeping a ready supply of viral emigrés on one's fingertips, the part of the body most likely to come in contact with another host, one is unconsciously selecting for just such a mode of transmission by one's myopic and uncomprehending preference for scratching when one itches--and this does not depend on scratching having any fitness-enhancing benefits for you: it may be, like the ant's hankering for the top of the grass stem, a desire that benefits the parasite, not the host. Similarly, if varying tempo and pitch of one's hums feels good, and also happens to create a ready supply of more attention-holding noises for spreading to conspecifics, one's primitive aesthetic preference can begin to shape, unconsciously, the lineages of humming habit that spread through one's community.
Brains in the community begin to be infected by a variety of these memes. Competition for time and space in these brains becomes more severe. The infected brains begin to take on a structure, as the memes that enter "learn" to cooperate on the task of turning a brain into a proper meme-nest, with lots of opportunities for entrance and exit (and hence replication).  Meanwhile, any memes out there "looking for" hosts, will have to compete for available space therein. Just like germs.
Methodical Selection of memes.
As the structure grows, it begins to take on a more active role in selecting. That is to say, the brains of the hosts, like the brains of the owners of domesticated animals, become ever more potent and discerning selective agencies--still largely unwitting, but nevertheless having a powerful influence. Some people, it turns out, are better at this than others. As Darwin says of animal breeders,
Not one man in a thousand has accuracy
of eye and judgment sufficient to become an eminent breeder. p32
And Bach, like the one man in a thousand who has the discernment to be an eminent animal breeder, knew how to breed new strains of music from old. Consider, for instance, his hugely successful chorale cantatas. Bach shrewdly chose, for his breeding stock, chorales--hymn melodies that had already proven themselves to be robust inhabitors of their human hosts, already domesticated tunes his audiences had been humming for generations, building up associations and memories, memes that had already sunk their hooks deeply into the emotional habits and triggers of the brains where they had been replicating for years. Then he used his technology to create variations on these memes, seeking to strengthen their strengths and damp their weaknesses, putting them in new environments, inducing new hybrids.
What about memetic engineering? Was Bach, in virtue of his highly sophisticated approach to the design of replicable musical memes, not just a meme-breeder but a memetic engineer? In the light of Darwin's admiring comment on the rare skill--the genius--of the good breeder, it is interesting to note how sharply our prevailing attitudes distinguish between our honoring the "art" of selective breeding and our deep suspicion and disapproval of the "technology" of gene-splicing. Let's hear it for art, but not for technology, we say, forgetting that the words share a common ancestor, techné, the Greek word for art, skill, or craft in any work. We retreat in horror from genetically engineered tomatoes, and turn up our noses at "artificial" fibers in our clothing, while extolling such "organic" and "natural" products as whole grain flour or cotton and wool, forgetting that grains and cotton plants and sheep are themselves products of human technology, of skillful hybridization and rearing techniques. He who would clothe himself in fibers unimproved by technology and live on food from non-domesticated sources is going to be cold and hungry indeed.
Besides, just as genetic engineers, for all their foresight and insight into the innards of things, are still at the mercy of natural selection when it comes to the fate of their creations (that is why, after all, we are so cautious about letting them release their brainchildren on the outside world), so too the memetic engineer, no matter how sophisticated, still has to contend with the daunting task of winning the replication tournaments in the memosphere. One of the most sophisticated musical memetic engineers of the age, Leonard Bernstein, wryly noted this in a wonderful piece he published in 1955 entitled:
"Why don't you run
upstairs and write a nice Gershwin tune?"
Bernstein had credentials and academic honors aplenty in 1955, but no songs on the Hit Parade.
weeks ago a serious composer-friend and I . . . got boiling mad about it. Why
shouldn't we be able to come up with a hit, we said, if the standard is as low
as it seems to be? We decided that all we had to do was to put ourselves into
the mental state of an idiot and write a ridiculous hillbilly tune.
just that it would be nice to hear someone accidentally whistle something of mine,
somewhere, just once." [p54]
There is surely much, much more to be said--to
be discovered--about the evolution of music. I chose it as my topic because it
so nicely illustrates the way the traditional perspective on culture and the evolutionary
perspective can join forces, instead of being seen to be in irresolvable conflict.
If you believe that music is sui generis, a wonderful, idiosyncratic feature
of our species that we prize in spite of the fact that it has not been
created to enhance our chances of having more offspring, you may well be right--and
if so, there is an evolutionary explanation of how this can be true. You cannot
evade the obligation to explain how such an expensive, time-consuming activity
came to flourish in this cruel world, and a Darwinian theory of culture is an
ally, not an opponent, in this investigation.
Some memes are like domesticated animals; they are prized for their benefits, and their replication is closely fostered and relatively well understood by their human owners. Some memes are more like rats; they thrive in the human environment in spite of being positively selected against--ineffectually--by their unwilling hosts. And some are more like bacteria or viruses, commandeering aspects of human behavior (provoking sneezing, for instance) in their "efforts" to propagate from host to host. There is artificial selection of "good" memes--like the memes of arithmetic and writing, the theory of counterpoint, and Bach's cantatas, which are carefully taught to each new generation. And there is unconscious selection of memes of all sorts--like the subtle mutations in pronunciation that spread through linguistic groups, presumably with some efficiency advantage, but perhaps just hitchhiking on some quirk of human preference. And there is unconscious selection of memes that are positively a menace, but which prey on flaws in the human decision-making apparatus, as provided for in the genome and enhanced and adjusted by other cultural innovations--such as the abducted-by-aliens meme, which makes perfect sense when its own fitness as a cultural replicator is considered. Only the meme's-eye perspective unites all these possibilities under one view.
Finally, one of the most persistent sources of discomfort about memes is the dread suspicion that an account of human minds in terms of brains being parasitized by memes will undermine the precious traditions of human creativity. On the contrary, I think it is clear that only an account of creativity in terms of memes has much of a chance of giving us any way to identify with the products of our own minds. We human beings extrude other products, on a daily basis, but after childhood, we don't tend to view our feces with the pride of an author or artist. These are mere biological byproducts, and although they have their own modest individuality and idiosyncracy, it is not anything we cherish. How could we justify viewing the secretions of our poor infected brains with any more pride? Because we identify with some subset of the memes we harbor. Why? Because among the memes we harbor are those that put a premium on identifying with just such a subset of memes! Lacking that meme-borne attitude, we would be mere loci of interaction, but we have such memes--that is who we are.