Biological evolution proceeds through the mutation of genes and natural selection. Genes that confer an adaptive advantage to their host organism get reproduced at a greater rate than genes that fail to confer an adaptive advantage, because the more adaptive organism survives and reproduces, while the non-adaptive organism does not. In a sense, adaptive genes are rewarded by natural selection by being reproduced, while non-adaptive genes are punished by being weeded out.
In his book, The Selfish Gene, evolutionary biologist Richard Dawkins has argued forcibly that something similar can occur with ideas and social practices. Dawkins coined the term "memes" for ideas and social practices that compete for believers and practitioners. Of course, strictly speaking, humans compete with ideas and behavior. Ideas are not autonomous beings, rather they are chunks of information in the minds of humans and cannot compete on their own. Similarly, practices are always embodied in physical things like humans, and so practices cannot compete on their own either. Nonetheless, some ideas and practices promote their own success by encouraging their own survival and accurate reproduction in large numbers of believers, often with rewards for the reproduction of the memes and punishments for the failure to do so. Such memes will successfully compete against memes that do not promote their reproduction as aggressively. In the competition for believers, memes that aggressively promote their own survival and reproduction propagate while those that do not die out. This is what we mean by memic selection.
In real life, memes are usually sets of related ideas that we can call belief systems, and they must do more than simply require their reproduction in people. I can easily create a meme by simply defining a new and aggressively competitive idea. Take the idea of the "apple seed demon," which I define as a demon that inhabits all apple seeds and that commands us to believe in its existence and to persuade other people to believe in its existence. The apple seed demon also will reward those who embrace the idea with an abundance of apples fed to the believers during their sleep, and it will punish those who fail to embrace the idea by turning their brains into runny eggs. This idea is in competition with standard ideas about apples, and I suspect it would lose out against the standard view. I'd be surprised if anyone accepted it no matter how strenuous I advocated it. The idea will not succeed in propagating itself because it does not fit in with our many other beliefs about the world, and the alleged reward is insufficient to attract most people. But if a meme is more consistent with popular beliefs, and is psychologically or emotionally attractive, it will be a serious competitor.
As Susan Blackmore (The Meme Machine) and others have pointed out, many religious beliefs exemplify memic selection, especially those that advocate spreading the "message." Christianity and Islam are both highly competitive and successful memes. Through the idea of a father-like god, they offer a sense of grander purpose, of security, and of belonging that are familiar and attractive to all humans. At the social level, they provide cultural identity, legitimize social structure, and sanctify moral codes. As memes, they are similar in several crucial regards. Both claim to be the only true religious teaching. This teaching must be reproduced (copied) in the minds of as many people as possible, the fidelity of the teaching must be preserved (and hence the beliefs of the religion are unquestionable dogmas), and embracing the teaching is rewarded with salvation in an afterlife, while failure to do so results in punishment via death or eternal suffering. In addition, both of these religious memes include the idea that any doubting thoughts that bring the meme into question and all opposition to its reproduction are due to an evil force that opposes the meme, which must be vanquished.
Clearly these memes are very competitive because they include their own, built-in reproduction drive (you must go out and spread the teaching), reproduction/copy error protection (new comers must accept the teaching as an unalterable dogma), and defensives against competing memes (doubting the teaching and opposition to it are evil and must be vanquished). Either of these sectarian memes will win out over less aggressive memes because their believers will work harder at winning converts and more staunchly resist doctrinal deviations and opposition. This is why evangelical Christianity is the most rapidly growing religion in America today, as opposed to Unitarianism, which is non-sectarian and non-proselytizing. In the Islamic world, it is one or another variant of fundamentalism that is ascendant, not the more liberal variants of Islam.
I do not mean to say that this is all there is to competition among different religious memes. Tradition, language, customs, geography, population differences, economics and politics all play a role in the battle for minds and souls. Sometimes these are the deciding factors, as they have in fact often been in the long struggle between Christianity and Islam. But when a very aggressive meme competes with a less aggressive meme, the more aggressive meme has the advantage by fact of its reproductive aggressiveness.
Are we then to give up on tolerance, the idea that many different memes should coexist? The very idea of tolerance is a very non-aggressive meme. For example, Unitarians don't exert much effort in trying to convert evangelical Christians. On the other hand, it is part of the tolerance belief system that over the long run, tolerance is superior to intolerance for reducing conflict and producing greater happiness. And tolerant people do encourage intolerant people to try to be more tolerant, so the tolerance meme does have a reproductive drive, even if not as aggressive as that of its sectarian competitors. The idea being that if enough of us understand the advantages of tolerance, the tolerance meme will succeed against the sectarian memes. Indeed, those who already embrace the tolerance meme are very resistant to sectarian political, ethnic and religious memes. In spite of the efforts of proselytizing Bible thumpers, few Unitarians (or philosophers for that matter!) are converted. In fact, it is more common that fundamentalists switch to other fundamentalist variants or to a more tolerant meme, be it a liberal religious variant or a non-theist belief system. With a sufficiently educated population, the tolerance meme is a sure winner. But in an uneducated and superstition-bound populace, it will lose out. Look around you. Look inside you. What kind of memes do you see?