``A great amount of variability, under which term individual differences are always included, will evidently be favourable [for the production of new forms through Natural Selection]. A large number of individuals, by giving a better chance within any given period for the appearance of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, a highly important element of success.''Charles Darwin, [2, chapter 4]
Even though variability is the main theme in natural selection, Charles Darwin concedes that a large number of individuals may compensate for some lack of individual variability. Free Software, unlike typical proprietary software, can be freely copied, not only in terms of freedom, but also in terms of cost. Even though the GNU GPL  does not require anyone to give away software for free, someone who legally obtains a copy of the software is entitled to do so. This means Free Software individuals have an enormous potential to leave many descendants, tending to proliferate.
Proprietary software organisms, in comparison, only breed in confinement, and most often solely by means of cloning. The only situation in which proprietary software is not cloned is when a new version of the software is released. Given that such pieces of software aren't released very often, because of the increased maintenance costs, a large number of copies would be necessary for a software release to enjoy a favorable position over software whose individuals present a higher amount of variability. The immense number of copies explains the difficulty in displacing MS-Office in its marketplace: because it is so much widespread, users fear compatibility problems and difficulties adopting alternatives, therefore they keep using MS-Office.
It might be argued that Free Software for the masses, with binary distributions of packages accompanied by source code that most people don't use, could fall prey to the same problem of low variability. It is certainly true that the variation is lowered, but if enough people make use of the freedom to see and modify the code, they may keep the variability levels of Free Software higher than those of proprietary software, and the combination of high variability with increased reproduction rates may be able to offset a huge number of cloned individuals.
Besides Free and proprietary software licenses, there are other varieties of licenses worth mentioning. Most Freeware allows unlimited copying and use, but no modification, because, in general, source-code is not provided. Note that Freeware and Free Software are very distinct concepts: Freeware most often comes only in binary, pre-compiled form, generally for MS-Windows. Freeware suffers from the same cloning problems as proprietary software.
Other licenses do include the sources, but do not allow them to be freely redistributed, modified or not, which is almost like selling sterile software. One example is the Sun Community Source License , that allows modification for research purposes, but requires a separate commercial agreement for distribution.
Microsoft Shared Source  doesn't enjoy the benefit of increased reproduction rates, since it disallows copying, nor of the increased variation, since it forbids modification. In these senses, it is no different from a proprietary closed-source software license, and it does not benefit the software they cover any more than a closed-source license does. In fact, it may even hamper the development of alternatives, since anyone who ever looks at source code licensed under such strict use terms is tainted for life, risking being sued for copyright violation should they ever create similar software.
The issue of procreation only in confinement may not seem a disadvantage at first, given that it allows for man's selection, and man's-directed selection can be much more effective than random natural selection over short periods of time. However, it must be pointed out that the man that chooses the features of the software to be released, in this case, is not the user, but the company that produces the proprietary software, that wishes to hold control over the software and its users. For example, a company may introduce changes in their software with the sole purpose of making it difficult for competitors, that don't have access to their code, to create software that competes or inter-operates with it.
As in nature, the excessive dominance of a life form may prevent the survival of new life forms that attempt to occupy the same niche, even if the new life forms are better-fitted for the niche. However, if the new life form survives long enough to establish itself in a niche, increased reproduction rates and advantageous variations favor the domination of the niche by the new life form.