There is a type of natural selection pressure on language, mentioned above, where language items that are understandable and replicable and transmittable and memorable are passed on while others are not - I think there's a weak selective pressure for this (language items that are already in use have already gone through this filter - it would probably apply more to neologisms)

Much of the contemporary research in phonological theory is heading in these directions. The most promising new work right now is probably Blevins's book Evolutionary Phonology (2008). The argument there takes evolution as a lot more than a metaphor, though for her it's phonological systems rather than cognitive faculties that are evolving.

The idea is basically that articulation and perception are inherently tied to noisy material channels, and thus the sounds encountered by a speaker of a language intrinsically include a broad scope of quasi-random variation. This scope of variation converges over time on relatively more stable, more contrastively adaptive type-categories in highly patterned (though completely atelic) ways, and tracking these pattens allows us to explain both the synchronic system and the diachronic change of languages simultaneously.

You may be interested in this approach on the evolutionary character of some cultural items:

http://rstb.royalsocietypublishing.org/content/369/1642/20130368.short

ABSTRACT:
Darwin-inspired population thinking suggests approaching culture as a population of items of different types, whose relative frequencies may change over time. Three nested subtypes of populational models can be distinguished: evolutionary, selectional and replicative. Substantial progress has been made in the study of cultural evolution by modelling it within the selectional frame. This progress has involved idealizing away from phenomena that may be critical to an adequate understanding of culture and cultural evolution, particularly the constructive aspect of the mechanisms of cultural transmission. Taking these aspects into account, we describe cultural evolution in terms of cultural attraction, which is populational and evolutionary, but only selectional under certain circumstances. As such, in order to model cultural evolution, we must not simply adjust existing replicative or selectional models but we should rather generalize them, so that, just as replicator-based selection is one form that Darwinian selection can take, selection itself is one of several different forms that attraction can take. We present an elementary formalization of the idea of cultural attraction.