Book excerpt: This Research Topic is the second volume of "The Adaptive Value of Languages: Non-Linguistic Causes of Language Diversity". Please see the first volume here.The goal of this Research Topic is to shed light on the non-linguistic causes of language diversity and, specifically, to explore the possibility that some aspects of the structure of languages may result from an adaptation to the natural and/or human-made environment. Traditionally, language diversity has been claimed to result from random, internally-motivated changes in language structure. Ongoing research suggests instead that different factors that are external to language can promote language change and ultimately account for aspects of language diversity. Accordingly, linguistic complexity has been found to correlate with features of the social environment, such as the absence of cross-cultural exchanges or the number of native speakers. Likewise, language structure could be influenced by the physical environment, as the effect of dry climates on tone seemingly shows. Finally, core properties of human languages, like duality of patterning, have been argued to result from iterative learning and cultural evolution, as research in village sign languages illustrates. On the whole this means that some aspects of languages could be an adaptation to ecological, social, or even technological niches. Eventually, certain gene alleles, provided that they bias language acquisition or processing, may affect language change through iterated cultural transmission, and ultimately, to language structure.