Co-Evolution of Language-Size and the Critical Period

Abstract

Species evolve, very slowly, through selection of genes which give rise to phenotypes well adapted to their environments. The cultures, including the languages, of human communities evolve, much faster, maintaining at least a minimum level of adaptedness to the external, non- cultural environment. In the phylogenetic evolution of species, the transmission of information across generations is via copying of molecules, and innovation is by mutation and sexual recombination. In cultural evolution, the transmission of information across generations is by learning, and innovation is by sporadic invention or borrowing from other cultures. This much is the foundational bedrock of evolutionary theory. But things get more complicated; there can be gene-culture co-evolution. Prior to the rise of culture, the physical environment is the only force shaping biological evolution from outside the organism, and cultures themselves are clearly constrained by the evolved biological characteristics of their members. But cultures become part of the external environment, and influence the course of biological evolution. For example, altruistic cultures with developed medical knowledge reduce the cost to the individual of carrying genes disposing to certain pathologies (such as diabetes); and such genes become more widespread in the populations maintaining such cultures. Assortative mating can affect biological evolution, and particular cultures may influence the factors which are sorted for in mating. (For a careful discussion of the effects of cultural evolution on natural selection, see Cavalli-Sforza and Bodmer, 1971:774- 804). This paper examines mechanisms involved in the co-evolution of a biological trait, the critical period for language acquisition, and a property of human cultures, the size of their languages. A gene/culture interaction will be shown that can be described as a kind of symbiosis, but perhaps more aptly as an `arms race'. In this introduction, we will sketch the basic mechanics of the interaction in very broad terms; the rest of the paper will explain and justify the details. The implications of our model for second language acquisition are given toward the end of the paper.