Pleiotropy and preadaptation in the evolution of human language capacity.

The capacity for spoken language in the human is a genetic trait, but the information communicated by this means is to a large extent culturally determined. Using a gene-culture coevolutionary approach, we model the hypothesis that speech evolved as a channel for the communication of adaptive cultural traits from parent to offspring. The motivation for this paper is a condition obtained previously that initial increase of communication would require at least a two-fold advantage for the transmitted trait. Here, we show that under reasonable assumptions the invasion condition becomes less stringent. In Model 1, we assume that two adaptive cultural traits can be transmitted. A gene which permits communication of the second adaptive trait. In Model 2, we assume that a related function such as greater memory capacity is a prerequisite for speech, and that this function confers an advantage independent of its association with speech. In both models we assume haploid sexual genetics and a simple scheme of vertical transmission. The stability properties of all corner and edge equilibria of the models are analyzed. The two models taken together suggest a possible scenario for the initial stages of the evolution of speech.