[POS] Evolution and Genetics

[POS-51] Rate of language evolution in a growing population in a stochastic process approach

*Raiki Nakano¹, Hisashi Ohtsuki¹ (1. The Graduate University for Advanced Studies (Japan))

Keywords:

Cultural evolution,Language evolution

Around 7,000 languages are spoken in this world. How languages diverge and when this language diversity emerged are central questions in linguistics. As speciation occurs through accumulation of mutations in DNA, language divergence occurs through accumulation of mutations in linguistic forms such as in words. A mutated word—a new word—is generated either by cultural innovation or cultural transmission error. Basically, language does not permit the existence of absolute synonymy (a pair of words that has exactly the same meaning but has distinct forms). For this reason, the emergence of a new word triggers cultural evolutionary dynamics of words, which could lead to the extinction of its synonym. In the dynamics of words, learning, forgetting, and recalling play key roles in the change of frequency of words. Here, we hypothesize that serial migration of humans that started from Out of Africa, along with the spread of small populations across the world, accelerated the rate of language evolution due to initial small population size. This is because the dynamics of word replacement in a small population are more affected by random drift than by frequency-dependent selection. To examine this hypothesis, we construct a stochastic model of word change in a growing population. We assume that individuals have a word representing a meaning or do not have such a word. A new word is produced either through innovation among individuals who do not know such a word, or through a mistake in learning process. The frequency of a word can increase through learning in a conversation between two randomly chosen individuals when one knows the word and the other does not. If no such conversation occurs, the word is forgotten with a given probability. Additionally, the population grows until it reaches a certain population size. We then compare the rate of word change in a growing population with that in a population of constant size.