From the perspective of artificial life, language can be viewed as a complex adaptive system emerging from linguistic interactions between individuals. Language and the human brain have evolved in parallel and by interacting with each other. In this study, we propose a model of language evolution based on biological evolution and learning. In our model, the linguistic space is expressed in the polar coordinate system in which each possible language is expressed as a point. We conduct evolutionary experiments based on this model, and visualize the results in linguistic space. The distribution trajectory of innate linguistic abilities shows the diversification and complexity of language growth. In the extended experiment, in which the angular coordinates represent the additional effect of the cost of plasticity, we observe a general tendency that the cost of plasticity evolves to become smaller. However, it never evolves to be zero, which might suggest that some cost of plasticity producing the Baldwin effect is adaptive in language evolution.