Figure 4. Early protolanguage.

Here the probability l that a linguistic principle changes increases gradually across generations by a small amount δl. At generation 1000, the population, M, splits. The number of neutral alleles continues to grow in both subpopulations, A and B, which therefore become increasingly genetically similar (their genetic divergence decreases). Yet their languages diverge rapidly. Parameters: δl = 0.1/W, with W = 2000, l(t = 0) = 0, l(t = 2000) = 0.1; others are as in Figure 1.