Split Dating
Dating of language splits can be determined if it is known how the characters evolve along each branch of a tree. The simplest assumption is that all characters evolve at a single constant rate with time and that this is independent of the tree branch. This was the assumption made in glottochronology. However, studies soon showed that there was variation between languages, some probably due to the presence of unrecognised borrowing. A better approach is to allow rate variation, and the gamma distribution is usually used because of its mathematical convenience. Studies have also been carried out that show that the character replacement rate depends on the frequency of use. Widespread borrowing can bias divergence time estimates by making languages seem more similar and hence younger. However, this also makes the ancestor's branch length longer so that the root is unaffected.
This aspect is the most controversial part of quantitative comparative linguistics.
Read more about this topic: Quantitative Comparative Linguistics
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