Biological evolution and languages changing over time. They're so similar that we can create a lot of parallels between both that are useful for both disciplines:

• "species" as defined by the shared genetic pool vs. "language" as defined by mutual intelligibility
• ring species vs. dialect continuum
• horizontal gene transmission vs. borrowing of vocabulary, grammatical features, and phonological features
• cladograms are pretty much the same, including their pros and cons
• latest common ancestor being a hypothetical construct vs. "proto-" reconstructions (e.g. Proto-Romance vs. Latin)

Because the underlying reasoning is exactly the same. You have an abstract system that piles up small changes over time, and those changes may be shared across different populations within that pool.

You need to watch out for a few differences too. For most part, biological evolution is driven by the interaction between phenotype and the environment, while linguistic evolution is more often than not some mutation with no intrinsic selective value, piggybacking on something outside language (e.g. speaker prestige).