As we discovered in Chapter 11, there are times and places where the tree of life has grown more rapidly than others. For example, islands and island-like habitats are sometimes described as hotspots of speciation (Losos and Schluter 2000; Hughes and Eastwood 2006), and diversification rates in such habitats can proceed at an extremely rapid pace. On a broader scale, many studies have shown that speciation rates are elevated and/or extinction rates depressed following mass extinctions (e.g. Sepkoski 1984). Finally, some clades seem to diversity much more rapidly than others. In my corner of the world, the Pacific Northwest of the United States, this variation is best seen in our local amphibians. We have species like the spotted frog and the Pacific tree frog, which represent two very diverse frog lineages with high diversification rates (Ranidae and Hylidae, respectively; Roelants et al. 2007). At the same time, if one drives a bit to the high mountain streams, you can find frogs with tiny tails. These Inland Tailed Frogs are members of Ascaphidae, a genus with only two species, one coastal and one inland. (As an aside, the tail, found only in males, is an intromittent organ used for internal fertilization - analogous to a penis, but different!) These two tailed frog species are the sister group to a small radiation of four species frogs in New Zealand (Leopelmatidae, which have no tails). These two clades together - just six species - make up the sister clade to all other frogs, nearly 7000 species (Roelants et al. 2007; Jetz and Pyron 2018). We seek to explain patterns like this contrast in the diversity of two groups which decend from a common ancestor and are, thus, the same age.
Simple, constant-rate birth-death models are not adequate to capture the complexity and dynamics of speciation and extinction across the tree of life. Speciation and extinction rates vary through time, across clades, and among geographic regions. We can sometimes predict this variation based on what we know about the mechanisms that lead to speciation and/or extinction.
In this chapter, I will explore some extensions to birth-death models that allow us to explore diversification in more detail. This chapter also leads naturally to the next, chapter 13, which will consider the case where diversification rates depend on species’ traits.