Exercise 19.1

Does Species Richness Increase with Clade Age?

(This exercise is based on Rabosky, D. L., G. J. Slater, and M. E. Alfaro. 2012. Clade age and species richness are decoupled across the eukaryotic tree of life. PLoS Biol 10(8): e1001381.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)


Do older major clades have more species clades than younger ones? If the diversification rate (the speciation rate minus the extinction rate) remains positive, then older clades should be more species-rich than younger ones. The empirical results have been mixed; some studies find evidence supporting this prediction, while others do not.

In a large-scale analysis making use of the “Tree of Life,” Daniel Rabosky and his colleagues examined 1,397 major clades (e.g., insect families) for which information existed on both the age of the clade and estimates of known species richness. More than 1.2 million described species were included, representing most of the known biodiversity.

Use the information in Figure 1 to answer questions 1 and 2.


Figure 1  The taxonomic distribution of the species included in Rabosky’s study.


Question 1. What is the largest color-coded group in the study? How many species does it include?


Question 2. About how many vertebrates were included in the analysis?


Use the information in Figure 2 to answer questions 3 and 4.

Figure 2  The relationship between clade age and number of species. (Note log scale on y-axis.)


Question 3. Does species richness increase or decrease with clade age?


Question 4. Is the trend in question 3 statistically significant? How can you tell?


Use the information in Figure 3 to answer questions 5 through 8.

Figure 3 The relationship between species richness and clade age in twelve groups.


Question 5. In which animal group(s) is there a strong positive relationship between species richness and clade age?


Question 6. What is the relationship between species richness and clade age in mammals?


Question 7. One difficulty in estimating species richness is that many species have yet to be described. Suppose that the number of undiscovered species varies among taxa and that old clades tend to have more undiscovered species. In which direction would the observed relationship between species diversity and clade age be biased?


Question 8. Suppose species in old clades were more likely to go extinct. How would this change the relationship between clade age and species richness?