Some genera have far more species than others. Is this just luck, or have some genera hit upon a “key innovation” that gives them a benefit and allows more species to accumulate? One possible key innovation in plants is the ability to climb like a vine, which makes it possible to reach above other plants to compete for light. A study counted the number of species in 48 genera that had all evolved climbing (Gianoli 2004). For each of these genera, the researchers also found the most closely related non-climbing genus and counted the number of species in each. The numbers of species in these pairs of closely related genera are listed in the table below. The number of species does not have a normal distribution in either climbing genera or non-climbing genera, and the differences between climbing genera and their most closely related non-climbing genera are also not normally distributed.

a. Plot a histogram of the difference in number of species for each pair.

b. Carry out an appropriate test of the difference in the number of species in climbing and non-climbing genera.

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Climbing 1414 2020 1313 4949 300300 358358 Non-climbing 33 2323 33 4242 99 2626 Climbing 267267 124124 1717 4242 850850 6161 Non-climbing 160160 3636 44 260260 3434 240240