Evolution, 4th Edition

486 CHAPTER 18

clades first inhabited the region (FIGURE 18.20B). It appears that tropical regions

have simply had more time to accumulate species [46]. In agreement with this

hypothesis, Paul Fine and Richard Ree determined that the number of tree species

in tropical, temperate, and boreal ecosystems on each continent is correlated with

an index that integrates the area that each of these ecosystems has occupied since

the Miocene, Oligocene, or even as far back as the Eocene [13]. A similar model

accounts for much of the regional variation in the species richness of vertebrates

across the world [20]. Tropical environments and vegetation have occupied larger

areas for a longer time than other environments, so that is where most genera and

species arose.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_18.20.ai Date 12-08-2016

60 50 40 30 20 10 0

Time (Mya)

Hyla (N. A. and Eurasia)

Smilisca clade

Ptychohyla clade

Plectrohyla clade

Acris/Pseudacris (N. A.)

Lophiohylini

Cophomantini

Phyllomedusinae

Pelodryadinae (Australia)

Scinax clade

Dendropsophus clade

(A)

(B)

Number of species

100 20 30 40 50 60 70

Time of colonization of region (Mya)

10

100

1000

1

Hyla versicolor

(temperate)

Agalychnis callidryas

(Phyllomedusinae, tropical)

Tropical America

Temperate N. America, Eurasia, Australia

FIGURE 18.20 Tree frogs (Hylidae) are much less diverse in the

temperate zone than in the tropics because only a few lineages have

adapted to the temperate zone, and those only recently. (A) A molecu-

lar phylogeny of major lineages of Hylidae shows that this family has

radiated in the American tropics and rather recently gave rise to the

three lineages (in blue) that invaded the temperate zone. (B) The num-

ber of tree frog species in a continental region is strongly correlated

with the time since the Hylidae first started diversifying in that region,

as estimated by applying a molecular clock to the branch points in the

phylogeny. (After [46].)

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