Evolution, 4th Edition

THE GEoGRAPHy of EvoluTion 485

longer time in tropical than in extratropical environments. During the Cretaceous

and the first 60 My of the Cenozoic, Earth was warmer than it is today, and much

more of the globe had a tropical climate than now. For that reason, most lineages

originated in tropical climates, and the relatively few lineages that have evolved

adaptations to the stressful temperatures and seasonal fluctuations in food sup-

ply that are typical of the temperate zone are younger lineages that have not had

time to become as diverse. Thus this hypothesis is based on phylogenetic niche

conser vatism [44].

Although productivity seems to have an important effect, the time and area

hypothesis has been supported by many recent studies. For example, the phylo-

geny of tree frogs (Hylidae) indicates that all the major lineages of tree frogs and

their common ancestors were distributed in tropical America (FIGURE 18.20A).

The temperate zone has been invaded by only three lineages. Moreover, the num-

ber of species in each region is positively correlated with the time since tree frog

Q: Orientation of map altered to be vertical with Equator on a horizontal line.

I’ve added countries and either (°N) or (°S) to help place location on a mental map.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_18.18.ai Date 02-02-2017

10° 20° 30° 40° 50° 60°

0

10,000

2000

4000

6000

8000

Number of vascular plant species per 10,000 km

2

Equator Latitude (°N or °S)

Manaus, Brazil (°S)Caracas, Venezuela (°N)Mexico City, Mexico (°N)Rio de Janeiro, Brazil (°S)Miami, U.S. (°N)Buenos Aires, Argentina (°S)New York City, U.S. (°N)Vancouver, Canada (°N)Punta Arenas, Chile (°S)(Chilean Patagonia) Anchorage, U.S. (°N)

FIGURE 18.18 An example of the latitu-

dinal diversity gradient. The number of

species of vascular plants in various regions

of North and South America drops more

than tenfold between the Equator and

high-latitude regions. The cities provide a

latitudinal frame of reference; they do not

correspond to the data points. (After [17].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_18.19.ai Date 12-08-2016

Species richness

Time

Tropical

Temperate

Time for

diversication

Ttrop > Ttemp

(C)

Species richness

Diversication rate

(S – E)trop > (S – E)temp

(B)

Species richness

Species carrying

capacity

Ktrop > Ktemp

(A)

FIGURE 18.19 Three models of species accumulation that have been proposed to ac-

count for the latitudinal diversity gradient. (A) Some ecological hypotheses propose that

tropical locations can support a higher equilibrium number of species (“carrying capacity,”

K) than temperate localities. (B) The diversification rate (difference between the specia-

tion rate, S, and the extinction rate, E) might be higher in the tropics. Species numbers

have not necessarily reached an equilibrium carrying capacity. (C) Lineages diversify at the

same rate, but started to diversify more recently in the temperate zone than in the tropics,

perhaps because they originated in tropical environments and only recently adapted to

the temperate zone. (After [28].)

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