484 CHAPTER 18Related species often exhibit phylogenetic niche conservatism: similar ecologi-
cal requirements that they have inherited from their common ancestor. For exam-
ple, Robert Ricklefs and colleagues have found that congeneric plants in eastern
Asia and eastern North America have similar latitudinal and climate distributions,
as do genera shared between North America and Europe [33, 38]. Similarly, many
lineages of herbivorous insects have remained associated with the same genus
or family of food plant; some of these associations have remained unchanged for
more than 40 My [47].
Niche conservatism contributes to our understanding of the geographic distri-
butions of many clades [45]. For instance, oaks (Quercus) and dogwoods (Cornus)
are among the many plant taxa that occur in temperate regions of eastern North
America, Asia, and Europe but have not adapted to warm tropical environments
[11]. Niche conservatism underlies the observation that many species shifted their
geographic ranges during the Pleistocene, rather than adapting in situ to changes
in climate (see Chapter 17). These observations raise important questions about
the ability of species to adapt to new environmental conditions.
What, however, accounts for niche conservatism? Why do species not evolve
broader tolerances, and steadily expand their geographic range or the range of
habitats or resources they use? We discussed these questions in Chapters 8 and 11
and saw that they have been only partly answered.Geographic Patterns of Diversity
The field of community ecology is concerned with explaining the species diversity,
species composition, and trophic structure of assemblages of coexisting species
(often called communities). Both ecological and historical biogeography bear on
these topics, since the geographic ranges of species determine whether or not they
might coexist.
A long-standing topic in community ecology is a pattern called the latitudi-
nal diversity gradient: the numbers of species (and of higher taxa such as genera
and families) decline with increasing latitude, both on land and in the ocean
(FIGURE 18.18). Most taxa of terrestrial animals and plants are far more diverse
in tropical regions, especially in lowlands with abundant rainfall, than in extra-
tropical regions.
Three major hypotheses have been proposed to account for this pattern [12,
28]. First, ecological factors might enable more tropical species to coexist in a
stable community (FIGURE 18.19A). These factors might include high produc-
tivity because of abundant solar energy, or fine partitioning of food resources
among many species. Alternatively, the pattern might be explained by evolu-
tionary dynamics over many millions of years [37, 44]. One of the leaders of
the evolutionary synthesis, the botanist G. Ledyard Stebbins, took this perspec-
tive when he suggested that tropical areas might be a “cradle,” in which new
species arise at a high rate, or a “museum,” in which ancient lineages persist
[42]. Related to the “cradle” idea, the “diversification rate hypothesis” holds that
the rate of increase in diversity has been greater in the tropics for a long time
because of a higher speciation rate, a lower extinction rate, or both (FIGURE
18.19B). For example, David Jablonski and colleagues determined that new gen-
era of marine bivalves have arisen mostly in tropical areas throughout the last
11 My and have spread from there toward higher latitudes while persisting in
tropical regions as well [19].
The “museum” idea is expressed today by the “time and area hypothesis” (FIG-
URE 18.19C), which holds that most lineages have been accumulating species for a18_EVOL4E_CH18.indd 484 3/22/17 1:39 PM