Conclusions

There is perhaps no issue more central to an understanding of the relationship between ecology and evolution than the problem of scale. Do processes and phenomena that occur and are observable on ecological timescales have the same kinds of effects when applied over geological timescales? If not, what are the effects that emerge at these longer timescales that would not be expected from examining the shorter timescales alone? What processes led to these faunal changes? We believe that these and many other similar cases can be usefully approached using an explicit analysis of the stages of the speciation process, and that when they are it will be clear that changes in nutrient conditions have important effects on the evolutionary process; that they are, in fact, one means by which ecological processes scale up to qualitatively different macroevolu-tionary processes.

Nutrients do not provide just a passive "backdrop" for evolution; they can affect the evolutionary process directly by affecting, or even controlling, the processes of speciation and extinction. They can affect speciation in separate but related ways; nutrient supply can affect the probability of the formation of isolated populations, the persistence of those populations, and the genetic differentiation of those populations. A given taxon may respond differently at different times, depending on the environmental conditions. Different taxa may respond differently based on their food requirements, among other factors. The point is that we have at least the potential to analyze episodes of

B CAI formation

Change in circulation/decrease in nutrients

Increase in speciation

Increase in extinction

FIGURE 7.3. (A) Simple model of possible effects of change in temperature and nutrient supply on origination and extinction in the late Cenozoic of the Western Atlantic. (a) Possible effects of temperature change on species origination (e.g., creation of isolates by local extirpation and fragmentation of geographic range, especially of thermophilic taxa); (b) Possible effects of temperature change on species extinction (e.g., through cooling and selective killing off of thermophiles); (c) Possible effects of nutrient change on species origination (e.g., creation of isolates by local extirpation and fragmentation of geographic range, especially of nutriphilic taxa); (d) Possible effects of nutrient change on species extinction (e.g., through selective killing off of nutriphiles) (Allmon, in press). (B) Simple model for effects of formation of the Central American Isthmus (CAI) on origination and extinction in the late Cenozoic of the Western Atlantic (Allmon, in press).

diversification in much greater depth than we have before by breaking the speciation process down into its component stages and analyzing the potential and actual effects of nutrients on each stage.

The nutrient paradox results from the differing scales of ecological and evolutionary processes. Speciation, in general, does not happen on ecological timescales. It can be affected by processes that do, but it may be misleading to extrapolate smoothly from one to the other. High nutrient levels may produce locally lower taxonomic diversity in ecological communities, but at least among some taxa, may also lead to higher taxonomic diversity in evolutionary time.

Because energy flow is so important in ecological systems, analysis of the effects of energy flow in evolution is an important step in forging a more adequate view of exactly how ecology "matters" in evolution (cf., Jackson 1988). Unless we apply an explicit mechanistic linkage between ecological and evolutionary scales, we risk being stuck in making broad and untestable correlations, and in not exploring more thoroughly this essential question in evolutionary biology.

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