Predators are extremely effective agents of selection. After all, if an individual member of a prey species does not survive long enough to reproduce, it will have lost its chance (kin selection considerations apart) to bequeath its genes to future generations. It is not surprising, therefore, that many cases of population difference have been attributed to geographic variation in risk. These population differences can take a variety of forms and may, for example, involve modifications to morphology or to life-history traits. The correlation between armor and predation in the three-spined stickleback, Gasterosteus aculeatus, is one case that has been well documented (see Reimchen 1994 for a review and discussion), while another is the association between reproductive allotment and risk (Reznick and Endler 1982) and male color pattern and risk (Endler 1980) in the Trinidadian guppy, Poecilia reticulata. However, such adaptations can be futile if they are not accompanied by effective antipredator behavior. For instance, a cryptic color pattern confers no advantage if its holder chooses the “wrong” background or behaves in a conspicuous manner. Behavior is also flexible in a way that life histories or morphology may not be, and it allows moment-to-moment changes in response as risk increases or decreases. Because it is such an important weapon in the evolutionary arms race, antipredator behavior provides important insights into the causes and consequences of natural selection. Some of the best examples of geographically variable antipredator responses occur in populations of freshwater fish (see, e.g., Bell and Foster 1994). The predation regime of these populations is relatively easy to classify—at least in terms of the presence and absence of predatory species—and the distribution of key predators can explain much of the documented variation in antipredator behavior (see p. 140). Covariance in predation regime and antipredator responses is compelling evidence for natural selection. Moreover, because predation regimes can change (or be manipulated) over relatively short periods of time, there is an opportunity to record heritable changes in antipredator responses—in other words, to watch evolution in action.
Characidium xanthopterum (Ostariophysi: Characiformes: Crenuchidae): a new species from the Central Brazilian Plateau