One way to examine how strongly natural selection has operated on antipredator behavior is to compare the antipredator behavior of individuals that live in populations that differ in terms of predation pressure. The between-population approach generally works as follows: find two (or more) populations of the same species that live in environments that are similar, but differ in one significant way -predation. In population 1, we might find the species we are studying under strong predation pressure, while in population 2, we might have individuals under little, perhaps even no such predation pressure. We then look for differences in the antipredator behaviors across these populations. If such differences are uncovered, it suggests that natural selection has been operating on antipredator behaviors.
The population comparison method for studying selection and antipredator behavior has been employed in many animal systems including (but not limited to) ground squirrels, guppies, sticklebacks, and minnows. For example, researchers have examined antipredator behavior in two different populations of minnows (Phoxinus phoxinus). Minnows from the Dorset area of southern England and the Gwynedd area of northern Wales were chosen, as the Dorset population is under strong predation pressure from pike predators, while pike are absent from the Gwynedd population of minnows. While individual fish from southern England and northern Wales both look like minnows, and are generally the same size, their antipredator repertoires are quite different.
In the laboratory, before exposure to a predator, Dorset minnows (those from high-pike-predation areas) were found to swim around in larger groups than Gwynedd minnows. Individuals in such large groups often have greater safety from predators. Dorset minnows also seemed to have more stable groups, with less movement of individuals from group to group than the Gwynedd fish. Once a predator was added to the protocol, both minnow populations dramatically increased their group sizes. What is particularly interesting, though, is that once the predator was removed, it took the Gwynedd minnows significantly longer to adjust their group size back to normal. So, not only did the high-predation Dorset minnows have generally stronger antipredator responses, but they were also quicker to respond to the removal of danger by resuming normal, nonpredator-based activities.
Returning to the foraging/predation tradeoff we discussed earlier, Gwynedd minnows completely ceased eating once a predator was presented, while Dorset minnows, who are accustomed to foraging in the face of danger, curtailed their foraging activity, but not nearly to the extent of Gwynedd fish. With respect to inspection behavior, Dorset minnows inspected more often than did Gwynedd minnows, but they were also much more likely to stop inspecting if a conspecific was eaten by a pike. When adult minnows from Gwynedd and Dorset populations were captured in the wild, and their offspring were raised in the laboratory, results suggest that natural selection has produced 'hardwired' differences in antipredator behavior across these two populations.
Interpopulation differences in antipredator behavior have also been well studied in guppies native to the Northern Mountains of Trinidad and Tobago. In many of these streams, guppies can be found both upstream and downstream of a series of waterfalls. These waterfalls, however, act as a barrier to many of the guppy's predators. Upstream of such waterfalls, guppies are typically under only slight predation pressure from larger species of fish; while downstream populations of guppies are often under severe predation pressure from numerous piscine (i.e., fish) predators.
Guppies from high-predation sites mature faster, produce more broods of (smaller) offspring, and tend to channel their resources to reproduction when compared to guppies from low-predation sites. These are all antipredator adaptations. At high predation sites, guppy predators tend to be large and can eat a guppy no matter how large it gets. At such sites, producing many smaller fish should be favored by natural selection, as this is akin to buying lots of lottery tickets and hoping that one is a winner. At low predation sites, only a single small fish predator (Rivulus hartii) of guppies exists. If guppies can get past a certain size threshold, they are safe from R. hartii. As such, natural selection favors females producing fewer, but larger offspring, who can quickly grow large enough to be out of the zone of the danger associated with R. hartii, and this is precisely what researchers have found. What's more, transplant experiments demonstrate that when low-predation fish are transferred to high-predation sites, natural selection quickly acts, and after only a handful of generations, descendants of the transplanted fish have converged on the traits associated with living in high predation. Reciprocal experiments have found the same result when high-preda-tion fish are transferred to low-predation sites.
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