Evolution Evolutionary consequences of niche construction and their implications for ecology ( ecosystem engineering y adaptation )

Organisms regularly modify local resource distributions, inf luencing both their ecosystems and the evolution of traits whose fitness depends on such alterable sources of natural selection in environments. We call these processes niche construction. We explore the evolutionary consequences of niche construction using a two-locus population genetic model, which extends earlier analyses by allowing resource distributions to be inf luenced both by niche construction and by independent processes of renewal and depletion. The analysis confirms that niche construction can be a potent evolutionary agent by generating selection that leads to the fixation of otherwise deleterious alleles, supporting stable polymorphisms where none are expected, eliminating what would otherwise be stable polymorphisms, and generating unusual evolutionary dynamics. Even small amounts of niche construction, or niche construction that only weakly affects resource dynamics, can significantly alter both ecological and evolutionary patterns. There is increasing recognition that all organisms modify their environments (1), a process that we call ‘‘niche construction’’ (2) but is elsewhere described as ‘‘ecosystem engineering’’ (3). Such modifications can have profound effects on the distribution and abundance of organisms, the influence of keystone species, the control of energy and material f lows, residence and return times, ecosystem resilience, and specific trophic relationships (3–6). The consequences of environment modification by organisms, however, are not restricted to ecology, and organisms can affect both their own and each other’s evolution by modifying sources of natural selection in their environments (2, 7). We have argued that niche construction is responsible for hitherto-neglected forms of feedback in evolution, whereby legacies of ancestrally modified natural selection pressures affect the subsequent evolution of later generations of organisms (7–9). There are numerous examples of organisms choosing or changing their habitats or constructing artifacts, leading to an evolutionary, as well as an ecological, response (see refs. 2, 3, and 7–9). For instance, orb-web spiders construct webs, which have led to the subsequent evolution of camouflage, defense, and communication behavior on the web (10). Similarly, ants, bees, wasps, and termites construct nests that are themselves the source of selection for many nest regulatory, maintenance, and defense behavior patterns. For example, many ant and termite species regulate temperature by plugging nest entrances at night or in the cold, by adjusting the height or shape of their mounds to optimize the intake of the sun’s rays, or by carrying their brood around their nest to the place with the optimal temperature and humidity for the brood’s development (11, 12). The construction of artifacts is equally common among vertebrates. Many mammals (including badgers, gophers, ground squirrels, hedgehogs, marmots, moles, mole rats, opossum, prairie dogs, rabbits, and rats) construct burrow systems, some with underground passages, interconnected chambers, and multiple entrances (13). Here, too, there is evidence that burrow defense, maintenance, and regulation behaviors have evolved in response to selection pressures that were initiated by the construction of the burrow (11, 13). Most cases of niche construction, however, do not involve the building of artifacts but merely the selection or modification of habitats. For example, as a result of the accumulated effects of past generations of earthworm niche construction, present generations of earthworms inhabit radically altered environments where they are exposed to modified selection pressures (14, 15). Previously, we described this legacy of modified selection pressures as an ‘‘ecological inheritance.’’ Nor is niche construction confined to animals. Plants change the chemical nature, the pattern of nutrient cycling, the temperature, the humidity, the fertility, the acidity, and the salinity of their soils and the patterns of light and shade in their habitats (16–19). For instance, pine and chaparral species increase the likelihood of forest fires by accumulating oils or litter, with the probable evolutionary consequence of having evolved a resistance to fire and in some species a dependency