Түрлердің қабаттасуынан туындаған эволюциялық өзгерістер

Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co occur, but are minimized or lost where the species' distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource (e. g. food). The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition. Character displacement was first explicitly explained by William L. Brown Jr. and E. O. Wilson in 1956: "Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the populations of that species are similar to the other species and may even be very difficult to distinguish from it. In the area of overlap, where the two species occur together, the populations are more divergent and easily distinguished, i. e., they 'displace' one another in one or more characters. The characters involved can be morphological, ecological, behavioral, or physiological; they are assumed to be genetically based." Brown and Wilson used the term character displacement to refer to instances of both reproductive character displacement, or reinforcement of reproductive barriers, and ecological character displacement driven by competition. including adaptive radiations like the cichlid fish faunas in the rift lakes of East Africa, it also plays an important role in structuring communities. It also plays a role in speciation by reinforcement in such that allopatric populations overlapping in sympatry exhibit greater trait divergence. The results of numerous studies contribute evidence that character displacement often influences the evolution of resource acquisition among members of an ecological guild. Competitive release, defined as the expansion of an ecological niche in the absence of a competitor, is essentially the mirror image of character displacement. It too was described by Brown and Wilson: "Two closely related species are distinct where they occur together, but where one member of the pair occurs alone it converges toward the second, even to the extent of being nearly identical with it in some characters." The existence of character displacement is evidence that the two species do not completely overlap in their niche requirement. Following the dissemination of the concept, character displacement was viewed as an important force in structuring ecological communities, and biologists identified numerous examples. During the late 1970s and early 1980s, however, the role of competition and character displacement in structuring communities was questioned and its importance greatly downgraded. Many found the early examples unconvincing and suggested it to be a rare phenomenon. Criticisms with earlier studies included the lack of rigor in statistical analyses and the use of poorly rationalized characters. These include: (1) differences between sympatric taxa are greater than expected by chance; (2) differences in character states are related to differences in resource use; (3) resources are limiting, and interspecific competition for these resources is a function of character similarity; (4) resource distribution are the same in sympatry and allopatry such that differences in character states are not due to differences in resource availability; (5) differences must have evolved in situ; (6) differences must be genetically based.

Studies have been performed in a wide variety of taxa—a few groups having disproportionately contributed to the understanding of character displacement: mammalian carnivores, Galapagos finches, anole lizards on islands, three spined stickleback fish, and snails. When either one occurred by itself on a smaller island, however, the beak size was intermediate in size relative to when the two co occurred. Most character displacement studies focus on morphological differences in feeding apparatus rather than on those relating to habitat use. However, comparisons of micro habitat use and morphological adaptations of Western and Eastern Rock Nuthatches indicate that these two species show spatial niche segregation in addition to trophic niche segregation. It is often assumed that closely related species are more likely to compete than are more distantly related species, and hence many researchers investigate character displacement among species in the same genus. Two finch species (Geospiza fuliginosa and G. difficilis) exploit more flower nectar on islands where the lager carpenter bee (Xylocopa darwini) is absent than on islands with the bees. Individual finches that harvest nectar are smaller than members of the same species that do not. Many studies have measured niche (often seen in diet) overlap between closely related species, sometimes finding strong niche divergence; seen even in broad niche overlaps. The specific periods of diet divergence are seen as the main cause of adaptive divergence in morphology and performance of a bird species; which can be connected to periods of scarcity. Between the sets of Finches there were low competition. These results are due to correlation between the vast differences in diet coupled with large and adaptive differences in beak morphology. However, with similar levels of Finch phylogeny showed ongoing divergence, diet overlap and competition.

The lizard genus Anolis on the islands in the Caribbean has also been the subject of numerous studies investigating the role of competition and character displacement in community structure. Lesser Antilles islands can only support Anolis species of different sizes, and the relative importance of character displacement versus size at colonization in determining invasion success has been explored and debated.

The Appalachian salamanders Plethodon hoffmani and P. cinereus display no morphological differences, eating habits, or resource use exploitation differences among allopatric populations; when the species occurs in sympatry; however, they exhibit morphological differentiation that is associated with segregation in prey size. Where these two species co occur, P. hoffmani has a faster closing jaw required for larger prey, and P. cinereus has a slower, stronger jaw for smaller prey. Other studies have found Plethodon salamander species that demonstrate character displacement from aggressive behavioral interference rather than exploitation. That is, morphological character displacement between the two species is due to aggressive interaction between them rather than the exploitation of different food resources.

On Okinawa Island, the snail species Satsuma largillierti lives on the eastern half of the island, while Satsuma eucosmia lives on the western half. Both populations overlap in sympatry along the middle of the island, where the penis length of the species differs significantly where they meet in sympatry. The snails' penis lengths exhibit divergence, suggesting reproductive character displacement of this trait.

Threespine sticklebacks (Gasterosteus spp.) in post glacial lakes in western Canada have contributed significantly to recent research of character displacement. Both observations of natural populations and manipulative experiments show that when two recently evolved species occur in a single lake, two morphologies are selected for: a limnetic form that feeds in open water and a benthic form that feeds at the lake bottom. They differ in size, shape and the number and length of gill rakers, all of which is related to divergence in their diet. Hybrids between the two forms are selected against. When only one species inhabits a lake, that fish displays an intermediate morphology. Studies on other fish species have shown similar patterns of selection for benthic and limnetic morphologies,

Introduced species have also provided recent "natural experiments" to investigate how rapidly character displacement can affect evolutionary change. This displacement was observed within a ten year study, demonstrating that competition can drive rapid evolutionary change.