|A simplified version of the difference between allopatry and sympatry|
Subsequent differences in mutation and selection between those two populations drives them apart to the point that they can be considered new species - an ancestral branch splits into two. There are two broad mechanisms for how this splitting occurs.
The first, allopatry, involves a geographical mating barrier such as a mountain range or a river which cannot be physically traversed and so is an effective means of preventing gene flow. The second, sympatry, is more contentious.
In sympatric speciation there is no geographical barrier. Instead a population becomes divided from within by a pre-mating barrier. This is any biological mechanism which ensures that groups of individuals within the population breed at different times. A common argument is that small variations in the microhabitat result in allopatric divisions, creating an illusion of sympatry when the habitat and its population is viewed as a whole. If sympatry is considered as a spatial rather than a biological variable, however, then preferences between microhabitats cannot be classed as sympatric effects, but as examples of niche displacement. True sympatry must therefore be another kind of phenomenon entirely.
A problem exists between the nature of sympatry - two genetically distinct populations occupying the same area and so its resources - and the basic ecological principle of competition. Interspecific competition between meta-populations (distinct groups within an over-arching population) and true species, and competitive exclusion (the better adapted meta-population or species will out-compete the other and either force it out of the area or to extinction) should mean that two meta-populations or species should not be able to sympatrically coexist. The logical conclusion of nature is read as being 'red in tooth and claw.'
A solution to the problem of the functional nature of sympatry has been proposed by Roberto Cazzolla Gatti from Tomsk State University, Russia; a solution which would add a profound twist into the nature of evolutionary theory.
'My model predicts that the coexistence of two species in a sympatric way can happen only if there is low competition or weak competitive exclusion between them and a kind of avoidance of competition that leads to a slight shift of the niche of a meta-population, which accumulated a series phenotypic difference due to genomic inclusions coming from other sources of genes. Thus, eventually, it's the avoidance of competition that drives the expansion of the diversity of living beings' said Gatti in his 2016 paper.
Gatti has proposed that this process of avoidance should be called endogenosymbiosis after the idea of endosymbiosis proposed by Lynn Margulis in 1960, as it relies on cooperation rather than conflict. In this new model, cooperation by the avoidance of conflict is responsible for the generation of biological diversity. Meanwhile the classical drives of natural selection, mutation and competition, become responsible for preserving and adapting lineages after their sympatric creation. Gatti proposed this model in 2015 but it is only recently that empirical evidence to support it has been found.
|An example of a stickleback species|
'We cannot know for sure that the Lake Constance sticklebacks will continue evolving until they become two non-interbreeding species' said Marques. 'But evidence for sympatric speciation is growing, from mole rats in Israel to palms on Lord Howe Island, Australia, and apple maggots evolved from hawthorn maggots in North America, leading some evolutionary biologists to think it could be surprisingly common.'
This example of sympatry clearly does not fit with the previous requirement of a pre-mating barrier, but as it does not involve a geographical mating barrier, neither can it be considered allopatric. Yet it fits with Gatti's conceptualization of sympatry within his cooperative model of evolution. This model will likely spark considerable debate. It may well be able to stand alone as an explanation for the functional mechanism behind sympatry. The potential for integration with other ideas in evolution, such as punctuated equilibrium or hierarchies of selection, on the other hand, remains unclear and is ripe for further study.