islets. A sympatrically distributed triplet of diving beetle (Paroster) species living in aquifers of Australia's Yilgarn region have likely speciated microallopatrically within a 3.5 km2 area. The term was originally proposed by Hobart M. Smith to describe a level of geographic resolution. A sympatric population may exist in low resolution, whereas viewed with a higher resolution (i.e. on a small, localized scale within the population) it is "microallopatric". Ben Fitzpatrick and colleagues contend that this original definition, "is misleading because it confuses geographical and ecological concepts". Modes with secondary contact Ecological speciation can occur allopatrically, sympatrically, or parapatrically; the only requirement being that it occurs as a result of adaptation to different ecological or micro-ecological conditions. Ecological allopatry is a reverse-ordered form of allopatric speciation in conjunction with reinforcement. First, divergent selection separates a non-allopatric population emerging from pre-zygotic barriers, from which genetic differences evolve due to the obstruction of complete gene flow. The terms allo-parapatric and allo-sympatric have been used to describe speciation scenarios where divergence occurs in allopatry but speciation occurs only upon secondary contact. These are effectively models of reinforcement or "mixed-mode" speciation events. Observational evidence As allopatric speciation is widely accepted as a common mode of speciation, the scientific literature is abundant with studies documenting its existence. The biologist Ernst Mayr was the first to summarize the contemporary literature of the time in 1942 and 1963. Many of the examples he set forth remain conclusive; however, modern research supports geographic speciation with molecular phylogenetics—adding a level of robustness unavailable to early researchers. The most recent thorough treatment of allopatric speciation (and speciation research in general) is Jerry Coyne and H. Allen Orr's 2004 publication Speciation. They list six mainstream arguments that lend support to the concept of vicariant speciation: Closely related species pairs, more often than not, reside in geographic ranges adjacent to one another, separated by a geographic or climatic barrier. Young species pairs (or sister species) often occur in allopatry, even without a known barrier. In occurrences where several pairs of related species share a range, they are distributed in abutting patterns, with borders exhibiting zones of hybridization. In regions where geographic isolation is doubtful, species do not exhibit sister pairs. Correlation of genetic differences between an array of distantly related species that correspond to known current or historical geographic barriers. Measures of reproductive isolation increase with the greater geographic distance of separation between