that explicitly test for parapatric speciation. However, research concerning sympatric speciation often lends support to the occurrence of parapatry. This is due to the fact that, in symaptric speciation, gene flow within a population is unrestricted; whereas in parapatric speciation, gene flow is limited—thus allowing reproductive isolation to evolve easier. Ödeen and Florin complied 63 laboratory experiments conducted between the years 1950–2000 (many of which were discussed by Rice and Hostert previously) concerning sympatric and parapatric speciation. They contend that the laboratory evidence is more robust than often suggested, citing laboratory populations sizes as the primary shortcoming. Observational evidence Parapatric speciation is very difficult to observe in nature. This is due to one primary factor: patterns of parapatry can easily be explained by an alternate mode of speciation. Particularly, documenting closely related species sharing common boundaries does not imply that parapatric speciation was the mode that created this geographic distribution pattern. Coyne and Orr assert that the most convincing evidence of parapatric speciation comes in two forms. This is described by the following criteria: Species populations that join, forming an ecotone can be interpreted as convincingly forming in parapatry if: No evidence exists for a period of geographic separation between two closely related species Different loci are not in agreement along the cline Phylogenies including sister groups support different divergence times An endemic species that exists within a specialized habitat next to its sister species that does not reside in the specialized habitat strongly suggests parapatric speciation. This has been exemplified by the grass species Agrostis tenuis that grows on soil contaminated with high levels of copper leeched from an unused mine. Adjacent is the non-contaminated soil. The populations are evolving reproductive isolation due to differences in flowering. The same phenomenon has been found in Anthoxanthum odoratum in lead and zinc contaminated soils. Speciation may be caused by allochrony. Clines are often cited as evidence of parapatric speciation and numerous examples have been documented to exist in nature; many of which contain hybrid zones. These clinal patterns, however, can also often be explained by allopatric speciation followed by a period of secondary contact—causing difficulty for researchers attempting to determine their origin. Thomas B. Smith and colleagues posit that large ecotones are "centers for speciation" (implying parapatric speciation) and are involved in the production of biodiversity in tropical rainforests. They cite patterns of morphologic and genetic divergence of the passerine species