such as the 105 superspecies in Melanesia, comprising 66 percent of all bird species in the region. Patagonia is home to 17 superspecies of forest birds, while North America has 127 superspecies of both land and freshwater birds. Sub-Saharan Africa has 486 passerine birds grouped into 169 superspecies. Australia has numerous bird superspecies as well, with 34 percent of all bird species grouped into superspecies. Laboratory evidence Experiments on allopatric speciation are often complex and do not simply divide a species population into two. This is due to a host of defining parameters: measuring reproductive isolation, sample sizes (the number of matings conducted in reproductive isolation tests), bottlenecks, length of experiments, number of generations allowed, or insufficient genetic diversity. Various isolation indices have been developed to measure reproductive isolation (and are often employed in laboratory speciation studies) such as here (index and index ): Here, and represent the number of matings in heterogameticity where and represent homogametic matings. and is one population and and is the second population. A negative value of denotes negative assortive mating, a positive value denotes positive assortive mating (i. e. expressing reproductive isolation), and a null value (of zero) means the populations are experiencing random mating. The experimental evidence has solidly established the fact that reproductive isolation evolves as a by-product of selection. Reproductive isolation has been shown to arise from pleiotropy (i.e. indirect selection acting on genes that code for more than one trait)—what has been referred to as genetic hitchhiking. Limitations and controversies exist relating to whether laboratory experiments can accurately reflect the long-scale process of allopatric speciation that occurs in nature. Experiments often fall beneath 100 generations, far less than expected, as rates of speciation in nature are thought to be much larger. Furthermore, rates specifically concerning the evolution of reproductive isolation in Drosophila are significantly higher than what is practiced in laboratory settings. Using index Y presented previously, a survey of 25 allopatric speciation experiments (included in the table below) found that reproductive isolation was not as strong as typically maintained and that laboratory environments have not been well-suited for modeling allopatric speciation. Nevertheless, numerous experiments have shown pre-zygotic and post-zygotic isolation in vicariance, some in less than 100 generations. Below is a non-exhaustive table of the laboratory experiments conducted on allopatric speciation. The first column indicates the species used in the referenced study, where the "Trait" column refers to the