Many species exhibit high levels of   intraspecific morphological and genetic variation. Variation in mitochondrial   DNA (mtDNA) is particularly prevalent, due to its faster evolutionary rate   compared with nuclear DNA (Avise   2000). Usually, such variation is confined to allopatric populations   and can be explained by long periods of isolation with differing selection   pressures and/or divergence due to genetic drift (Coyne and Orr 2004; Price   2008). High intraspecific mtDNA variation between individuals living in   sympatry is less common, and more difficult to explain. Upon closer   inspection, such divergent sympatric lineages often show evidence of   divergence in other parts of the genome as well as reproductive isolation   between the lineages, implying that they are in fact cryptic species (Hebert   et al. 2004a; Haine et al. 2006).

许多物种表现出高度种内的形态和遗传变异。线粒体DNA（mtDNA）的变异尤其突出，因为它的进化速度比核DNA更快（Avise 2000）。通常，这种变异仅限于异域分布群体，其原因可归纳为长时间的隔离和受到不同的选择压力和/或遗传漂变导致的分化（Coyne and Orr 2004; Price 2008）。共同生活的个体之间的高mtDNA变异不太常见，更难以解释。经过仔细观察，这种不同的同域谱系往往在基因组的其他部分显示出的分化以及谱系之间的生殖隔离，这意味着它们实际上是隐秘的物种分化（Hebert et al. 2004a; Haine et al. 2006）。