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Distribution, Adaptation and Probable Origin of an All-Female Form of Poeciliopsis (Pisces: Poeciliidae) in Northwestern Mexico

William S. Moore, Robert Rush Miller and R. Jack Schultz
Evolution
Vol. 24, No. 4 (Dec., 1970), pp. 789-795
DOI: 10.2307/2406558
Stable URL: http://www.jstor.org/stable/2406558
Page Count: 7
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Distribution, Adaptation and Probable Origin of an All-Female Form of Poeciliopsis (Pisces: Poeciliidae) in Northwestern Mexico
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Abstract

The distribution of the all-female "species" P. monacha-occidentalis over the range of its "host species" P. occidentalis is presented. The all-female form achieves greatest concentration at the southern limit of its range in the Rio Mayo and is successively diminished in the four northerly drainages. Within a given drainage the proportion of P. monacha-occidentalis increases from very low levels at its western extent, along the eastern edge of the Sonoran Desert, to maximum levels midway between the eastern and western boundaries of its range. To the east of this maximum, considerable localized fluctuation in proportional levels of P. monacha-occidentalis occurs. The hypothesis which most parsimoniously explains the origin and distribution of the unisexual form, P. monacha-occidentalis, is: 1) P. monacha-occidentalis arose from P. monacha-lucida, a unisexual form of a more southern origin which has migrated north into the range of P. occidentalis. 2) It sustains itself by mating with males of the indigenous P. occidentalis. In so doing, it inherits not only some of its morphological characters but also some of its ecological requirements. All-femaleness and the maternal genome are retained, however, via hybridogenesis. 3) The level at which P. monacha-occidentalis females are found in a population is probably determined by the slight differences in niche requirements of P. monacha-occidentalis and the host species P. occidentalis. A stable equilibrium between the two forms appears to exist within a given drainage and possibly within local populations. However, the causative factors in this equilibrium cannot be resolved from these data.

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