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Introgression in Planktonic Rotatoria with Some Points of View on Its Causes and Conceivable Results

Birger Pejler
Evolution
Vol. 10, No. 3 (Sep., 1956), pp. 246-261
DOI: 10.2307/2406010
Stable URL: http://www.jstor.org/stable/2406010
Page Count: 16
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Introgression in Planktonic Rotatoria with Some Points of View on Its Causes and Conceivable Results
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Abstract

1. Intermediate forms have been observed between Polyarthra vulgaris and P. dolichoptera, and between Conochilus hippocrepis and C. unicornis. 2. 'The loose association of most of the variables' (Anderson, 1953, p. 296) within my material, but particularly comparisons with more thoroughly studied organisms (especially plants), make it probable that the intermediate forms are due to introgressive hybridization. It is a question of sympatric introgression. 3. Polyarthra vulgaris behaves as eurythermal, P. dolichoptera as cold-tolerant stenothermal. Polyarthra dolichoptera can, in contrast to P. vulgaris, live in water poor in oxygen, and occurs as a rule only in the neighborhood of the bottom. Intermediate forms have been encountered only in tarns (not in lakes). For P. vulgaris the sexual period occurs, as a rule, during the late summer, autumn or early winter, for P. dolichoptera at about the time for the breaking up of the ice. In only two cases have exceptions from this rule been observed, both in tarns with intermediate forms. 4. The two species of Conochilus exhibit isolation of habitat in the waters of Central Europe, but not in the two lakes within the region of Tornetrask (Lapponia, Sweden) where I have found both of them; here they both live eulimnoplanktonic, apparently without hybridizing. Introgressive forms have rarely been found together with one of the parent species (in contrast to the conditions in the species of Polyarthra). They have only been found in lakes or tarns with poor plankton. For this reason it appears probable that in the localities of the hybrids the supply of food is too scant to sustain more than one population of Conochilus. By their wider variation, introgressive forms frequently should possess the best qualifications for the colonization of these localities poor in food or more generally, for the colonization of biotopes which are more extreme than those of the parent species. 5. A feeble isolation of the species has been observed in the Scandinavian mountain range for two couples of species of butterflies. It is possible that there exists a common cause. 6. The repeatedly mentioned local morphological variation of planktonic Rotatoria has sometimes (often?) been caused by introgressive hybridization. 7. The introgressive forms have been found mainly in smaller waters, poor in niches, where, often, both parent species are missing. It appears probable that such forms can occasionally develop in such a direction that they come to constitute new species living side by side with their parent species if they come into new localities which are rich in niches.

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