Selection for melanism and for low reproductive rate in Trichosurus vulpecula (Marsupialia).
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Agouti grey pelage, as found in the opossum and many other mammals, is a generalised concealing coloration. In dense vegetation melanic pelage also is concealing, but the melanic phase displaces the grey one in rain forests where, in the current absence of predation, selection is not for colour. This local dominance suggests close linkage of genes for melanism with ones for toleration of humidity.
Since food trees of the opossum are killed by heavy browsing, a low birth rate conserves limited resources. Elimination of surplus young would not be effective in this because there is a long period of maternal protection, and young do not face direct competition for food before reaching an age of one and a half years.
Increase of reproductive rate is beneficial to the species when vacant habitat is available, but such an increase is dangerous unless curbed after population establishment. Field evidence in New Zealand shows that, despite abundance of food, a low reproductive rate is usual. This would be expected if the pioneer and the established opossum belong to alternative genetic phases, analagous to the visually distinct melanic and grey ones