Mutual Facilitation and the Fitness of Polymorphic Populations

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Abstract
Estimates of the viability from first instar to eclosion have been made for single generation vial population of Drosophila pseudoobscura. The populations used were monomorphic ("pure") or dimorphic or trimorphic ("mixed") for individuals with the Chiricahua and/or Arrowhead gene arrangements of the third chromosome, the initial population size being (30-larvae per vial). The viability of the "mixed" populations was compared with the expected values derived from the "pure" populations. The dimorphic combination AR/AR+AR/CH and AR/AR+CH/CH were more productive than expected, the combination AR/CH + CH/CH less productive. The trimorphic combination AR/AR + AR/CH +CH/CH was the most viable, showed the highest positive deviation from the expected value and showed markedly less between-culture variance in mean viability. These findings are interpreted to mean that there exists differential utilization of niches by the different morphs, though whether the niches are controlled by the presence of the morphs or exist in the physical environment is not known. These results show that the greater fitness of polymorphic Drosophila populations compared with monomorphic is in part determined by mutal facilitation. The relevance of these results to genetic diversity within populations and selection between populations is discussed.