Differential Survival of Genetic Variants in Crowded Populations of Phlox

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Abstract
The nature of change in genetic structure during thinning of crowded mixtures of P. drummondii Hook. genotypes was investigated in 2 experiments; one with cultivars and the other with a natural population having variable flower color and eye-markers under 2 nutrient levels. The cultivars differed in maximum percentage germination and in number of days to reach maximum germination. After 38 days of growth the frequency of the genotypes (cultivars) in both nutrient treatments was not significantly different from the expected frequencies based on the seed germination data. However, 60 days and 95 days after seedling emergence the frequency of cultivars were significantly different from the expected frequencies from seed germination as well as from their frequencies at the 1st census. There were striking differences in survivorship of the different genotypes and the relative survivorship of genotypes varied as a function of population age and soil nutrient level. In the experiment with the natural population, there was also genotype-specific thinning. For example, the frequency of the white flower morph was significantly lower than expected. The results of these 2 experiments show that there are genotype-specific survivorship curves which are differentially modified by nutrient status and that thinning in crowded populations is determined by genetic identity of the individuals as well as extrinsic factors of the environment.